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Date: Thu, 21 Mar 2024 11:07:52 -0600
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Subject: Re: [PATCH V4] blk-mq: don't schedule block kworker on isolated CPUs
Content-Language: en-US
To: Ming Lei <ming.lei@redhat.com>, linux-block@vger.kernel.org
Cc: linux-kernel@vger.kernel.org, Tim Chen <tim.c.chen@linux.intel.com>,
 Juri Lelli <juri.lelli@redhat.com>, Andrew Theurer <atheurer@redhat.com>,
 Joe Mario <jmario@redhat.com>, Sebastian Jug <sejug@redhat.com>,
 Frederic Weisbecker <frederic@kernel.org>,
 Bart Van Assche <bvanassche@acm.org>, Tejun Heo <tj@kernel.org>
References: <20240320023446.882006-1-ming.lei@redhat.com>
From: Jens Axboe <axboe@kernel.dk>
In-Reply-To: <20240320023446.882006-1-ming.lei@redhat.com>
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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:

> 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.

>  	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

> +	 * check if all CPUs mapped to this hctx are offline
> +	 */

-- 
Jens Axboe