Subject: Re: [PATCH AUTOSEL 5.15 20/29] block: reduce
 kblockd_mod_delayed_work_on() CPU consumption
To:     "Michael Kelley (LINUX)" <mikelley@microsoft.com>,
        Sasha Levin <sashal@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>
Cc:     Dexuan Cui <decui@microsoft.com>, Ming Lei <ming.lei@redhat.com>,
        "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>
References: <20211221015751.116328-1-sashal@kernel.org>
 <20211221015751.116328-20-sashal@kernel.org>
 <MWHPR21MB1593141494C76CF5A0BDDB11D77C9@MWHPR21MB1593.namprd21.prod.outlook.com>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <ad76826e-73b2-b2f0-3cd4-8481645a6568@kernel.dk>
Date:   Tue, 21 Dec 2021 08:36:33 -0700
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On 12/21/21 8:35 AM, Michael Kelley (LINUX) wrote:
> From: Sasha Levin <sashal@kernel.org> Sent: Monday, December 20, 2021 5:58 PM
>>
>> From: Jens Axboe <axboe@kernel.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@microsoft.com>
>> Link: https://lore.kernel.org/linux-block/BYAPR21MB1270C598ED214C0490F47400BF719@BYAPR21MB1270.namprd21.prod.outlook.com/
>> Reviewed-by: Ming Lei <ming.lei@redhat.com>
>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>> Signed-off-by: Sasha Levin <sashal@kernel.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...

-- 
Jens Axboe