Doug / smarc-fsl-linux-kernel

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Commit ca74a6f84e68b44867022f4a4f3ec17c087c864e

Authored by Andi Kleen
Committed by Ingo Molnar
1 parent 7517527891
Exists in master and in 7 other branches imx_3.0.35_4.1.0, imx_3.10.17_1.0.1_ga, imx_3.10.53_1.1.0_ga, imx_3.14.28_1.0.0_ga, smarc-imx_3.10.53_1.1.0_ga, smarc-imx_3.14.28_1.0.0_ga, smarc-l5.0.0_1.0.0-ga

x86: optimize lock prefix switching to run less frequently 

On VMs implemented using JITs that cache translated code changing the lock
prefixes is a quite costly operation that forces the JIT to throw away and
retranslate a lot of code.

Previously a SMP kernel would rewrite the locks once for each CPU which
is quite unnecessary. This patch changes the code to never switch at boot in
 the normal case (SMP kernel booting with >1 CPU) or only once for SMP kernel
on UP.

This makes a significant difference in boot up performance on AMD SimNow!
Also I expect it to be a little faster on native systems too because a smp
switch does a lot of text_poke()s which each synchronize the pipeline.

v1->v2: Rename max_cpus
v1->v2: Fix off by one in UP check (Thomas Gleixner)

Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

Showing 3 changed files with 24 additions and 10 deletions Side-by-side Diff

arch/x86/kernel/alternative.c
Diff comments   View file @ ca74a6f
... ... @@ -273,6 +273,7 @@
273 273 };
274 274 static LIST_HEAD(smp_alt_modules);
275 275 static DEFINE_SPINLOCK(smp_alt);
  276 +static int smp_mode = 1; /* protected by smp_alt */
276 277  
277 278 void alternatives_smp_module_add(struct module *mod, char *name,
278 279 void *locks, void *locks_end,
... ... @@ -354,7 +355,14 @@
354 355 BUG_ON(!smp && (num_online_cpus() > 1));
355 356  
356 357 spin_lock_irqsave(&smp_alt, flags);
357   - if (smp) {
  358 +
  359 + /*
  360 + * Avoid unnecessary switches because it forces JIT based VMs to
  361 + * throw away all cached translations, which can be quite costly.
  362 + */
  363 + if (smp == smp_mode) {
  364 + /* nothing */
  365 + } else if (smp) {
358 366 printk(KERN_INFO "SMP alternatives: switching to SMP code\n");
359 367 clear_cpu_cap(&boot_cpu_data, X86_FEATURE_UP);
360 368 clear_cpu_cap(&cpu_data(0), X86_FEATURE_UP);
... ... @@ -369,6 +377,7 @@
369 377 alternatives_smp_unlock(mod->locks, mod->locks_end,
370 378 mod->text, mod->text_end);
371 379 }
  380 + smp_mode = smp;
372 381 spin_unlock_irqrestore(&smp_alt, flags);
373 382 }
374 383  
... ... @@ -441,7 +450,10 @@
441 450 alternatives_smp_module_add(NULL, "core kernel",
442 451 __smp_locks, __smp_locks_end,
443 452 _text, _etext);
444   - alternatives_smp_switch(0);
  453 +
  454 + /* Only switch to UP mode if we don't immediately boot others */
  455 + if (num_possible_cpus() == 1 || setup_max_cpus <= 1)
  456 + alternatives_smp_switch(0);
445 457 }
446 458 #endif
447 459 apply_paravirt(__parainstructions, __parainstructions_end);
... ... @@ -78,6 +78,8 @@
78 78 */
79 79 void smp_prepare_boot_cpu(void);
80 80  
  81 +extern unsigned int setup_max_cpus;
  82 +
81 83 #else /* !SMP */
82 84  
83 85 /*
... ... @@ -128,7 +128,7 @@
128 128  
129 129 #ifdef CONFIG_SMP
130 130 /* Setup configured maximum number of CPUs to activate */
131   -static unsigned int __initdata max_cpus = NR_CPUS;
  131 +unsigned int __initdata setup_max_cpus = NR_CPUS;
132 132  
133 133 /*
134 134 * Setup routine for controlling SMP activation
... ... @@ -146,7 +146,7 @@
146 146  
147 147 static int __init nosmp(char *str)
148 148 {
149   - max_cpus = 0;
  149 + setup_max_cpus = 0;
150 150 disable_ioapic_setup();
151 151 return 0;
152 152 }
... ... @@ -155,8 +155,8 @@
155 155  
156 156 static int __init maxcpus(char *str)
157 157 {
158   - get_option(&str, &max_cpus);
159   - if (max_cpus == 0)
  158 + get_option(&str, &setup_max_cpus);
  159 + if (setup_max_cpus == 0)
160 160 disable_ioapic_setup();
161 161  
162 162 return 0;
... ... @@ -164,7 +164,7 @@
164 164  
165 165 early_param("maxcpus", maxcpus);
166 166 #else
167   -#define max_cpus NR_CPUS
  167 +#define setup_max_cpus NR_CPUS
168 168 #endif
169 169  
170 170 /*
... ... @@ -393,7 +393,7 @@
393 393  
394 394 /* FIXME: This should be done in userspace --RR */
395 395 for_each_present_cpu(cpu) {
396   - if (num_online_cpus() >= max_cpus)
  396 + if (num_online_cpus() >= setup_max_cpus)
397 397 break;
398 398 if (!cpu_online(cpu))
399 399 cpu_up(cpu);
... ... @@ -401,7 +401,7 @@
401 401  
402 402 /* Any cleanup work */
403 403 printk(KERN_INFO "Brought up %ld CPUs\n", (long)num_online_cpus());
404   - smp_cpus_done(max_cpus);
  404 + smp_cpus_done(setup_max_cpus);
405 405 }
406 406  
407 407 #endif
... ... @@ -824,7 +824,7 @@
824 824 __set_special_pids(1, 1);
825 825 cad_pid = task_pid(current);
826 826  
827   - smp_prepare_cpus(max_cpus);
  827 + smp_prepare_cpus(setup_max_cpus);
828 828  
829 829 do_pre_smp_initcalls();
830 830