linux-kernel - Re: [PATCH 4/5] workqueue: Automatically mark CPU-hogging work items CPU

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Message-ID: <ZEvj-eet6anRUR6o@slm.duckdns.org>
Date:   Fri, 28 Apr 2023 05:19:21 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     jiangshanlai@...il.com, torvalds@...ux-foundation.org,
        linux-kernel@...r.kernel.org, kernel-team@...a.com
Subject: Re: [PATCH 4/5] workqueue: Automatically mark CPU-hogging work items
 CPU_INTENSIVE

Hello, Peter.

On Tue, Apr 25, 2023 at 03:12:54PM +0200, Peter Zijlstra wrote:
> On Tue, Apr 18, 2023 at 10:51:58AM -1000, Tejun Heo wrote:
> > If a per-cpu work item hogs the CPU, it can prevent other work items from
> > starting through concurrency management. A per-cpu workqueue which intends
> > to host such CPU-hogging work items can choose to not participate in
> > concurrency management by setting %WQ_CPU_INTENSIVE; however, this can be
> > error-prone and difficult to debug when missed.
> 
> Well; you could use this very heuristic, and instead of silently trying
> to fix up, complain about the missing CPU_INTENSIVE thing.
...
> But why not keep it a debug mechanism? Now you're got a heuristic with
> all the down-sides that they bring.

I'm working on improving the locality of unbound workqueues and it isn't
clear whether there's enough space for per-cpu CPU_INTENSIVE work items -
ie. if it's gonna saturate the CPU for extended periods of time to the point
of requiring CPU_INTENSIVE, it might as well be an unbound work item if the
baseline locality can be good enough. There aren't that many users of
CPU_INTENSIVE in tree and we can decide based on how unbound workqueues
actually work out for them.

As for warnings, yeah, that'd be great to have regardless of how
CPU_INTENSIVE turns out. Just gotta make sure it doesn't fire spuriously and
become a nuisance.

Thanks.

-- 
tejun