linux-kernel - Re: [RFC PATCH] workqueue: Unbind workers before sending them to exit()

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Date:   Tue, 26 Jul 2022 21:36:06 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Lai Jiangshan <jiangshanlai@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Phil Auld <pauld@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [RFC PATCH] workqueue: Unbind workers before sending them to
 exit()

On 26/07/22 07:30, Tejun Heo wrote:
> Hello,
>
> On Mon, Jul 25, 2022 at 11:21:37AM +0100, Valentin Schneider wrote:
>> Hm so my choice of words in the changelog wasn't great - "initial setup"
>> can be kernel init, but *also* setup of whatever workload is being deployed
>> onto the system.
>>
>> So you can be having "normal" background activity (I've seen some IRQs end
>> up with schedule_work() on isolated CPUs, they're not moved away at boot
>> time but rather shortly before launching the latency-sensitive app), some
>> preliminary stats collection / setup to make sure the CPU will be quiet
>> (e.g. refresh_vm_stats()), and *then* the application starts with
>> fresh-but-no-longer-required extra pcpu kworkers assigned to its CPU.
>
> Ah, I see. I guess we'll need to figure out how to unbind the workers then.
>

I've been playing with different ways to unbind & wake the workers in a
sleepable context, but so far I haven't been happy with any of my
experiments.

What hasn't changed much between my attempts is transferring to-be-destroyed
kworkers from their pool->idle_list to a reaper_list which is walked by
*something* that does unbind+wakeup. AFAIA as long as the kworker is off
the pool->idle_list we can play with it (i.e. unbind+wake) off the
pool->lock.

It's the *something* that's annoying to get right, I don't want it to be
overly complicated given most users are probably not impacted by what I'm
trying to fix, but I'm getting the feeling it should still be a per-pool
kthread. I toyed with a single reaper kthread but a central synchronization
for all the pools feels like a stupid overhead.

If any of that sounds ludicrous please shout, otherwise I'm going to keep
tinkering :)

> Thanks.
>
> --
> tejun