Messages in this thread | | | From | Paul Turner <> | Date | Wed, 1 May 2013 04:50:12 -0700 | Subject | Re: [PATCH 09/10] sched: task_sched_runtime introduce micro optimization |
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On Wed, May 1, 2013 at 4:00 AM, Peter Zijlstra <peterz@infradead.org> wrote: > On Mon, Apr 29, 2013 at 11:17:17PM -0400, kosaki.motohiro@gmail.com wrote: >> From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> >> >> rq lock in task_sched_runtime() is necessary for two reasons. 1) >> accessing se.sum_exec_runtime is inatomic on 32bit and 2) >> do_task_delta_exec() require it. >> >> And then, 64bit can avoid holds rq lock when add_delta is false. >> >> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> >> --- >> kernel/sched/core.c | 6 ++++++ >> 1 files changed, 6 insertions(+), 0 deletions(-) >> >> diff --git a/kernel/sched/core.c b/kernel/sched/core.c >> index b817e6d..24ba1c6 100644 >> --- a/kernel/sched/core.c >> +++ b/kernel/sched/core.c >> @@ -2657,6 +2657,12 @@ unsigned long long task_sched_runtime(struct task_struct *p, bool add_delta) >> struct rq *rq; >> u64 ns = 0; >> >> + /* Micro optimization. */ > > Instead of the above; how about something like: > > /* 64-bit doesn't need locks to atomically read a 64bit value */ > >> +#ifdef CONFIG_64BIT >> + if (!add_delta) >> + return p->se.sum_exec_runtime; >> +#endif
Stronger:
+#ifdef CONFIG_64BIT + if (!p->on_cpu) + return p->se.sum_exec_runtime; +#endif
[ Or !p->on_cpu || !add_delta ].
We can take the racy read versus p->on_cpu since: If we race with it leaving cpu: we take lock, we're correct If we race with it entering cpu: unaccounted time ---> 0, this is indistinguishable from the read occurring a few cycles earlier.
>> >> rq = task_rq_lock(p, &flags); >> ns = p->se.sum_exec_runtime; >> if (add_delta) >> -- >> 1.7.1 >> > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/
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