Re: [PATCH REPOST] blk-mq: Don't disable preemption around __blk_mq_run_hw_queue().

[Ming Lei <ming.lei@redhat.com>]

On Tue, Jun 21, 2022 at 09:39:19AM +0200, Sebastian Andrzej Siewior wrote:
> __blk_mq_delay_run_hw_queue() disables preemption to get a stable
> current CPU number and then invokes __blk_mq_run_hw_queue() if the CPU
> number is part the mask.
> 
> __blk_mq_run_hw_queue() acquires a spin_lock_t which is a sleeping lock
> on PREEMPT_RT and can't be acquired with disabled preemption.
> 
> If it is important that the current CPU matches the requested CPU mask
> and that the context does not migrate to another CPU while
> __blk_mq_run_hw_queue() is invoked then it possible to achieve this by
> disabling migration and keeping the context preemptible.
> 
> Disable only migration while testing the CPU mask and invoking
> __blk_mq_run_hw_queue().
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Link: https://lore.kernel.org/r/YnQHqx/5+54jd+U+@linutronix.de
> Link: https://lore.kernel.org/r/YqISXf6GAQeWqcR+@linutronix.de
> ---
>  block/blk-mq.c |    8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2083,14 +2083,14 @@ static void __blk_mq_delay_run_hw_queue(
>  		return;
>  
>  	if (!async && !(hctx->flags & BLK_MQ_F_BLOCKING)) {
> -		int cpu = get_cpu();
> -		if (cpumask_test_cpu(cpu, hctx->cpumask)) {
> +		migrate_disable();
> +		if (cpumask_test_cpu(raw_smp_processor_id(), hctx->cpumask)) {
>  			__blk_mq_run_hw_queue(hctx);
> -			put_cpu();
> +			migrate_enable();

I think you can replace it with raw_smp_processor_id() directly without
disabling migration.

Both async run queue and direct issue can run on cpu not on hctx->cpumask,
so it is fine for sync run queue too. 

Thanks,
Ming