linux-kernel - Re: [PATCH 5/7] workqueue: Automatically mark CPU-hogging work items CPU

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Message-ID: <ZF1wg7-Lczi0APsP@slm.duckdns.org>
Date:   Thu, 11 May 2023 12:47:31 -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 5/7] workqueue: Automatically mark CPU-hogging work items
 CPU_INTENSIVE

Hello,

On Thu, May 11, 2023 at 11:23:58PM +0200, Peter Zijlstra wrote:
> > * Even when detection is working, the 10ms detection delays can add up if
> >   many CPU-hogging work items are queued at the same time.
> 
> HZ=100 assumption there :-) My HZs are bigger 'n yours etc.

Oh, I was referring to the default threshold, but yeah, lower HZ can
definitely be a factor.

> > However, in vast majority of cases, this should be able to detect violations
> > reliably and provide reasonable protection with a small increase in code
> > complexity.
> > 
> > If some work items trigger this condition repeatedly, the bigger problem
> > likely is the CPU being saturated with such per-cpu work items and the
> > solution would be making them UNBOUND. The next patch will add a debug
> > mechanism to help spot such cases.
> > 
> > v3: Switch to use wq_worker_tick() instead of hooking into preemptions as
> >     suggested by Peter.
> > 
> > v2: Lai pointed out that wq_worker_stopping() also needs to be called from
> >     preemption and rtlock paths and an earlier patch was updated
> >     accordingly. This patch adds a comment describing the risk of infinte
> >     recursions and how they're avoided.
> 
> I tend to prefer these changelog-changelogs to go below the --- so that
> they go away on applying, they're not really relevant when reading the
> patch in a year's time when trying to figure out wtf this patch did.

I tried that and promptly lost the version logs while iterating / posting
cuz that involved going through git commits. But yeah, the above version
logs aren't useful. I'll drop them when applying.

> Anyway, this seems entirely reasonable.
> 
> Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>

Thanks.

-- 
tejun