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

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Date:   Mon, 25 Jul 2022 11:21:37 +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 22/07/22 19:16, Tejun Heo wrote:
> On Thu, Jul 21, 2022 at 02:53:43PM +0100, Valentin Schneider wrote:
>> > I think it needs something like task_set_cpumask_possible() which is
>> > documented as being usable in (raw) spinlocks and set the task's cpumask
>> > to cpu_possible_mask and let the later ttwu help migrate it to a
>> > proper non-isolated CPU or let it keep running.
>> 
>> I'll see what I can come up with, thanks for the suggestion.
>
> Alternatively, we can just kill all the idle kworkers on isolated cpus at
> the end of the booting process.
>

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.