Messages in this thread | | | From | KOSAKI Motohiro <> | Subject | Re: [PATCH] sched: cpuacct: Use bigger percpu counter batch values for stats counters | Date | Mon, 18 Jan 2010 13:55:33 +0900 (JST) |
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> > Hi, > > Another try at this percpu_counter batch issue with CONFIG_VIRT_CPU_ACCOUNTING > and CONFIG_CGROUP_CPUACCT enabled. Thoughts? > > -- > > When CONFIG_VIRT_CPU_ACCOUNTING and CONFIG_CGROUP_CPUACCT are enabled we can > call cpuacct_update_stats with values much larger than percpu_counter_batch. > This means the call to percpu_counter_add will always add to the global count > which is protected by a spinlock and we end up with a global spinlock in > the scheduler. > > Based on an idea by KOSAKI Motohiro, this patch scales the batch value by > cputime_one_jiffy such that we have the same batch limit as we would > if CONFIG_VIRT_CPU_ACCOUNTING was disabled. His patch did this once at boot > but that initialisation happened too early on PowerPC (before time_init) > and it was never updated at runtime as a result of a hotplug cpu add/remove. > > This patch instead scales percpu_counter_batch by cputime_one_jiffy at > runtime, which keeps the batch correct even after cpu hotplug operations. > We cap it at INT_MAX in case of overflow. > > For architectures that do not support CONFIG_VIRT_CPU_ACCOUNTING, > cputime_one_jiffy is the constant 1 and gcc is smart enough to > optimise min(s32 percpu_counter_batch, INT_MAX) to just percpu_counter_batch > at least on x86 and PowerPC. So there is no need to add an #ifdef. > > On a 64 thread PowerPC box with CONFIG_VIRT_CPU_ACCOUNTING and > CONFIG_CGROUP_CPUACCT enabled, a context switch microbenchmark is 234x faster > and almost matches a CONFIG_CGROUP_CPUACCT disabled kernel: > > CONFIG_CGROUP_CPUACCT disabled: 16906698 ctx switches/sec > CONFIG_CGROUP_CPUACCT enabled: 61720 ctx switches/sec > CONFIG_CGROUP_CPUACCT + patch: 16663217 ctx switches/sec > > Tested with: > > wget http://ozlabs.org/~anton/junkcode/context_switch.c > make context_switch > for i in `seq 0 63`; do taskset -c $i ./context_switch & done > vmstat 1 > > Signed-off-by: Anton Blanchard <anton@samba.org>
Looks fine to me. Thanks Anton! Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> --- > > Note: ccing ia64 and s390 who have not yet added code to statically > initialise cputime_one_jiffy at boot. > See a42548a18866e87092db93b771e6c5b060d78401 (cputime: Optimize > jiffies_to_cputime(1) for details). Adding this would help optimise not only > this patch but many other areas of the scheduler when > CONFIG_VIRT_CPU_ACCOUNTING is enabled. > > Index: linux.trees.git/kernel/sched.c > =================================================================== > --- linux.trees.git.orig/kernel/sched.c 2010-01-18 14:27:12.000000000 +1100 > +++ linux.trees.git/kernel/sched.c 2010-01-18 15:21:59.000000000 +1100 > @@ -10894,6 +10894,7 @@ static void cpuacct_update_stats(struct > enum cpuacct_stat_index idx, cputime_t val) > { > struct cpuacct *ca; > + int batch; > > if (unlikely(!cpuacct_subsys.active)) > return; > @@ -10901,8 +10902,9 @@ static void cpuacct_update_stats(struct > rcu_read_lock(); > ca = task_ca(tsk); > > + batch = min_t(long, percpu_counter_batch * cputime_one_jiffy, INT_MAX); > do { > - percpu_counter_add(&ca->cpustat[idx], val); > + __percpu_counter_add(&ca->cpustat[idx], val, batch); > ca = ca->parent; > } while (ca); > rcu_read_unlock();
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