Messages in this thread Patch in this message | | | From | kosaki.motohiro@gmail ... | Subject | [PATCH 6/8] sched: task_sched_runtime introduce micro optimization | Date | Sun, 26 May 2013 17:35:47 -0400 |
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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 not atomic on 32bit and 2) do_task_delta_exec() require it.
So, 64bit can avoid holding rq lock when add_delta is false and delta_exec is 0.
Cc: Olivier Langlois <olivier@trillion01.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Frederic Weisbecker <fweisbec@gmail.com> Cc: Ingo Molnar <mingo@kernel.org> Suggested-by: Paul Turner <pjt@google.com> Acked-by: Peter Zijlstra <peterz@infradead.org> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> --- kernel/sched/core.c | 15 +++++++++++++++ 1 files changed, 15 insertions(+), 0 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 96512e9..0f859cc 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2692,6 +2692,21 @@ unsigned long long task_sched_runtime(struct task_struct *p, bool add_delta) struct rq *rq; u64 ns = 0; +#ifdef CONFIG_64BIT + /* + * 64-bit doesn't need locks to atomically read a 64bit value. So we + * have two optimization chances, 1) when caller doesn't need + * delta_exec and 2) when the task's delta_exec is 0. The former is + * obvious. The latter is complicated. 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 (!add_delta || !p->on_cpu) + return p->se.sum_exec_runtime; +#endif + rq = task_rq_lock(p, &flags); ns = p->se.sum_exec_runtime; if (add_delta) -- 1.7.1
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