linux-kernel - Re: [PATCH] blk-mq: create correct map for fallback case

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Message-ID: <20250123101843.1a682cea@gandalf.local.home>
Date: Thu, 23 Jan 2025 10:18:43 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Daniel Wagner <wagi@...nel.org>
Cc: Jens Axboe <axboe@...nel.dk>, Hannes Reinecke <hare@...e.de>, Ming Lei
 <ming.lei@...hat.com>, John Garry <john.g.garry@...cle.com>,
 linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] blk-mq: create correct map for fallback case

On Thu, 23 Jan 2025 14:08:29 +0100
Daniel Wagner <wagi@...nel.org> wrote:

> The fallback code in blk_mq_map_hw_queues is original from
> blk_mq_pci_map_queues and was added to handle the case where
> pci_irq_get_affinity will return NULL for !SMP configuration.
> 
> blk_mq_map_hw_queues replaces besides blk_mq_pci_map_queues also
> blk_mq_virtio_map_queues which used to use blk_mq_map_queues for the
> fallback.
> 
> It's possible to use blk_mq_map_queues for both cases though.
> blk_mq_map_queues creates the same map as blk_mq_clear_mq_map for !SMP
> that is CPU 0 will be mapped to hctx 0.
> 
> The WARN_ON_ONCE has to be dropped for virtio as the fallback is also
> taken for certain configuration on default. Though there is still a
> WARN_ON_ONCE check in lib/group_cpus.c:
> 
>        WARN_ON(nr_present + nr_others < numgrps);
> 
> which will trigger if the caller tries to create more hardware queues
> than CPUs. It tests the same as the WARN_ON_ONCE in
> blk_mq_pci_map_queues did.
> 
> Fixes: a5665c3d150c ("virtio: blk/scsi: replace blk_mq_virtio_map_queues with blk_mq_map_hw_queues")
> Reported-by: Steven Rostedt <rostedt@...dmis.org>

Tested-by: Steven Rostedt (Google) <rostedt@...dmis.org>

-- Steve

> Closes: https://lore.kernel.org/all/20250122093020.6e8a4e5b@gandalf.local.home/
> Signed-off-by: Daniel Wagner <wagi@...nel.org>
> ---
>  block/blk-mq-cpumap.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>