linux-kernel - Re: [PATCH 2/2] sched: Cache task_struct::flags in sched_submit

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Date:   Wed, 19 Aug 2020 22:11:27 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, io-uring@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Jens Axboe <axboe@...nel.dk>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 2/2] sched: Cache task_struct::flags in
 sched_submit_work()

On Wed, Aug 19, 2020 at 10:00:25PM +0200, Sebastian Andrzej Siewior wrote:
> sched_submit_work() is considered to be a hot path. The preempt_disable()
> instruction is a compiler barrier and forces the compiler to load
> task_struct::flags for the second comparison.
> By using a local variable, the compiler can load the value once and keep it in
> a register for the second comparison.
> 
> Verified on x86-64 with gcc-10.
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
> 
> Optimisation at molecule level, part two. Drop this in case this branch
> isn't consider *that* hot and the cache hot value can be loaded again.
> But then the value is around and be speculated early on :)

That's fine, task->flags can only be changed by current.

Patches look good to me, I'll stuff them in tomorrow. Thanks!