On Thu, Mar 21, 2024 at 11:07:52AM -0600, Jens Axboe wrote:
> On 3/19/24 8:34 PM, Ming Lei wrote:
> > Kernel parameter of `isolcpus=` or 'nohz_full=' are used to isolate CPUs
> > for specific task, and it isn't expected to let block IO disturb these CPUs.
> > blk-mq kworker shouldn't be scheduled on isolated CPUs. Also if isolated
> > CPUs is run for blk-mq kworker, long block IO latency can be caused.
> >
> > Kernel workqueue only respects CPU isolation for WQ_UNBOUND, for bound
> > WQ, the responsibility is on user because CPU is specified as WQ API
> > parameter, such as mod_delayed_work_on(cpu), queue_delayed_work_on(cpu)
> > and queue_work_on(cpu).
> >
> > So not run blk-mq kworker on isolated CPUs by removing isolated CPUs
> > from hctx->cpumask. Meantime use queue map to check if all CPUs in this
> > hw queue are offline instead of hctx->cpumask, this way can avoid any
> > cost in fast IO code path, and is safe since hctx->cpumask are only
> > used in the two cases.
>
> In general, I think the fix is fine. Only thing that's a bit odd is:
Thanks for the review!
>
> > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > index 555ada922cf0..187fbfacb397 100644
> > --- a/block/blk-mq.c
> > +++ b/block/blk-mq.c
> > @@ -28,6 +28,7 @@
> > #include <linux/prefetch.h>
> > #include <linux/blk-crypto.h>
> > #include <linux/part_stat.h>
> > +#include <linux/sched/isolation.h>
> >
> > #include <trace/events/block.h>
> >
> > @@ -2179,7 +2180,11 @@ 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)
> > + /*
> > + * Switch to unbound work if all CPUs in this hw queue fall
> > + * into isolated CPUs
> > + */
> > + if (hctx->queue->nr_hw_queues == 1 || next_cpu >= nr_cpu_ids)
> > return WORK_CPU_UNBOUND;
>
> This relies on find_next_foo() returning >= nr_cpu_ids if the set is
> empty, which is a lower level implementation detail that someone reading
> this code may not know.
Indeed, looks it is more readable to add one helper:
static bool blk_mq_hctx_empty_cpumask(struct blk_mq_hw_ctx *hctx)
{
return hctx->next_cpu >= nr_cpu_ids;
}
>
> > if (--hctx->next_cpu_batch <= 0) {
> > @@ -3488,14 +3493,30 @@ static bool blk_mq_hctx_has_requests(struct blk_mq_hw_ctx *hctx)
> > return data.has_rq;
> > }
> >
> > -static inline bool blk_mq_last_cpu_in_hctx(unsigned int cpu,
> > - struct blk_mq_hw_ctx *hctx)
> > +static bool blk_mq_hctx_has_online_cpu(struct blk_mq_hw_ctx *hctx,
> > + unsigned int this_cpu)
> > {
> > - if (cpumask_first_and(hctx->cpumask, cpu_online_mask) != cpu)
> > - return false;
> > - if (cpumask_next_and(cpu, hctx->cpumask, cpu_online_mask) < nr_cpu_ids)
> > - return false;
> > - return true;
> > + enum hctx_type type = hctx->type;
> > + int cpu;
> > +
> > + /*
> > + * hctx->cpumask has rule out isolated CPUs, but userspace still
> ^^
>
> has to
>
> > + * might submit IOs on these isolated CPUs, so use queue map to
> ^^
>
> use the queue map
OK, will fix them in V5.
thanks,
Ming
