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    SubjectRe: [PATCH v2] cpuacct: VIRT_CPU_ACCOUNTING don't prevent percpu cputime count

    * Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:

    > On Thu, 2009-04-30 at 11:48 +0530, Balbir Singh wrote:
    >
    > > > Subject: [PATCH v2] cpuacct: VIRT_CPU_ACCOUNTING don't prevent percpu cputime count
    > > >
    > > > cpuacct_update_stats() is called at every tick updating. and it use percpu_counter
    > > > for avoiding performance degression.
    > > >
    > > > For archs which define VIRT_CPU_ACCOUNTING, every tick would result
    > > > in >1000 units of cputime updates and since this is much much greater
    > > > than percpu_batch_counter, we end up taking spinlock on every tick.
    > > >
    > > > This patch change batch rule. now, any cpu can store "percpu_counter_bach * jiffies"
    > > > cputime in per-cpu cache.
    > > > it mean this patch don't have behavior change if VIRT_CPU_ACCOUNTING=n.
    > > >
    > > > Cc: Bharata B Rao <bharata@linux.vnet.ibm.com>
    > > > Cc: Balaji Rao <balajirrao@gmail.com>
    > > > Cc: Dhaval Giani <dhaval@linux.vnet.ibm.com>
    > > > Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    > > > Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
    > > > Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
    > > > Cc: Ingo Molnar <mingo@elte.hu>
    > > > Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
    > > > Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
    > > > ---
    > > > kernel/sched.c | 8 ++++++--
    > > > 1 file changed, 6 insertions(+), 2 deletions(-)
    > > >
    > > > Index: b/kernel/sched.c
    > > > ===================================================================
    > > > --- a/kernel/sched.c 2009-04-30 11:37:47.000000000 +0900
    > > > +++ b/kernel/sched.c 2009-04-30 14:17:00.000000000 +0900
    > > > @@ -10221,6 +10221,7 @@ struct cpuacct {
    > > > };
    > > >
    > > > struct cgroup_subsys cpuacct_subsys;
    > > > +static s32 cpuacct_batch;
    > > >
    > > > /* return cpu accounting group corresponding to this container */
    > > > static inline struct cpuacct *cgroup_ca(struct cgroup *cgrp)
    > > > @@ -10250,6 +10251,9 @@ static struct cgroup_subsys_state *cpuac
    > > > if (!ca->cpuusage)
    > > > goto out_free_ca;
    > > >
    > > > + if (!cpuacct_batch)
    > > > + cpuacct_batch = jiffies_to_cputime(percpu_counter_batch);
    > > > +
    > > > for (i = 0; i < CPUACCT_STAT_NSTATS; i++)
    > > > if (percpu_counter_init(&ca->cpustat[i], 0))
    > > > goto out_free_counters;
    > > > @@ -10376,7 +10380,7 @@ static int cpuacct_stats_show(struct cgr
    > > > int i;
    > > >
    > > > for (i = 0; i < CPUACCT_STAT_NSTATS; i++) {
    > > > - s64 val = percpu_counter_read(&ca->cpustat[i]);
    > > > + s64 val = percpu_counter_sum(&ca->cpustat[i]);
    > > > val = cputime64_to_clock_t(val);
    > > > cb->fill(cb, cpuacct_stat_desc[i], val);
    > > > }
    > > > @@ -10446,7 +10450,7 @@ static void cpuacct_update_stats(struct
    > > > ca = task_ca(tsk);
    > > >
    > > > do {
    > > > - percpu_counter_add(&ca->cpustat[idx], val);
    > > > + __percpu_counter_add(&ca->cpustat[idx], val, cpuacct_batch);
    > > > ca = ca->parent;
    > > > } while (ca);
    > > > rcu_read_unlock();
    > > >
    > > >
    > >
    > > What do the test results look like with this? I'll see if I can
    > > find some time to test this patch. On a patch read level this
    > > seems much better to me, Peter?
    >
    > I don't really fancy percpu_counter_sum() usage. I'm thinking its
    > ok to degrate accuracy on larger machines and simply use
    > percpu_counter_read().

    yes - and the values will converge anyway, right? So it's just a
    small delay, not even any genuine loss of accuracy.

    Ingo


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