linux-kernel - Re: [PATCH 6/7] workqueue: Report work funcs that trigger automatic CPU

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Message-ID: <ZF1xwy-gtR6F5jUC@slm.duckdns.org>
Date:   Thu, 11 May 2023 12:52:51 -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 6/7] workqueue: Report work funcs that trigger automatic
 CPU_INTENSIVE mechanism

Hello,

On Thu, May 11, 2023 at 11:26:06PM +0200, Peter Zijlstra wrote:
> On Thu, May 11, 2023 at 08:19:30AM -1000, Tejun Heo wrote:
> > Workqueue now automatically marks per-cpu work items that hog CPU for too
> > long as CPU_INTENSIVE, which excludes them from concurrency management and
> > prevents stalling other concurrency-managed work items. If a work function
> > keeps running over the thershold, it likely needs to be switched to use an
> > unbound workqueue.
> > 
> > This patch adds a debug mechanism which tracks the work functions which
> > trigger the automatic CPU_INTENSIVE mechanism and report them using
> > pr_warn() with exponential backoff.
> > 
> > Signed-off-by: Tejun Heo <tj@...nel.org>
> > Suggested-by: Peter Zijlstra <peterz@...radead.org>
> 
> I did do wonder why you chose for external storage for this -- I figured
> it was to keep the cost down since it shouldn't really be happening, so
> storage in the normal data structures is a waste etc..?

The only releveant data structures are workqueue and work_struct. The former
is too coarse because a given workqueue can run any number of different work
items (e.g. system_wq). The latter is too transient to record anything on.
In a lot of cases, the only meaningfully identifiable thing is the work
function pointer, which doesn't have any data structure attached by default,
so...

Thanks.

-- 
tejun