31 Oct, 2011
1 commit
-
The changed files were only including linux/module.h for the
EXPORT_SYMBOL infrastructure, and nothing else. Revector them
onto the isolated export header for faster compile times.Nothing to see here but a whole lot of instances of:
-#include
+#includeThis commit is only changing the kernel dir; next targets
will probably be mm, fs, the arch dirs, etc.Signed-off-by: Paul Gortmaker
29 Sep, 2011
18 commits
-
It is possible for the CPU that noted the end of the prior grace period
to not need a new one, and therefore to decide to propagate ->completed
throughout the rcu_node tree without starting another grace period.
However, in so doing, it releases the root rcu_node structure's lock,
which can allow some other CPU to start another grace period. The first
CPU will be propagating ->completed in parallel with the second CPU
initializing the rcu_node tree for the new grace period. In theory
this is harmless, but in practice we need to keep things simple.This commit therefore moves the propagation of ->completed to
rcu_report_qs_rsp(), and refrains from marking the old grace period
as having been completed until it has finished doing this. This
prevents anyone from starting a new grace period concurrently with
marking the old grace period as having been completed.Of course, the optimization where a CPU needing a new grace period
doesn't bother marking the old one completed is still in effect:
In that case, the marking happens implicitly as part of initializing
the new grace period.Signed-off-by: Paul E. McKenney
-
The purpose of rcu_needs_cpu_flush() was to iterate on pushing the
current grace period in order to help the current CPU enter dyntick-idle
mode. However, this can result in failures if the CPU starts entering
dyntick-idle mode, but then backs out. In this case, the call to
rcu_pending() from rcu_needs_cpu_flush() might end up announcing a
non-existing quiescent state.This commit therefore removes rcu_needs_cpu_flush() in favor of letting
the dyntick-idle machinery at the end of the softirq handler push the
loop along via its call to rcu_pending().Signed-off-by: Paul E. McKenney
-
RCU boost threads start life at RCU_BOOST_PRIO, while others remain
at RCU_KTHREAD_PRIO. While here, change thread names to match other
kthreads, and adjust rcu_yield() to not override the priority set by
the user. This last change sets the stage for runtime changes to
priority in the -rt tree.Signed-off-by: Mike Galbraith
Signed-off-by: Paul E. McKenney -
CPUs set rdp->qs_pending when coming online to resolve races with
grace-period start. However, this means that if RCU is idle, the
just-onlined CPU might needlessly send itself resched IPIs. Adjust
the online-CPU initialization to avoid this, and also to correctly
cause the CPU to respond to the current grace period if needed.Signed-off-by: Paul E. McKenney
Tested-by: Josh Boyer
Tested-by: Christian Hoffmann -
It is possible for an RCU CPU stall to end just as it is detected, in
which case the current code will uselessly dump all CPU's stacks.
This commit therefore checks for this condition and refrains from
sending needless NMIs.And yes, the stall might also end just after we checked all CPUs and
tasks, but in that case we would at least have given some clue as
to which CPU/task was at fault.Signed-off-by: Paul E. McKenney
-
Greater use of RCU during early boot (before the scheduler is operating)
is causing RCU to attempt to start grace periods during that time, which
in turn is resulting in both RCU and the callback functions attempting
to use the scheduler before it is ready.This commit prevents these problems by prohibiting RCU grace periods
until after the scheduler has spawned the first non-idle task.Signed-off-by: Paul E. McKenney
Signed-off-by: Paul E. McKenney -
There isn't a whole lot of point in poking the scheduler before there
are other tasks to switch to. This commit therefore adds a check
for rcu_scheduler_fully_active in __rcu_pending() to suppress any
pre-scheduler calls to set_need_resched(). The downside of this approach
is additional runtime overhead in a reasonably hot code path.Signed-off-by: Paul E. McKenney
Signed-off-by: Paul E. McKenney -
The trigger_all_cpu_backtrace() function is a no-op in architectures that
do not define arch_trigger_all_cpu_backtrace. On such architectures, RCU
CPU stall warning messages contain no stack trace information, which makes
debugging quite difficult. This commit therefore substitutes dump_stack()
for architectures that do not define arch_trigger_all_cpu_backtrace,
so that at least the local CPU's stack is dumped as part of the RCU CPU
stall warning message.Signed-off-by: Paul E. McKenney
-
When the ->dynticks field in the rcu_dynticks structure changed to an
atomic_t, its size on 64-bit systems changed from 64 bits to 32 bits.
The local variables in rcu_implicit_dynticks_qs() need to change as
well, hence this commit.Signed-off-by: Paul E. McKenney
-
The in_irq() check in rcu_enter_nohz() is redundant because if we really
are in an interrupt, the attempt to re-enter dyntick-idle mode will invoke
rcu_needs_cpu() in any case, which will force the check for RCU callbacks.
So this commit removes the check along with the set_need_resched().Suggested-by: Frederic Weisbecker
Signed-off-by: Paul E. McKenney -
There is often a delay between the time that a CPU passes through a
quiescent state and the time that this quiescent state is reported to the
RCU core. It is quite possible that the grace period ended before the
quiescent state could be reported, for example, some other CPU might have
deduced that this CPU passed through dyntick-idle mode. It is critically
important that quiescent state be counted only against the grace period
that was in effect at the time that the quiescent state was detected.Previously, this was handled by recording the number of the last grace
period to complete when passing through a quiescent state. The RCU
core then checks this number against the current value, and rejects
the quiescent state if there is a mismatch. However, one additional
possibility must be accounted for, namely that the quiescent state was
recorded after the prior grace period completed but before the current
grace period started. In this case, the RCU core must reject the
quiescent state, but the recorded number will match. This is handled
when the CPU becomes aware of a new grace period -- at that point,
it invalidates any prior quiescent state.This works, but is a bit indirect. The new approach records the current
grace period, and the RCU core checks to see (1) that this is still the
current grace period and (2) that this grace period has not yet ended.
This approach simplifies reasoning about correctness, and this commit
changes over to this new approach.Signed-off-by: Paul E. McKenney
Signed-off-by: Paul E. McKenney -
Add trace events to record grace-period start and end, quiescent states,
CPUs noticing grace-period start and end, grace-period initialization,
call_rcu() invocation, tasks blocking in RCU read-side critical sections,
tasks exiting those same critical sections, force_quiescent_state()
detection of dyntick-idle and offline CPUs, CPUs entering and leaving
dyntick-idle mode (except from NMIs), CPUs coming online and going
offline, and CPUs being kicked for staying in dyntick-idle mode for too
long (as in many weeks, even on 32-bit systems).Signed-off-by: Paul E. McKenney
Signed-off-by: Paul E. McKenneyrcu: Add the rcu flavor to callback trace events
The earlier trace events for registering RCU callbacks and for invoking
them did not include the RCU flavor (rcu_bh, rcu_preempt, or rcu_sched).
This commit adds the RCU flavor to those trace events.Signed-off-by: Paul E. McKenney
-
We now have kthreads only for flavors of RCU that support boosting,
so update the now-misleading comments accordingly.Signed-off-by: Paul E. McKenney
Signed-off-by: Paul E. McKenney -
Add a string to the rcu_batch_start() and rcu_batch_end() trace
messages that indicates the RCU type ("rcu_sched", "rcu_bh", or
"rcu_preempt"). The trace messages for the actual invocations
themselves are not marked, as it should be clear from the
rcu_batch_start() and rcu_batch_end() events before and after.Signed-off-by: Paul E. McKenney
Signed-off-by: Paul E. McKenney -
In order to allow event tracing to distinguish between flavors of
RCU, we need those names in the relevant RCU data structures. TINY_RCU
has avoided them for memory-footprint reasons, so add them only if
CONFIG_RCU_TRACE=y.Signed-off-by: Paul E. McKenney
Signed-off-by: Paul E. McKenney -
This commit adds the trace_rcu_utilization() marker that is to be
used to allow postprocessing scripts compute RCU's CPU utilization,
give or take event-trace overhead. Note that we do not include RCU's
dyntick-idle interface because event tracing requires RCU protection,
which is not available in dyntick-idle mode.Signed-off-by: Paul E. McKenney
Signed-off-by: Paul E. McKenney -
There was recently some controversy about the overhead of invoking RCU
callbacks. Add TRACE_EVENT()s to obtain fine-grained timings for the
start and stop of a batch of callbacks and also for each callback invoked.Signed-off-by: Paul E. McKenney
Signed-off-by: Paul E. McKenney -
Pull the code that waits for an RCU grace period into a single function,
which is then called by synchronize_rcu() and friends in the case of
TREE_RCU and TREE_PREEMPT_RCU, and from rcu_barrier() and friends in
the case of TINY_RCU and TINY_PREEMPT_RCU.Signed-off-by: Paul E. McKenney
13 Jul, 2011
1 commit
-
Under some rare but real combinations of configuration parameters, RCU
callbacks are posted during early boot that use kernel facilities that
are not yet initialized. Therefore, when these callbacks are invoked,
hard hangs and crashes ensue. This commit therefore prevents RCU
callbacks from being invoked until after the scheduler is fully up and
running, as in after multiple tasks have been spawned.It might well turn out that a better approach is to identify the specific
RCU callbacks that are causing this problem, but that discussion will
wait until such time as someone really needs an RCU callback to be invoked
(as opposed to merely registered) during early boot.Reported-by: julie Sullivan
Reported-by: RKK
Signed-off-by: Paul E. McKenney
Tested-by: Konrad Rzeszutek Wilk
Tested-by: julie Sullivan
Tested-by: RKK
17 Jun, 2011
1 commit
-
The commit "use softirq instead of kthreads except when RCU_BOOST=y"
just applied #ifdef in place. This commit is a cleanup that moves
the newly #ifdef'ed code to the header file kernel/rcutree_plugin.h.Signed-off-by: Paul E. McKenney
Signed-off-by: Paul E. McKenney
16 Jun, 2011
1 commit
-
This patch #ifdefs RCU kthreads out of the kernel unless RCU_BOOST=y,
thus eliminating context-switch overhead if RCU priority boosting has
not been configured.Signed-off-by: Paul E. McKenney
15 Jun, 2011
2 commits
-
Commit a26ac2455ffcf3(rcu: move TREE_RCU from softirq to kthread)
introduced performance regression. In an AIM7 test, this commit degraded
performance by about 40%.The commit runs rcu callbacks in a kthread instead of softirq. We observed
high rate of context switch which is caused by this. Out test system has
64 CPUs and HZ is 1000, so we saw more than 64k context switch per second
which is caused by RCU's per-CPU kthread. A trace showed that most of
the time the RCU per-CPU kthread doesn't actually handle any callbacks,
but instead just does a very small amount of work handling grace periods.
This means that RCU's per-CPU kthreads are making the scheduler do quite
a bit of work in order to allow a very small amount of RCU-related
processing to be done.Alex Shi's analysis determined that this slowdown is due to lock
contention within the scheduler. Unfortunately, as Peter Zijlstra points
out, the scheduler's real-time semantics require global action, which
means that this contention is inherent in real-time scheduling. (Yes,
perhaps someone will come up with a workaround -- otherwise, -rt is not
going to do well on large SMP systems -- but this patch will work around
this issue in the meantime. And "the meantime" might well be forever.)This patch therefore re-introduces softirq processing to RCU, but only
for core RCU work. RCU callbacks are still executed in kthread context,
so that only a small amount of RCU work runs in softirq context in the
common case. This should minimize ksoftirqd execution, allowing us to
skip boosting of ksoftirqd for CONFIG_RCU_BOOST=y kernels.Signed-off-by: Shaohua Li
Tested-by: "Alex,Shi"
Signed-off-by: Paul E. McKenney -
Make the functions creating the kthreads wake them up. Leverage the
fact that the per-node and boost kthreads can run anywhere, thus
dispensing with the need to wake them up once the incoming CPU has
gone fully online.Signed-off-by: Paul E. McKenney
Tested-by: Daniel J Blueman
31 May, 2011
1 commit
-
Commit cc3ce5176d83 (rcu: Start RCU kthreads in TASK_INTERRUPTIBLE
state) fudges a sleeping task' state, resulting in the scheduler seeing
a TASK_UNINTERRUPTIBLE task going to sleep, but a TASK_INTERRUPTIBLE
task waking up. The result is unbalanced load calculation.The problem that patch tried to address is that the RCU threads could
stay in UNINTERRUPTIBLE state for quite a while and triggering the hung
task detector due to on-demand wake-ups.Cure the problem differently by always giving the tasks at least one
wake-up once the CPU is fully up and running, this will kick them out of
the initial UNINTERRUPTIBLE state and into the regular INTERRUPTIBLE
wait state.[ The alternative would be teaching kthread_create() to start threads as
INTERRUPTIBLE but that needs a tad more thought. ]Reported-by: Damien Wyart
Signed-off-by: Peter Zijlstra
Acked-by: Paul E. McKenney
Link: http://lkml.kernel.org/r/1306755291.1200.2872.camel@twins
Signed-off-by: Ingo Molnar
28 May, 2011
4 commits
-
Upon creation, kthreads are in TASK_UNINTERRUPTIBLE state, which can
result in softlockup warnings. Because some of RCU's kthreads can
legitimately be idle indefinitely, start them in TASK_INTERRUPTIBLE
state in order to avoid those warnings.Suggested-by: Peter Zijlstra
Signed-off-by: Paul E. McKenney
Signed-off-by: Paul E. McKenney
Tested-by: Yinghai Lu
Signed-off-by: Ingo Molnar -
It is not necessary to use waitqueues for the RCU kthreads because
we always know exactly which thread is to be awakened. In addition,
wake_up() only issues an actual wakeup when there is a thread waiting on
the queue, which was why there was an extra explicit wake_up_process()
to get the RCU kthreads started.Eliminating the waitqueues (and wake_up()) in favor of wake_up_process()
eliminates the need for the initial wake_up_process() and also shrinks
the data structure size a bit. The wakeup logic is placed in a new
rcu_wait() macro.Signed-off-by: Peter Zijlstra
Signed-off-by: Paul E. McKenney
Signed-off-by: Ingo Molnar -
This commit switches manipulations of the rcu_node ->wakemask field
to atomic operations, which allows rcu_cpu_kthread_timer() to avoid
acquiring the rcu_node lock. This should avoid the following lockdep
splat reported by Valdis Kletnieks:[ 12.872150] usb 1-4: new high speed USB device number 3 using ehci_hcd
[ 12.986667] usb 1-4: New USB device found, idVendor=413c, idProduct=2513
[ 12.986679] usb 1-4: New USB device strings: Mfr=0, Product=0, SerialNumber=0
[ 12.987691] hub 1-4:1.0: USB hub found
[ 12.987877] hub 1-4:1.0: 3 ports detected
[ 12.996372] input: PS/2 Generic Mouse as /devices/platform/i8042/serio1/input/input10
[ 13.071471] udevadm used greatest stack depth: 3984 bytes left
[ 13.172129]
[ 13.172130] =======================================================
[ 13.172425] [ INFO: possible circular locking dependency detected ]
[ 13.172650] 2.6.39-rc6-mmotm0506 #1
[ 13.172773] -------------------------------------------------------
[ 13.172997] blkid/267 is trying to acquire lock:
[ 13.173009] (&p->pi_lock){-.-.-.}, at: [] try_to_wake_up+0x29/0x1aa
[ 13.173009]
[ 13.173009] but task is already holding lock:
[ 13.173009] (rcu_node_level_0){..-...}, at: [] rcu_cpu_kthread_timer+0x27/0x58
[ 13.173009]
[ 13.173009] which lock already depends on the new lock.
[ 13.173009]
[ 13.173009]
[ 13.173009] the existing dependency chain (in reverse order) is:
[ 13.173009]
[ 13.173009] -> #2 (rcu_node_level_0){..-...}:
[ 13.173009] [] check_prevs_add+0x8b/0x104
[ 13.173009] [] validate_chain+0x36f/0x3ab
[ 13.173009] [] __lock_acquire+0x369/0x3e2
[ 13.173009] [] lock_acquire+0xfc/0x14c
[ 13.173009] [] _raw_spin_lock+0x36/0x45
[ 13.173009] [] rcu_read_unlock_special+0x8c/0x1d5
[ 13.173009] [] __rcu_read_unlock+0x4f/0xd7
[ 13.173009] [] rcu_read_unlock+0x21/0x23
[ 13.173009] [] cpuacct_charge+0x6c/0x75
[ 13.173009] [] update_curr+0x101/0x12e
[ 13.173009] [] check_preempt_wakeup+0xf7/0x23b
[ 13.173009] [] check_preempt_curr+0x2b/0x68
[ 13.173009] [] ttwu_do_wakeup+0x76/0x128
[ 13.173009] [] ttwu_do_activate.constprop.63+0x57/0x5c
[ 13.173009] [] scheduler_ipi+0x48/0x5d
[ 13.173009] [] smp_reschedule_interrupt+0x16/0x18
[ 13.173009] [] reschedule_interrupt+0x13/0x20
[ 13.173009] [] rcu_read_unlock+0x21/0x23
[ 13.173009] [] find_get_page+0xa9/0xb9
[ 13.173009] [] filemap_fault+0x6a/0x34d
[ 13.173009] [] __do_fault+0x54/0x3e6
[ 13.173009] [] handle_pte_fault+0x12c/0x1ed
[ 13.173009] [] handle_mm_fault+0x1cd/0x1e0
[ 13.173009] [] do_page_fault+0x42d/0x5de
[ 13.173009] [] page_fault+0x1f/0x30
[ 13.173009]
[ 13.173009] -> #1 (&rq->lock){-.-.-.}:
[ 13.173009] [] check_prevs_add+0x8b/0x104
[ 13.173009] [] validate_chain+0x36f/0x3ab
[ 13.173009] [] __lock_acquire+0x369/0x3e2
[ 13.173009] [] lock_acquire+0xfc/0x14c
[ 13.173009] [] _raw_spin_lock+0x36/0x45
[ 13.173009] [] __task_rq_lock+0x8b/0xd3
[ 13.173009] [] wake_up_new_task+0x41/0x108
[ 13.173009] [] do_fork+0x265/0x33f
[ 13.173009] [] kernel_thread+0x6b/0x6d
[ 13.173009] [] rest_init+0x21/0xd2
[ 13.173009] [] start_kernel+0x3bb/0x3c6
[ 13.173009] [] x86_64_start_reservations+0xaf/0xb3
[ 13.173009] [] x86_64_start_kernel+0xf0/0xf7
[ 13.173009]
[ 13.173009] -> #0 (&p->pi_lock){-.-.-.}:
[ 13.173009] [] check_prev_add+0x68/0x20e
[ 13.173009] [] check_prevs_add+0x8b/0x104
[ 13.173009] [] validate_chain+0x36f/0x3ab
[ 13.173009] [] __lock_acquire+0x369/0x3e2
[ 13.173009] [] lock_acquire+0xfc/0x14c
[ 13.173009] [] _raw_spin_lock_irqsave+0x44/0x57
[ 13.173009] [] try_to_wake_up+0x29/0x1aa
[ 13.173009] [] wake_up_process+0x10/0x12
[ 13.173009] [] rcu_cpu_kthread_timer+0x44/0x58
[ 13.173009] [] call_timer_fn+0xac/0x1e9
[ 13.173009] [] run_timer_softirq+0x1aa/0x1f2
[ 13.173009] [] __do_softirq+0x109/0x26a
[ 13.173009] [] call_softirq+0x1c/0x30
[ 13.173009] [] do_softirq+0x44/0xf1
[ 13.173009] [] irq_exit+0x58/0xc8
[ 13.173009] [] smp_apic_timer_interrupt+0x79/0x87
[ 13.173009] [] apic_timer_interrupt+0x13/0x20
[ 13.173009] [] get_page_from_freelist+0x2aa/0x310
[ 13.173009] [] __alloc_pages_nodemask+0x178/0x243
[ 13.173009] [] pte_alloc_one+0x1e/0x3a
[ 13.173009] [] __pte_alloc+0x22/0x14b
[ 13.173009] [] handle_mm_fault+0x17e/0x1e0
[ 13.173009] [] do_page_fault+0x42d/0x5de
[ 13.173009] [] page_fault+0x1f/0x30
[ 13.173009]
[ 13.173009] other info that might help us debug this:
[ 13.173009]
[ 13.173009] Chain exists of:
[ 13.173009] &p->pi_lock --> &rq->lock --> rcu_node_level_0
[ 13.173009]
[ 13.173009] Possible unsafe locking scenario:
[ 13.173009]
[ 13.173009] CPU0 CPU1
[ 13.173009] ---- ----
[ 13.173009] lock(rcu_node_level_0);
[ 13.173009] lock(&rq->lock);
[ 13.173009] lock(rcu_node_level_0);
[ 13.173009] lock(&p->pi_lock);
[ 13.173009]
[ 13.173009] *** DEADLOCK ***
[ 13.173009]
[ 13.173009] 3 locks held by blkid/267:
[ 13.173009] #0: (&mm->mmap_sem){++++++}, at: [] do_page_fault+0x1f3/0x5de
[ 13.173009] #1: (&yield_timer){+.-...}, at: [] call_timer_fn+0x0/0x1e9
[ 13.173009] #2: (rcu_node_level_0){..-...}, at: [] rcu_cpu_kthread_timer+0x27/0x58
[ 13.173009]
[ 13.173009] stack backtrace:
[ 13.173009] Pid: 267, comm: blkid Not tainted 2.6.39-rc6-mmotm0506 #1
[ 13.173009] Call Trace:
[ 13.173009] [] print_circular_bug+0xc8/0xd9
[ 13.173009] [] check_prev_add+0x68/0x20e
[ 13.173009] [] ? save_stack_trace+0x28/0x46
[ 13.173009] [] check_prevs_add+0x8b/0x104
[ 13.173009] [] validate_chain+0x36f/0x3ab
[ 13.173009] [] __lock_acquire+0x369/0x3e2
[ 13.173009] [] ? try_to_wake_up+0x29/0x1aa
[ 13.173009] [] lock_acquire+0xfc/0x14c
[ 13.173009] [] ? try_to_wake_up+0x29/0x1aa
[ 13.173009] [] ? rcu_check_quiescent_state+0x82/0x82
[ 13.173009] [] _raw_spin_lock_irqsave+0x44/0x57
[ 13.173009] [] ? try_to_wake_up+0x29/0x1aa
[ 13.173009] [] try_to_wake_up+0x29/0x1aa
[ 13.173009] [] ? rcu_check_quiescent_state+0x82/0x82
[ 13.173009] [] wake_up_process+0x10/0x12
[ 13.173009] [] rcu_cpu_kthread_timer+0x44/0x58
[ 13.173009] [] ? rcu_check_quiescent_state+0x82/0x82
[ 13.173009] [] call_timer_fn+0xac/0x1e9
[ 13.173009] [] ? del_timer+0x75/0x75
[ 13.173009] [] ? rcu_check_quiescent_state+0x82/0x82
[ 13.173009] [] run_timer_softirq+0x1aa/0x1f2
[ 13.173009] [] __do_softirq+0x109/0x26a
[ 13.173009] [] ? tick_dev_program_event+0x37/0xf6
[ 13.173009] [] ? time_hardirqs_off+0x1b/0x2f
[ 13.173009] [] call_softirq+0x1c/0x30
[ 13.173009] [] do_softirq+0x44/0xf1
[ 13.173009] [] irq_exit+0x58/0xc8
[ 13.173009] [] smp_apic_timer_interrupt+0x79/0x87
[ 13.173009] [] apic_timer_interrupt+0x13/0x20
[ 13.173009] [] ? get_page_from_freelist+0x114/0x310
[ 13.173009] [] ? get_page_from_freelist+0x2aa/0x310
[ 13.173009] [] ? clear_page_c+0x7/0x10
[ 13.173009] [] ? prep_new_page+0x14c/0x1cd
[ 13.173009] [] get_page_from_freelist+0x2aa/0x310
[ 13.173009] [] __alloc_pages_nodemask+0x178/0x243
[ 13.173009] [] ? __pmd_alloc+0x87/0x99
[ 13.173009] [] pte_alloc_one+0x1e/0x3a
[ 13.173009] [] ? __pmd_alloc+0x87/0x99
[ 13.173009] [] __pte_alloc+0x22/0x14b
[ 13.173009] [] handle_mm_fault+0x17e/0x1e0
[ 13.173009] [] do_page_fault+0x42d/0x5de
[ 13.173009] [] ? sys_brk+0x32/0x10c
[ 13.173009] [] ? time_hardirqs_off+0x1b/0x2f
[ 13.173009] [] ? trace_hardirqs_off_caller+0x3f/0x9c
[ 13.173009] [] ? trace_hardirqs_off_thunk+0x3a/0x3c
[ 13.173009] [] page_fault+0x1f/0x30
[ 14.010075] usb 5-1: new full speed USB device number 2 using uhci_hcdReported-by: Valdis Kletnieks
Signed-off-by: Paul E. McKenney
Signed-off-by: Paul E. McKenney
Signed-off-by: Ingo Molnar -
…ck/linux-2.6-rcu into core/urgent
27 May, 2011
5 commits
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(Note: this was reverted, and is now being re-applied in pieces, with
this being the fifth and final piece. See below for the reason that
it is now felt to be safe to re-apply this.)Commit d09b62d fixed grace-period synchronization, but left some smp_mb()
invocations in rcu_process_callbacks() that are no longer needed, but
sheer paranoia prevented them from being removed. This commit removes
them and provides a proof of correctness in their absence. It also adds
a memory barrier to rcu_report_qs_rsp() immediately before the update to
rsp->completed in order to handle the theoretical possibility that the
compiler or CPU might move massive quantities of code into a lock-based
critical section. This also proves that the sheer paranoia was not
entirely unjustified, at least from a theoretical point of view.In addition, the old dyntick-idle synchronization depended on the fact
that grace periods were many milliseconds in duration, so that it could
be assumed that no dyntick-idle CPU could reorder a memory reference
across an entire grace period. Unfortunately for this design, the
addition of expedited grace periods breaks this assumption, which has
the unfortunate side-effect of requiring atomic operations in the
functions that track dyntick-idle state for RCU. (There is some hope
that the algorithms used in user-level RCU might be applied here, but
some work is required to handle the NMIs that user-space applications
can happily ignore. For the short term, better safe than sorry.)This proof assumes that neither compiler nor CPU will allow a lock
acquisition and release to be reordered, as doing so can result in
deadlock. The proof is as follows:1. A given CPU declares a quiescent state under the protection of
its leaf rcu_node's lock.2. If there is more than one level of rcu_node hierarchy, the
last CPU to declare a quiescent state will also acquire the
->lock of the next rcu_node up in the hierarchy, but only
after releasing the lower level's lock. The acquisition of this
lock clearly cannot occur prior to the acquisition of the leaf
node's lock.3. Step 2 repeats until we reach the root rcu_node structure.
Please note again that only one lock is held at a time through
this process. The acquisition of the root rcu_node's ->lock
must occur after the release of that of the leaf rcu_node.4. At this point, we set the ->completed field in the rcu_state
structure in rcu_report_qs_rsp(). However, if the rcu_node
hierarchy contains only one rcu_node, then in theory the code
preceding the quiescent state could leak into the critical
section. We therefore precede the update of ->completed with a
memory barrier. All CPUs will therefore agree that any updates
preceding any report of a quiescent state will have happened
before the update of ->completed.5. Regardless of whether a new grace period is needed, rcu_start_gp()
will propagate the new value of ->completed to all of the leaf
rcu_node structures, under the protection of each rcu_node's ->lock.
If a new grace period is needed immediately, this propagation
will occur in the same critical section that ->completed was
set in, but courtesy of the memory barrier in #4 above, is still
seen to follow any pre-quiescent-state activity.6. When a given CPU invokes __rcu_process_gp_end(), it becomes
aware of the end of the old grace period and therefore makes
any RCU callbacks that were waiting on that grace period eligible
for invocation.If this CPU is the same one that detected the end of the grace
period, and if there is but a single rcu_node in the hierarchy,
we will still be in the single critical section. In this case,
the memory barrier in step #4 guarantees that all callbacks will
be seen to execute after each CPU's quiescent state.On the other hand, if this is a different CPU, it will acquire
the leaf rcu_node's ->lock, and will again be serialized after
each CPU's quiescent state for the old grace period.On the strength of this proof, this commit therefore removes the memory
barriers from rcu_process_callbacks() and adds one to rcu_report_qs_rsp().
The effect is to reduce the number of memory barriers by one and to
reduce the frequency of execution from about once per scheduling tick
per CPU to once per grace period.This was reverted do to hangs found during testing by Yinghai Lu and
Ingo Molnar. Frederic Weisbecker supplied Yinghai with tracing that
located the underlying problem, and Frederic also provided the fix.The underlying problem was that the HARDIRQ_ENTER() macro from
lib/locking-selftest.c invoked irq_enter(), which in turn invokes
rcu_irq_enter(), but HARDIRQ_EXIT() invoked __irq_exit(), which
does not invoke rcu_irq_exit(). This situation resulted in calls
to rcu_irq_enter() that were not balanced by the required calls to
rcu_irq_exit(). Therefore, after these locking selftests completed,
RCU's dyntick-idle nesting count was a large number (for example,
72), which caused RCU to to conclude that the affected CPU was not in
dyntick-idle mode when in fact it was.RCU would therefore incorrectly wait for this dyntick-idle CPU, resulting
in hangs.In contrast, with Frederic's patch, which replaces the irq_enter()
in HARDIRQ_ENTER() with an __irq_enter(), these tests don't ever call
either rcu_irq_enter() or rcu_irq_exit(), which works because the CPU
running the test is already marked as not being in dyntick-idle mode.
This means that the rcu_irq_enter() and rcu_irq_exit() calls and RCU
then has no problem working out which CPUs are in dyntick-idle mode and
which are not.The reason that the imbalance was not noticed before the barrier patch
was applied is that the old implementation of rcu_enter_nohz() ignored
the nesting depth. This could still result in delays, but much shorter
ones. Whenever there was a delay, RCU would IPI the CPU with the
unbalanced nesting level, which would eventually result in rcu_enter_nohz()
being called, which in turn would force RCU to see that the CPU was in
dyntick-idle mode.The reason that very few people noticed the problem is that the mismatched
irq_enter() vs. __irq_exit() occured only when the kernel was built with
CONFIG_DEBUG_LOCKING_API_SELFTESTS.Signed-off-by: Paul E. McKenney
Reviewed-by: Josh Triplett -
The old version of rcu_enter_nohz() forced RCU into nohz mode even if
the nesting count was non-zero. This change causes rcu_enter_nohz()
to hold off for non-zero nesting counts.Signed-off-by: Paul E. McKenney
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Condition the set_need_resched() in rcu_irq_exit() on in_irq(). This
should be a no-op, because rcu_irq_exit() should only be called from irq.Signed-off-by: Paul E. McKenney
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Second step of partitioning of commit e59fb3120b.
Signed-off-by: Paul E. McKenney
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Add the memory barriers added by e59fb3120b.
Signed-off-by: Paul E. McKenney
21 May, 2011
1 commit
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Commit e66eed651fd1 ("list: remove prefetching from regular list
iterators") removed the include of prefetch.h from list.h, which
uncovered several cases that had apparently relied on that rather
obscure header file dependency.So this fixes things up a bit, using
grep -L linux/prefetch.h $(git grep -l '[^a-z_]prefetchw*(' -- '*.[ch]')
grep -L 'prefetchw*(' $(git grep -l 'linux/prefetch.h' -- '*.[ch]')to guide us in finding files that either need
inclusion, or have it despite not needing it.There are more of them around (mostly network drivers), but this gets
many core ones.Reported-by: Stephen Rothwell
Signed-off-by: Linus Torvalds
20 May, 2011
1 commit
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This reverts commit e59fb3120becfb36b22ddb8bd27d065d3cdca499.
This reversion was due to (extreme) boot-time slowdowns on SPARC seen by
Yinghai Lu and on x86 by Ingo
.
This is a non-trivial reversion due to intervening commits.Conflicts:
Documentation/RCU/trace.txt
kernel/rcutree.cSigned-off-by: Ingo Molnar
08 May, 2011
1 commit
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Avoid calling into the scheduler while holding core RCU locks. This
allows rcu_read_unlock() to be called while holding the runqueue locks,
but only as long as there was no chance of the RCU read-side critical
section having been preempted. (Otherwise, if RCU priority boosting
is enabled, rcu_read_unlock() might call into the scheduler in order to
unboost itself, which might allows self-deadlock on the runqueue locks
within the scheduler.)Signed-off-by: Paul E. McKenney
Signed-off-by: Paul E. McKenney
06 May, 2011
3 commits
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Provide rcu_virt_note_context_switch() for vitalization use to note
quiescent state during guest entry.Signed-off-by: Gleb Natapov
Signed-off-by: Paul E. McKenney -
Signed integer overflow is undefined by the C standard, so move
calculations to unsigned.Signed-off-by: Paul E. McKenney
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This commit marks a first step towards making call_rcu() have
real-time behavior. If irqs are disabled, don't dive into the
RCU core. Later on, this new early exit will wake up the
per-CPU kthread, which first must be modified to handle the
cases involving callback storms.Signed-off-by: Paul E. McKenney
Reviewed-by: Josh Triplett
