linux-kernel - Re: [RFC PATCH] blk-mq: avoid housekeeping CPUs scheduling a worker on a non-housekeeping CPU

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Date:   Thu, 23 Feb 2023 16:06:24 +0100
From:   Christoph Hellwig <hch@....de>
To:     Xiongfeng Wang <wangxiongfeng@...weicloud.com>
Cc:     axboe@...nel.dk, hch@....de, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, wangxiongfeng2@...wei.com,
        liwei391@...wei.com, wangkefeng.wang@...wei.com
Subject: Re: [RFC PATCH] blk-mq: avoid housekeeping CPUs scheduling a
 worker on a non-housekeeping CPU

On Thu, Feb 23, 2023 at 03:48:26PM +0800, Xiongfeng Wang wrote:
> From: Xiongfeng Wang <wangxiongfeng2@...wei.com>
> 
> 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.

This looks like an odd non-systemic bandaid.  Shouldn't we have a more
generic way nothing ever gets onto these non-housekeeping CPUs by making
sure they never show up in the cpumask, and never get completion IPIs?