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

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Message-ID: <c9ef4fa6-99a4-46fd-8623-b8979556566e@flourine.local>
Date: Mon, 8 Sep 2025 09:36:34 +0200
From: Daniel Wagner <dwagner@...e.de>
To: linux-nvme@...ts.infradead.org
Cc: 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>, Hannes Reinecke <hare@...e.de>, 
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, linux-kernel@...r.kernel.org, linux-block@...r.kernel.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 Fri, Sep 05, 2025 at 04:59:56PM +0200, Daniel Wagner wrote:
>  void blk_mq_map_queues(struct blk_mq_queue_map *qmap)
>  {
> -	const struct cpumask *masks;
> +	struct cpumask *masks __free(kfree) = NULL;
> +	const struct cpumask *constraint;
>  	unsigned int queue, cpu, nr_masks;
> +	cpumask_var_t active_hctx;
>  
> +	if (!zalloc_cpumask_var(&active_hctx, GFP_KERNEL))
> +		goto fallback;
> +

> +	free_cpumask_var(active_hctx);
> +
> +	return;
> +
> +free_fallback:
> +	free_cpumask_var(active_hctx);
> +
> +fallback:
> +	blk_mq_map_fallback(qmap);

I am not so happy that the cpumask_var_t and __free doesn't work to
together at this point due to the 'evil way' how cpumask_var_t is defined:

	ifdef CONFIG_CPUMASK_OFFSTACK
	typedef struct cpumask *cpumask_var_t;
	#else
	typedef struct cpumask cpumask_var_t[1];
	#endif /* CONFIG_CPUMASK_OFFSTACK */

In the previous version I used

	cpumask_var_t active_hctx __free(free_cpumask_var) = NULL;

which resulted in a way cleaner code. Though the kernel test robot
complained with

      >> block/blk-mq-cpumap.c:155:16: error: array initializer must be an initializer list
           155 |         cpumask_var_t active_hctx __free(free_cpumask_var) = NULL;

I try to figure out if it's possible to get this somehow working with
some witchcraft (aka pre compiler magic).