Messages in this thread | | | Date | Mon, 3 Oct 2022 10:54:23 -0700 | From | Yury Norov <> | Subject | Re: [PATCH bitmap-for-next 1/5] blk_mq: Fix cpumask_check() warning in blk_mq_hctx_next_cpu() |
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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@gmail.com>
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@redhat.com> > --- > 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
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