linux-kernel - Re: [RFC PATCH 1/2] workqueue: Introduce PF_WQ_RESCUE

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Message-ID: <20230729160712.GA5697@hirez.programming.kicks-ass.net>
Date:   Sat, 29 Jul 2023 18:07:12 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Aaron Tomlin <atomlin@...mlin.com>
Cc:     linux-kernel@...r.kernel.org, tj@...nel.org, jiangshanlai@...il.com
Subject: Re: [RFC PATCH 1/2] workqueue: Introduce PF_WQ_RESCUE_WORKER

On Sat, Jul 29, 2023 at 02:53:33PM +0100, Aaron Tomlin wrote:
> The Linux kernel does not provide a way to differentiate between a
> kworker and a rescue kworker for user-mode.
> From user-mode, one can establish if a task is a kworker by testing for
> PF_WQ_WORKER in a specified task's flags bit mask (or bitmap) via
> /proc/[PID]/stat. Indeed, one can examine /proc/[PID]/stack and search
> for the function namely "rescuer_thread". This is only available to the
> root user.
> 
> It can be useful to identify a rescue kworker since their CPU affinity
> cannot be modified and their initial CPU assignment can be safely ignored.
> Furthermore, a workqueue that was created with WQ_MEM_RECLAIM and
> WQ_SYSFS the cpumask file is not applicable to the rescue kworker.
> By design a rescue kworker should run anywhere.
> 
> This patch introduces PF_WQ_RESCUE_WORKER and ensures it is set and
> cleared appropriately.

Is the implication that PF_flags are considered ABI? We've been changing
them quite a bit over the years.

Also, while we have a few spare bits atm, we used to be nearly out for a
while, and I just don't think this is sane usage of them. We don't use
PF flags just for userspace.