linux-kernel - Re: [RFC PATCH 0/2] workqueue: Introduce PF_WQ_RESCUE

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Date:   Mon, 11 Dec 2023 15:51:57 +0100
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Aaron Tomlin <atomlin@...mlin.com>
Cc:     linux-kernel@...r.kernel.org, tj@...nel.org,
        jiangshanlai@...il.com, peterz@...radead.org
Subject: Re: [RFC PATCH 0/2] workqueue: Introduce PF_WQ_RESCUE_WORKER

Hi,

Just stumbled upon this series while looking into rescuers myself. :)

On 29/07/23 14:53, 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.

Guess this is a requirement because, if workqueue processing is stuck
for some reason, getting rescuers to run on the same set of cpus
workqueues have been restricted to already doesn't really have good
chances of making any progress?

Wonder if we still might need some sort of fail hard/warn mode in case
strict isolation is in place? Or maybe we have that already?

Thanks!
Juri