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

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Message-ID: <d11a0c60-1b75-49ec-a2f8-7df402c4adf2@flourine.local>
Date: Mon, 8 Sep 2025 09:26:05 +0200
From: Daniel Wagner <dwagner@...e.de>
To: Hannes Reinecke <hare@...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 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.