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    Subject[tip:sched/urgent] sched: Optimize task_sched_runtime()
    Commit-ID:  911b2898b3c9fe0048e9485ad1629ed4fce330fd
    Gitweb: http://git.kernel.org/tip/911b2898b3c9fe0048e9485ad1629ed4fce330fd
    Author: Peter Zijlstra <peterz@infradead.org>
    AuthorDate: Mon, 11 Nov 2013 18:21:56 +0100
    Committer: Ingo Molnar <mingo@kernel.org>
    CommitDate: Wed, 13 Nov 2013 13:33:54 +0100

    sched: Optimize task_sched_runtime()

    Large multi-threaded apps like to hit this using do_sys_times() and
    then queue up on the rq->lock.

    Avoid when possible.

    Larry reported ~20% performance increase his test case.

    Reported-by: Larry Woodman <lwoodman@redhat.com>
    Suggested-by: Paul Turner <pjt@google.com>
    Signed-off-by: Peter Zijlstra <peterz@infradead.org>
    Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Link: http://lkml.kernel.org/r/20131111172925.GG26898@twins.programming.kicks-ass.net
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    ---
    kernel/sched/core.c | 14 ++++++++++++++
    1 file changed, 14 insertions(+)

    diff --git a/kernel/sched/core.c b/kernel/sched/core.c
    index 1deccd7..c180860 100644
    --- a/kernel/sched/core.c
    +++ b/kernel/sched/core.c
    @@ -2253,6 +2253,20 @@ unsigned long long task_sched_runtime(struct task_struct *p)
    struct rq *rq;
    u64 ns = 0;

    +#if defined(CONFIG_64BIT) && defined(CONFIG_SMP)
    + /*
    + * 64-bit doesn't need locks to atomically read a 64bit value.
    + * So we have a optimization chance when the task's delta_exec is 0.
    + * Reading ->on_cpu is racy, but this is ok.
    + *
    + * If we race with it leaving cpu, we'll take a lock. So we're correct.
    + * If we race with it entering cpu, unaccounted time is 0. This is
    + * indistinguishable from the read occurring a few cycles earlier.
    + */
    + if (!p->on_cpu)
    + return p->se.sum_exec_runtime;
    +#endif
    +
    rq = task_rq_lock(p, &flags);
    ns = p->se.sum_exec_runtime + do_task_delta_exec(p, rq);
    task_rq_unlock(rq, p, &flags);

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