* Re: [PATCH] io_uring: unlock sqd->lock before sq thread release CPU
2023-05-24 5:28 ` [PATCH] io_uring: unlock sqd->lock before sq thread release CPU Wenwen Chen
@ 2023-05-24 14:08 ` Jens Axboe
0 siblings, 0 replies; 2+ messages in thread
From: Jens Axboe @ 2023-05-24 14:08 UTC (permalink / raw)
To: Wenwen Chen, asml.silence; +Cc: io-uring, Kanchan Joshi
On 5/23/23 11:28?PM, Wenwen Chen wrote:
> The sq thread actively releases CPU resources by calling the
> cond_resched() and schedule() interfaces when it is idle. Therefore,
> more resources are available for other threads to run.
>
> There exists a problem in sq thread: it does not unlock sqd->lock before
> releasing CPU resources every time. This makes other threads pending on
> sqd->lock for a long time. For example, the following interfaces all
> require sqd->lock: io_sq_offload_create(), io_register_iowq_max_workers()
> and io_ring_exit_work().
>
> Before the sq thread releases CPU resources, unlocking sqd->lock will
> provide the user a better experience because it can respond quickly to
> user requests.
>
> Signed-off-by: Kanchan Joshi<[email protected]>
> Signed-off-by: Wenwen Chen<[email protected]>
> ---
> io_uring/sqpoll.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/io_uring/sqpoll.c b/io_uring/sqpoll.c
> index 9db4bc1f521a..759c80fb4afa 100644
> --- a/io_uring/sqpoll.c
> +++ b/io_uring/sqpoll.c
> @@ -255,7 +255,9 @@ static int io_sq_thread(void *data)
> sqt_spin = true;
>
> if (sqt_spin || !time_after(jiffies, timeout)) {
> + mutex_unlock(&sqd->lock);
> cond_resched();
> + mutex_lock(&sqd->lock);
> if (sqt_spin)
> timeout = jiffies + sqd->sq_thread_idle;
> continue;
Since this is the spin case, and we expect (by far) most of these
to NOT need a reschedule, I think we should do:
if (need_resched()) {
mutex_unlock(&sqd->lock);
cond_resched();
mutex_lock(&sqd->lock);
}
to make that lock shuffle dependent on the need to reschedule. And
since we're marking the timeout at that point, timeout should be
assigned first as far as I can tell. So in total:
if (sqt_spin || !time_after(jiffies, timeout)) {
if (sqt_spin)
timeout = jiffies + sqd->sq_thread_idle;
if (unlikely(need_resched())) {
mutex_unlock(&sqd->lock);
cond_resched();
mutex_lock(&sqd->lock);
}
continue;
}
would probably be the better fix.
--
Jens Axboe
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