Re: [PATCH V4] blk-mq: don't schedule block kworker on isolated CPUs

[Ming Lei]

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