linux-kernel - Re: [PATCH v7 05/10] scsi: Use block layer helpers to constrain queue affinity

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Message-ID: <2e7576e4-442f-4000-817d-6253374f5818@flourine.local>
Date: Fri, 4 Jul 2025 11:37:09 +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 v7 05/10] scsi: Use block layer helpers to constrain
 queue affinity

On Thu, Jul 03, 2025 at 08:43:01AM +0200, Hannes Reinecke wrote:
> All of these drivers are not aware of CPU hotplug, and as such
> will not be notified when the number of CPUs changes.

Ah, this explains this part.

> But you use 'blk_mq_online_queue_affinity()' for all of these
> drivers.

All these drivers are also using blk_mq_num_online_queue. When I only
used cpu_possible_mask the resulting mapping was not usable.

> Wouldn't 'blk_mq_possible_queue_affinit()' a better choice here
> to insulate against CPU hotplug effects?

With this mask the queues will be distributed to all possible CPUs and
some of the hardware queues could be assigned to offline CPUs. I think
this would work but the question is, is this okay to leave some of the
perfomance on the road?

I am not against this, just saying it would change the existing
behavior.

> Also some drivers which are using irq affinity (eg aacraid, lpfc) are
> missing from these conversions. Why?

I was not aware of aacraid. I started to work on lpfc and well let's put
it this way, it's complicated. lpfc needs a lot of work to make it
isolcpus aware.