linux-kernel - Re: [PATCH v8 10/12] blk-mq: use hk cpus only when isolcpus=io

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Message-ID: <321277a3-7dcc-414b-a329-71acfb504e91@suse.de>
Date: Mon, 8 Sep 2025 09:51:34 +0200
From: Hannes Reinecke <hare@...e.de>
To: Daniel Wagner <dwagner@...e.de>
Cc: Daniel Wagner <wagi@...nel.org>, Jens Axboe <axboe@...nel.dk>,
 Keith Busch <kbusch@...nel.org>, Christoph Hellwig <hch@....de>,
 Sagi Grimberg <sagi@...mberg.me>, "Michael S. Tsirkin" <mst@...hat.com>,
 Aaron Tomlin <atomlin@...mlin.com>,
 "Martin K. Petersen" <martin.petersen@...cle.com>,
 Thomas Gleixner <tglx@...utronix.de>, Costa Shulyupin
 <costa.shul@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
 Valentin Schneider <vschneid@...hat.com>, Waiman Long <llong@...hat.com>,
 Ming Lei <ming.lei@...hat.com>, Frederic Weisbecker <frederic@...nel.org>,
 Mel Gorman <mgorman@...e.de>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
 linux-nvme@...ts.infradead.org, megaraidlinux.pdl@...adcom.com,
 linux-scsi@...r.kernel.org, storagedev@...rochip.com,
 virtualization@...ts.linux.dev, GR-QLogic-Storage-Upstream@...vell.com
Subject: Re: [PATCH v8 10/12] blk-mq: use hk cpus only when isolcpus=io_queue
 is enabled

On 9/8/25 09:26, Daniel Wagner wrote:
> On Mon, Sep 08, 2025 at 08:13:31AM +0200, Hannes Reinecke wrote:
>>>    const struct cpumask *blk_mq_online_queue_affinity(void)
>>>    {
>>> +	if (housekeeping_enabled(HK_TYPE_IO_QUEUE)) {
>>> +		cpumask_and(&blk_hk_online_mask, cpu_online_mask,
>>> +			    housekeeping_cpumask(HK_TYPE_IO_QUEUE));
>>> +		return &blk_hk_online_mask;
>>
>> Can you explain the use of 'blk_hk_online_mask'?
>> Why is a static variable?
> 
> The blk_mq_*_queue_affinity helpers return a const struct cpumask *, the
> caller doesn't need to free the return value. Because cpumask_and needs
> store its result somewhere, I opted for the global static variable.
> 
>> To my untrained eye it's being recalculated every time one calls
>> this function. And only the first invocation run on an empty mask,
>> all subsequent ones see a populated mask.
> 
> The cpu_online_mask might change over time, it's not a static bitmap.
> Thus it's necessary to update the blk_hk_online_mask. Doing some sort of
> caching is certainly possible. Given that we have plenty of cpumask
> logic operation in the cpu_group_evenly code path later, I am not so
> sure this really makes a huge difference.

Oh, that's okay. I'm perfectly fine to update the cpumask on every call.
What makes me wonder is the initialisation of blk_hk_online_mask;
it's zeroed _at boot_, and then all we do is calling cpumask_and()
for every invocation. So the mask will only increase in scope,
and never decrease.

Wouldn't it be better to call 'cpumask_zero' before 'cpumask_and'?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@...e.de                                +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
HRB 36809 (AG Nürnberg), GF: I. Totev, A. McDonald, W. Knoblich