linux-kernel - Re: [PATCH] io-wq: set task TASK_INTERRUPTIBLE state before schedule

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Message-ID: <bc138db4-4609-b8e6-717a-489cf2027fc0@kernel.dk>
Date:   Tue, 27 Oct 2020 07:35:00 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     qiang.zhang@...driver.com
Cc:     io-uring@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] io-wq: set task TASK_INTERRUPTIBLE state before
 schedule_timeout

On 10/26/20 9:09 PM, qiang.zhang@...driver.com wrote:
> From: Zqiang <qiang.zhang@...driver.com>
> 
> In 'io_wqe_worker' thread, if the work which in 'wqe->work_list' be
> finished, the 'wqe->work_list' is empty, and after that the
> '__io_worker_idle' func return false, the task state is TASK_RUNNING,
> need to be set TASK_INTERRUPTIBLE before call schedule_timeout func.

I don't think that's safe - what if someone added work right before you
call schedule_timeout_interruptible? Something ala:

io_wq_enqueue()
			set_current_state(TASK_INTERRUPTIBLE();
			schedule_timeout(WORKER_IDLE_TIMEOUT);

then we'll have work added and the task state set to running, but the
worker itself just sets us to non-running and will hence wait
WORKER_IDLE_TIMEOUT before the work is processed.

The current situation will do one extra loop for this case, as the
schedule_timeout() just ends up being a nop and we go around again
checking for work. Since we already unused the mm, the next iteration
will go to sleep properly unless new work came in.

-- 
Jens Axboe