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Date:   Wed, 19 Aug 2020 15:15:07 +0200
From:   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: [RFC PATCH] sched: Invoke io_wq_worker_sleeping() with enabled
 preemption
Message-ID: <20200819131507.GC2674@hirez.programming.kicks-ass.net>
References: <20200819123758.6v45rj2gvojddsnn@linutronix.de>
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On Wed, Aug 19, 2020 at 02:37:58PM +0200, Sebastian Andrzej Siewior wrote:

> I don't see a significant reason why this lock should become a
> raw_spinlock_t therefore I suggest to move it after the
> tsk_is_pi_blocked() check.

> Any feedback on this vs raw_spinlock_t?
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
>  fs/io-wq.c          |  8 ++++----
>  kernel/sched/core.c | 10 +++++-----
>  2 files changed, 9 insertions(+), 9 deletions(-)
> 

> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 3bbb60b97c73c..b76c0f27bd95e 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4694,18 +4694,18 @@ static inline void sched_submit_work(struct task_struct *tsk)
>  	 * in the possible wakeup of a kworker and because wq_worker_sleeping()
>  	 * requires it.
>  	 */
> -	if (tsk->flags & (PF_WQ_WORKER | PF_IO_WORKER)) {
> +	if (tsk->flags & PF_WQ_WORKER) {
>  		preempt_disable();
> -		if (tsk->flags & PF_WQ_WORKER)
> -			wq_worker_sleeping(tsk);
> -		else
> -			io_wq_worker_sleeping(tsk);
> +		wq_worker_sleeping(tsk);
>  		preempt_enable_no_resched();
>  	}
>  
>  	if (tsk_is_pi_blocked(tsk))
>  		return;
>  
> +	if (tsk->flags & PF_IO_WORKER)
> +		io_wq_worker_sleeping(tsk);
> +

Urgh, so this adds a branch in what is normally considered a fairly hot
path.

I'm thinking that the raw_spinlock_t option would permit leaving that
single:

	if (tsk->flags & (PF_WQ_WORKER | PF_IO_WORKER))

branch intact?