linux-kernel - Re: [PATCH AUTOSEL 5.15 20/29] block: reduce kblockd_mod_delayed_work

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Date:   Tue, 21 Dec 2021 11:11:43 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Sasha Levin <sashal@...nel.org>
Cc:     "Michael Kelley (LINUX)" <mikelley@...rosoft.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Dexuan Cui <decui@...rosoft.com>,
        Ming Lei <ming.lei@...hat.com>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>
Subject: Re: [PATCH AUTOSEL 5.15 20/29] block: reduce
 kblockd_mod_delayed_work_on() CPU consumption

On 12/21/21 10:58 AM, Sasha Levin wrote:
> On Tue, Dec 21, 2021 at 08:36:33AM -0700, Jens Axboe wrote:
>> On 12/21/21 8:35 AM, Michael Kelley (LINUX) wrote:
>>> From: Sasha Levin <sashal@...nel.org> Sent: Monday, December 20, 2021 5:58 PM
>>>>
>>>> From: Jens Axboe <axboe@...nel.dk>
>>>>
>>>> [ 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@...rosoft.com>
>>>> Link: https://lore.kernel.org/linux-block/BYAPR21MB1270C598ED214C0490F47400BF719@BYAPR21MB1270.namprd21.prod.outlook.com/
>>>> Reviewed-by: Ming Lei <ming.lei@...hat.com>
>>>> Signed-off-by: Jens Axboe <axboe@...nel.dk>
>>>> Signed-off-by: Sasha Levin <sashal@...nel.org>
>>>> ---
>>>>  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.
>>
>> Both this and the revert were backported. Which arguably doesn't make a
>> lot of sense, but at least it's consistent and won't cause any issues...
> 
> The logic behind it is that it makes it easy for both us as well as
> everyone else to annotate why a certain patch might be "missing" from
> the trees - in this case because it was reverted.
> 
> It looks dumb now, but it saves a lot of time as well as mitigates the
> risk of it being picked up again at some point in the future.

It's fine with me, when I saw the first patch yesterday I did get
worried, but then I saw the revert was picked too. As I said, as long
as the end result is sane, then there's no harm in doing it this way.

-- 
Jens Axboe