linux-kernel - Re: [PATCH v7 03/10] blk-mq: add blk_mq_{online|possible}_queue

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Message-ID: <9c7f50c9-dd43-4b75-83ee-eac6a6ee2093@suse.de>
Date: Thu, 3 Jul 2025 08:29:08 +0200
From: Hannes Reinecke <hare@...e.de>
To: 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>
Cc: 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 03/10] blk-mq: add
 blk_mq_{online|possible}_queue_affinity

On 7/2/25 18:33, Daniel Wagner wrote:
> Introduce blk_mq_{online|possible}_queue_affinity, which returns the
> queue-to-CPU mapping constraints defined by the block layer. This allows
> other subsystems (e.g., IRQ affinity setup) to respect block layer
> requirements.
> 
> It is necessary to provide versions for both the online and possible CPU
> masks because some drivers want to spread their I/O queues only across
> online CPUs, while others prefer to use all possible CPUs. And the mask
> used needs to match with the number of queues requested
> (see blk_num_{online|possible}_queues).
> 
Technically you are correct.
However, I do have the sneaking suspicion that most drivers just use
num_online_cpus() for convenience and to reduce the size of the
structures.
But I guess I'll comment on that in the patch modifying the drivers.

> Signed-off-by: Daniel Wagner <wagi@...nel.org>
> ---
>   block/blk-mq-cpumap.c  | 24 ++++++++++++++++++++++++
>   include/linux/blk-mq.h |  2 ++
>   2 files changed, 26 insertions(+)
> 
> diff --git a/block/blk-mq-cpumap.c b/block/blk-mq-cpumap.c
> index 705da074ad6c7e88042296f21b739c6d686a72b6..8244ecf878358c0b8de84458dcd5100c2f360213 100644
> --- a/block/blk-mq-cpumap.c
> +++ b/block/blk-mq-cpumap.c
> @@ -26,6 +26,30 @@ static unsigned int blk_mq_num_queues(const struct cpumask *mask,
>   	return min_not_zero(num, max_queues);
>   }
>   
> +/**
> + * blk_mq_possible_queue_affinity - Return block layer queue affinity
> + *
> + * Returns an affinity mask that represents the queue-to-CPU mapping
> + * requested by the block layer based on possible CPUs.
> + */
> +const struct cpumask *blk_mq_possible_queue_affinity(void)
> +{
> +	return cpu_possible_mask;
> +}
> +EXPORT_SYMBOL_GPL(blk_mq_possible_queue_affinity);
> +
> +/**
> + * blk_mq_online_queue_affinity - Return block layer queue affinity
> + *
> + * Returns an affinity mask that represents the queue-to-CPU mapping
> + * requested by the block layer based on online CPUs.
> + */
> +const struct cpumask *blk_mq_online_queue_affinity(void)
> +{
> +	return cpu_online_mask;
> +}
> +EXPORT_SYMBOL_GPL(blk_mq_online_queue_affinity);
> +
>   /**
>    * blk_mq_num_possible_queues - Calc nr of queues for multiqueue devices
>    * @max_queues:	The maximum number of queues the hardware/driver
> diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
> index 2a5a828f19a0ba6ff0812daf40eed67f0e12ada1..1144017dce47af82f9d010e42bfbf26fa4ddf33f 100644
> --- a/include/linux/blk-mq.h
> +++ b/include/linux/blk-mq.h
> @@ -947,6 +947,8 @@ int blk_mq_freeze_queue_wait_timeout(struct request_queue *q,
>   void blk_mq_unfreeze_queue_non_owner(struct request_queue *q);
>   void blk_freeze_queue_start_non_owner(struct request_queue *q);
>   
> +const struct cpumask *blk_mq_possible_queue_affinity(void);
> +const struct cpumask *blk_mq_online_queue_affinity(void);
>   unsigned int blk_mq_num_possible_queues(unsigned int max_queues);
>   unsigned int blk_mq_num_online_queues(unsigned int max_queues);
>   void blk_mq_map_queues(struct blk_mq_queue_map *qmap);
> 

Reviewed-by: Hannes Reinecke <hare@...e.de>

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