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From: Quentin Perret <qperret@google.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: mingo@kernel.org, vincent.guittot@linaro.org,
	dietmar.eggemann@arm.com, juri.lelli@redhat.com,
	rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
	linux-kernel@vger.kernel.org, valentin.schneider@arm.com,
	qais.yousef@arm.com, ktkhai@virtuozzo.com
Subject: Re: [PATCH 4/7] sched: Optimize pick_next_task()
Date: Fri, 8 Nov 2019 14:33:48 +0000	[thread overview]
Message-ID: <20191108143348.GB123156@google.com> (raw)
In-Reply-To: <20191108131909.603037345@infradead.org>

On Friday 08 Nov 2019 at 14:15:57 (+0100), Peter Zijlstra wrote:
> Ever since we moved the sched_class defenitions into their own files,

s/defenitions/definitions

> the constant expression {fair,idle}_sched_class.pick_next_task() is
> not in fact a compile time constant anymore and results in an indirect
> call (barring LTO).
> 
> Fix that by exposing pick_next_task_{fair,idle}() directly, this gets
> rid of the indirect call (and RETPOLINE) on the fast path.
> 
> Also remove the unlikely() from the idle case, it is in fact /the/ way
> we select idle -- and that is a very common thing to do.

I assumed this was to optimize the case where we did find a cfs task to
run. That is, we can afford to hit the unlikely case when there is no
work to do after, but when there is, we shouldn't spend time checking
the idle case. Makes sense ?

Thanks,
Quentin

  reply	other threads:[~2019-11-08 14:33 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-08 13:15 [PATCH 0/7] scheduler patches Peter Zijlstra
2019-11-08 13:15 ` [PATCH 1/7] sched: Fix pick_next_task() vs change pattern race Peter Zijlstra
2019-11-08 14:28   ` Quentin Perret
2019-11-08 16:05   ` Valentin Schneider
2019-11-08 20:49     ` Peter Zijlstra
2019-11-08 17:03   ` Qais Yousef
2019-11-08 13:15 ` [PATCH 2/7] sched/fair: Better document newidle_balance() Peter Zijlstra
2019-11-11  9:32   ` [tip: sched/core] " tip-bot2 for Peter Zijlstra
2019-11-08 13:15 ` [PATCH 3/7] sched: Make pick_next_task_idle() more consistent Peter Zijlstra
2019-11-11  9:32   ` [tip: sched/core] sched/core: " tip-bot2 for Peter Zijlstra
2019-11-08 13:15 ` [PATCH 4/7] sched: Optimize pick_next_task() Peter Zijlstra
2019-11-08 14:33   ` Quentin Perret [this message]
2019-11-08 16:46     ` Vincent Guittot
2019-11-08 20:46     ` Peter Zijlstra
2019-11-11  9:32   ` [tip: sched/core] sched/core: " tip-bot2 for Peter Zijlstra
2019-11-08 13:15 ` [PATCH 5/7] sched: Simplify sched_class::pick_next_task() Peter Zijlstra
2019-11-11  9:32   ` [tip: sched/core] sched/core: " tip-bot2 for Peter Zijlstra
2019-11-08 13:15 ` [PATCH 6/7] sched/fair: Use mul_u32_u32() Peter Zijlstra
2019-11-11  9:32   ` [tip: sched/core] " tip-bot2 for Peter Zijlstra
2019-11-08 13:16 ` [PATCH 7/7] sched: Further clarify sched_class::set_next_task() Peter Zijlstra
2019-11-11  9:32   ` [tip: sched/core] sched/core: " tip-bot2 for Peter Zijlstra

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