On Tue, Oct 24, 2023 at 09:53:49AM -0700, Bart Van Assche wrote:
>
> On 10/13/23 05:47, Ming Lei wrote:
> > @@ -3926,6 +3941,15 @@ static void blk_mq_map_swqueue(struct request_queue *q)
> > */
> > sbitmap_resize(&hctx->ctx_map, hctx->nr_ctx);
> > + /*
> > + * rule out isolated CPUs from hctx->cpumask for avoiding to
> > + * run wq worker on isolated CPU
> > + */
> > + for_each_cpu(cpu, hctx->cpumask) {
> > + if (cpu_is_isolated(cpu))
> > + cpumask_clear_cpu(cpu, hctx->cpumask);
> > + }
>
> What will happen if this code makes hctx->cpumask empty? Code like
> blk_mq_first_mapped_cpu() and blk_mq_hctx_next_cpu() assumes that
> hctx->cpumask is not empty. There may be other code that assumes that
> hctx->cpumask is not empty.
hctx->cpumask is only used for selecting next cpu to schedule run
queue now, so it is fine for hctx->cpumask to be empty.
But the patch has one hole, following the delta fix:
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 0917f8eabab9..1d9a7ded27af 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2159,7 +2159,7 @@ static int blk_mq_hctx_next_cpu(struct blk_mq_hw_ctx *hctx)
bool tried = false;
int next_cpu = hctx->next_cpu;
- if (hctx->queue->nr_hw_queues == 1)
+ if (hctx->queue->nr_hw_queues == 1 || next_cpu >= nr_cpu_ids)
return WORK_CPU_UNBOUND;
if (--hctx->next_cpu_batch <= 0) {
Thanks,
Ming
