linux-kernel - Re: [PATCH rfc] workqueue: honour cond

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Date:   Fri, 20 Nov 2020 10:23:44 +1100
From:   NeilBrown <neilb@...e.de>
To:     "tj@...nel.org" <tj@...nel.org>,
        Trond Myklebust <trondmy@...merspace.com>
Cc:     "jiangshanlai@...il.com" <jiangshanlai@...il.com>,
        "mhocko@...e.com" <mhocko@...e.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "juri.lelli@...hat.com" <juri.lelli@...hat.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH rfc] workqueue: honour cond_resched() more effectively.

On Mon, Nov 09 2020, tj@...nel.org wrote:

>                                                    Given that nothing on
> these types of workqueues can be latency sensitive

This caught my eye and it seems worth drilling in to.  There is no
mention of "latency" in workqueue.rst or workqueue.h.  But you seem to
be saying there is an undocumented assumption that latency-sensitive
work items much not be scheduled on CM-workqueues.
Is that correct?

NFS writes are latency sensitive to a degree as increased latency per
request will hurt overall throughput.  Does this mean that handling
write-completion in a CM-wq is a poor choice?
Would it be better to us WQ_HIGHPRI??  Is there any rule-of-thumb that
can be used to determine when WQ_HIGHPRI is appropriate?

Thanks,
NeilBrown

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