RE: [PATCH AUTOSEL 5.15 20/29] block: reduce kblockd_mod_delayed_work_on() CPU consumption

[Michael Kelley (LINUX)]

From: Sasha Levin <sashal@xxxxxxxxxx> Sent: Monday, December 20, 2021 5:58 PM
> 
> From: Jens Axboe <axboe@xxxxxxxxx>
> 
> [ Upstream commit cb2ac2912a9ca7d3d26291c511939a41361d2d83 ]
> 
> Dexuan reports that he's seeing spikes of very heavy CPU utilization when
> running 24 disks and using the 'none' scheduler. This happens off the
> sched restart path, because SCSI requires the queue to be restarted async,
> and hence we're hammering on mod_delayed_work_on() to ensure that the work
> item gets run appropriately.
> 
> Avoid hammering on the timer and just use queue_work_on() if no delay
> has been specified.
> 
> Reported-and-tested-by: Dexuan Cui <decui@xxxxxxxxxxxxx>
> Link: https://lore.kernel.org/linux-block/BYAPR21MB1270C598ED214C0490F47400BF719@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
> Reviewed-by: Ming Lei <ming.lei@xxxxxxxxxx>
> Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
> ---
>  block/blk-core.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index c2d912d0c976c..a728434fcff87 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -1625,6 +1625,8 @@ EXPORT_SYMBOL(kblockd_schedule_work);
>  int kblockd_mod_delayed_work_on(int cpu, struct delayed_work *dwork,
>  				unsigned long delay)
>  {
> +	if (!delay)
> +		return queue_work_on(cpu, kblockd_workqueue, &dwork->work);
>  	return mod_delayed_work_on(cpu, kblockd_workqueue, dwork, delay);
>  }
>  EXPORT_SYMBOL(kblockd_mod_delayed_work_on);
> --
> 2.34.1

Sasha -- there are reports of this patch causing performance problems.  See https://lore.kernel.org/lkml/1639853092.524jxfaem2.none@localhost/.
I would suggest *not* backporting it to any of the stable branches until the
issues are fully sorted out.

Michael