Re: [PATCH bitmap-for-next 1/5] blk_mq: Fix cpumask_check() warning in blk_mq_hctx_next_cpu()

[Yury Norov]

On Mon, Oct 03, 2022 at 04:34:16PM +0100, Valentin Schneider wrote:
> A recent commit made cpumask_next*() trigger a warning when passed
> n = nr_cpu_ids - 1. This means extra care must be taken when feeding CPU
> numbers back into cpumask_next*().
> 
> The warning occurs nearly every boot on QEMU:

[...]
 
> Fixes: 78e5a3399421 ("cpumask: fix checking valid cpu range")

No! It fixes blk-mq bug, which has been revealed after 78e5a3399421.

> Suggested-by: Yury Norov <yury.norov@xxxxxxxxx>

OK, maybe I suggested something like this. But after looking into the code
of blk_mq_hctx_next_cpu() code for more, I have a feeling that this should
be overridden deeper. 

Can you check - did this warning raise because hctx->next_cpu, or
because cpumask_next_and() was called twice after jumping on
select_cpu label?

> Signed-off-by: Valentin Schneider <vschneid@xxxxxxxxxx>
> ---
>  block/blk-mq.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index c96c8c4f751b..30ae51eda95e 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2046,8 +2046,13 @@ static int blk_mq_hctx_next_cpu(struct blk_mq_hw_ctx *hctx)
>  
>  	if (--hctx->next_cpu_batch <= 0) {
>  select_cpu:

Because we have backward looking goto, I have a strong feeling that the
code should be reorganized.

> -		next_cpu = cpumask_next_and(next_cpu, hctx->cpumask,
> -				cpu_online_mask);
> +		if (next_cpu == nr_cpu_ids - 1)
> +			next_cpu = nr_cpu_ids;
> +		else
> +			next_cpu = cpumask_next_and(next_cpu,
> +						    hctx->cpumask,
> +						    cpu_online_mask);
> +
>  		if (next_cpu >= nr_cpu_ids)
>  			next_cpu = blk_mq_first_mapped_cpu(hctx);

This simply means 'let's start from the beginning', and should be
replaced with cpumask_next_and_wrap().

>  		hctx->next_cpu_batch = BLK_MQ_CPU_WORK_BATCH;


Maybe something like this would work?

        if (--hctx->next_cpu_batch > 0 && cpu_online(next_cpu)) {
                hctx->next_cpu = next_cpu;
                return next_cpu;
        }

        next_cpu = cpumask_next_and_wrap(next_cpu, hctx->cpumask, cpu_online_mask)
        if (next_cpu < nr_cpu_ids) {
                hctx->next_cpu_batch = BLK_MQ_CPU_WORK_BATCH;
                hctx->next_cpu = next_cpu;
                return next_cpu;
        }

        /*
         * Make sure to re-select CPU next time once after CPUs
         * in hctx->cpumask become online again.
         */
        hctx->next_cpu = next_cpu;
        hctx->next_cpu_batch = 1;
        return WORK_CPU_UNBOUND;

I didn't test it and likely screwed some corner case. I'm just
trying to say that picking next cpu should be an easier thing.

Thanks,
Yury