From: Peter Zijlstra <peterz@infradead.org>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: linux-kernel@vger.kernel.org, io-uring@vger.kernel.org,
Ingo Molnar <mingo@redhat.com>,
Juri Lelli <juri.lelli@redhat.com>,
Vincent Guittot <vincent.guittot@linaro.org>,
Dietmar Eggemann <dietmar.eggemann@arm.com>,
Steven Rostedt <rostedt@goodmis.org>,
Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
Jens Axboe <axboe@kernel.dk>,
Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH 2/2] sched: Cache task_struct::flags in sched_submit_work()
Date: Wed, 19 Aug 2020 22:11:27 +0200 [thread overview]
Message-ID: <20200819201127.GP3982@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <20200819200025.lqvmyefqnbok5i4f@linutronix.de>
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@linutronix.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!
next prev parent reply other threads:[~2020-08-19 20:11 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-19 12:37 [RFC PATCH] sched: Invoke io_wq_worker_sleeping() with enabled preemption Sebastian Andrzej Siewior
2020-08-19 13:15 ` peterz
2020-08-19 13:18 ` Jens Axboe
2020-08-19 19:44 ` [PATCH] io_wq: Make io_wqe::lock a raw_spinlock_t Sebastian Andrzej Siewior
2020-09-01 8:41 ` [PATCH v2] " Sebastian Andrzej Siewior
2020-09-01 14:17 ` Jens Axboe
2020-08-19 13:33 ` [RFC PATCH] sched: Invoke io_wq_worker_sleeping() with enabled preemption Sebastian Andrzej Siewior
2020-08-19 14:21 ` peterz
2020-08-19 19:55 ` [PATCH 1/2] sched: Bring the PF_IO_WORKER and PF_WQ_WORKER bits closer together Sebastian Andrzej Siewior
2020-08-19 20:00 ` [PATCH 2/2] sched: Cache task_struct::flags in sched_submit_work() Sebastian Andrzej Siewior
2020-08-19 20:11 ` Peter Zijlstra [this message]
2020-09-07 12:58 ` [PATCH 1/2] sched: Bring the PF_IO_WORKER and PF_WQ_WORKER bits closer together Will Deacon
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