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Subject: Re: [RFC PATCH] blk-mq: avoid housekeeping CPUs scheduling a worker
 on a non-housekeeping CPU
To:     <axboe@kernel.dk>, <hch@lst.de>,
        Frederic Weisbecker <frederic@kernel.org>
CC:     <linux-block@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        <yuyufen@huawei.com>, <guohanjun@huawei.com>
References: <20220210093532.182818-1-wangxiongfeng2@huawei.com>
From:   Xiongfeng Wang <wangxiongfeng2@huawei.com>
Message-ID: <68800eb8-af54-a836-eb1d-eecde951ede4@huawei.com>
Date:   Tue, 15 Feb 2022 17:38:00 +0800
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Hi Frederic,

Sorry to disturb you. It's just that I think you may be interested in this
patch. I notice you are reviewing some other CPU isolation patches. Appreciate
it a lot if you can give it a look. Or just ignore it if you are not interested.

Thanks,
Xiongfeng

On 2022/2/10 17:35, Xiongfeng Wang wrote:
> When NOHZ_FULL is enabled, such as in HPC situation, CPUs are divided
> into housekeeping CPUs and non-housekeeping CPUs. Non-housekeeping CPUs
> are NOHZ_FULL CPUs and are often monopolized by the userspace process,
> such HPC application process. Any sort of interruption is not expected.
> 
> blk_mq_hctx_next_cpu() selects each cpu in 'hctx->cpumask' alternately
> to schedule the work thread blk_mq_run_work_fn(). When 'hctx->cpumask'
> contains housekeeping CPU and non-housekeeping CPU at the same time, a
> housekeeping CPU, which want to request a IO, may schedule a worker on a
> non-housekeeping CPU. This may affect the performance of the userspace
> application running on non-housekeeping CPUs.
> 
> So let's just schedule the worker thread on the current CPU when the
> current CPU is housekeeping CPU.
> 
> Signed-off-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
> ---
>  block/blk-mq.c | 15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 1adfe4824ef5..ff9a4bf16858 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -24,6 +24,7 @@
>  #include <linux/sched/sysctl.h>
>  #include <linux/sched/topology.h>
>  #include <linux/sched/signal.h>
> +#include <linux/sched/isolation.h>
>  #include <linux/delay.h>
>  #include <linux/crash_dump.h>
>  #include <linux/prefetch.h>
> @@ -2036,6 +2037,8 @@ static int blk_mq_hctx_next_cpu(struct blk_mq_hw_ctx *hctx)
>  static void __blk_mq_delay_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async,
>  					unsigned long msecs)
>  {
> +	int work_cpu;
> +
>  	if (unlikely(blk_mq_hctx_stopped(hctx)))
>  		return;
>  
> @@ -2050,7 +2053,17 @@ static void __blk_mq_delay_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async,
>  		put_cpu();
>  	}
>  
> -	kblockd_mod_delayed_work_on(blk_mq_hctx_next_cpu(hctx), &hctx->run_work,
> +	/*
> +	 * Avoid housekeeping CPUs scheduling a worker on a non-housekeeping
> +	 * CPU
> +	 */
> +	if (tick_nohz_full_enabled() && housekeeping_cpu(smp_processor_id(),
> +							 HK_FLAG_WQ))
> +		work_cpu = smp_processor_id();
> +	else
> +		work_cpu = blk_mq_hctx_next_cpu(hctx);
> +
> +	kblockd_mod_delayed_work_on(work_cpu, &hctx->run_work,
>  				    msecs_to_jiffies(msecs));
>  }
>  
>