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
