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    SubjectRe: [PATCH] sched: cpuacct: Use bigger percpu counter batch values for stats counters
    Date
    > 
    > 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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