Commit c072a388d59a1d48e36864d0e66f42d71745be1c
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394f4528c5
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rcu: demote SRCU_SYNCHRONIZE_DELAY from kernel-parameter status
Because the adaptive synchronize_srcu_expedited() approach has worked very well in testing, remove the kernel parameter and replace it by a C-preprocessor macro. If someone finds problems with this approach, a more complex and aggressively adaptive approach might be required. Longer term, SRCU will be merged with the other RCU implementations, at which point synchronize_srcu_expedited() will be event driven, just as synchronize_sched_expedited() currently is. At that point, there will be no need for this adaptive approach. Reported-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Showing 2 changed files with 13 additions and 17 deletions Side-by-side Diff
init/Kconfig
| ... | ... | @@ -497,21 +497,6 @@ |
| 497 | 497 | |
| 498 | 498 | Accept the default if unsure. |
| 499 | 499 | |
| 500 | -config SRCU_SYNCHRONIZE_DELAY | |
| 501 | - int "Microseconds to delay before waiting for readers" | |
| 502 | - range 0 20 | |
| 503 | - default 10 | |
| 504 | - help | |
| 505 | - This option controls how long SRCU delays before entering its | |
| 506 | - loop waiting on SRCU readers. The purpose of this loop is | |
| 507 | - to avoid the unconditional context-switch penalty that would | |
| 508 | - otherwise be incurred if there was an active SRCU reader, | |
| 509 | - in a manner similar to adaptive locking schemes. This should | |
| 510 | - be set to be a bit longer than the common-case SRCU read-side | |
| 511 | - critical-section overhead. | |
| 512 | - | |
| 513 | - Accept the default if unsure. | |
| 514 | - | |
| 515 | 500 | endmenu # "RCU Subsystem" |
| 516 | 501 | |
| 517 | 502 | config IKCONFIG |
kernel/srcu.c
| ... | ... | @@ -156,6 +156,16 @@ |
| 156 | 156 | EXPORT_SYMBOL_GPL(__srcu_read_unlock); |
| 157 | 157 | |
| 158 | 158 | /* |
| 159 | + * We use an adaptive strategy for synchronize_srcu() and especially for | |
| 160 | + * synchronize_srcu_expedited(). We spin for a fixed time period | |
| 161 | + * (defined below) to allow SRCU readers to exit their read-side critical | |
| 162 | + * sections. If there are still some readers after 10 microseconds, | |
| 163 | + * we repeatedly block for 1-millisecond time periods. This approach | |
| 164 | + * has done well in testing, so there is no need for a config parameter. | |
| 165 | + */ | |
| 166 | +#define SYNCHRONIZE_SRCU_READER_DELAY 10 | |
| 167 | + | |
| 168 | +/* | |
| 159 | 169 | * Helper function for synchronize_srcu() and synchronize_srcu_expedited(). |
| 160 | 170 | */ |
| 161 | 171 | static void __synchronize_srcu(struct srcu_struct *sp, void (*sync_func)(void)) |
| 162 | 172 | |
| ... | ... | @@ -207,11 +217,12 @@ |
| 207 | 217 | * will have finished executing. We initially give readers |
| 208 | 218 | * an arbitrarily chosen 10 microseconds to get out of their |
| 209 | 219 | * SRCU read-side critical sections, then loop waiting 1/HZ |
| 210 | - * seconds per iteration. | |
| 220 | + * seconds per iteration. The 10-microsecond value has done | |
| 221 | + * very well in testing. | |
| 211 | 222 | */ |
| 212 | 223 | |
| 213 | 224 | if (srcu_readers_active_idx(sp, idx)) |
| 214 | - udelay(CONFIG_SRCU_SYNCHRONIZE_DELAY); | |
| 225 | + udelay(SYNCHRONIZE_SRCU_READER_DELAY); | |
| 215 | 226 | while (srcu_readers_active_idx(sp, idx)) |
| 216 | 227 | schedule_timeout_interruptible(1); |
| 217 | 228 |