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

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Message-ID: <20230425131254.GF1335080@hirez.programming.kicks-ass.net>
Date:   Tue, 25 Apr 2023 15:12:54 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Tejun Heo <tj@...nel.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

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.

> This patch adds an automatic CPU usage based detection. If a
> concurrency-managed work item consumes more CPU time than the threshold (5ms
> by default), it's marked CPU_INTENSIVE automatically on schedule-out.
> 
> The mechanism isn't foolproof in that the 5ms detection delays can add up if
> many CPU-hogging work items are queued at the same time. However, in such
> situations, the bigger problem likely is the CPU being saturated with
> per-cpu work items and the solution would be making them UNBOUND.
> 
> For occasional CPU hogging, the new automatic mechanism should provide
> reasonable protection with minimal increase in code complexity.

But why not keep it a debug mechanism? Now you're got a heuristic with
all the down-sides that they bring.