linux-kernel - Re: [PATCH] cpu/hotplug: wait for cpuset_hotplug

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Date:   Thu, 11 Feb 2021 14:09:39 +0000
From:   Alexey Klimov <aklimov@...hat.com>
To:     Daniel Jordan <daniel.m.jordan@...cle.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        yury.norov@...il.com, tglx@...utronix.de,
        Joshua Baker <jobaker@...hat.com>, audralmitchel@...il.com,
        arnd@...db.de, gregkh@...uxfoundation.org, rafael@...nel.org,
        tj@...nel.org, lizefan@...wei.com,
        Qais Yousef <qais.yousef@....com>, hannes@...xchg.org,
        Alexey Klimov <klimov.linux@...il.com>
Subject: Re: [PATCH] cpu/hotplug: wait for cpuset_hotplug_work to finish on
 cpu onlining

On Fri, Feb 5, 2021 at 12:41 AM Daniel Jordan
<daniel.m.jordan@...cle.com> wrote:
>
> Peter Zijlstra <peterz@...radead.org> writes:

[...]

> >> > One concequence of this is that you'll now get a bunch of notifications
> >> > across things like suspend/hybernate.
> >>
> >> The patch doesn't change the number of kobject_uevent()s. The
> >> userspace will get the same number of uevents as before the patch (at
> >> least if I can rely on my eyes).
> >
> > bringup_hibernate_cpu() didn't used to generate an event, it does now.
> > Same for bringup_nonboot_cpus().
>
> Both of those call cpu_up(), which only gets a cpuset_wait_for_hotplug()
> in this patch.  No new events generated from that, right, it's just a
> wrapper for a flush_work()?
>
> > Also, looking again, you don't seem to be reinstating the OFFLINE event
> > you took out.
>
> It seems to be reinstated in cpuhp_smt_disable()?

Peter, what Daniel said.
cpuset_wait_for_hotplug() doesn't generate an event.

The offline event was moved below in the same function:

+
+ /* Tell user space about the state changes */
+ for_each_cpu(cpu, mask) {
+ dev = get_cpu_device(cpu);
+ kobject_uevent(&dev->kobj, KOBJ_OFFLINE);
+ }
+
+ free_cpumask_var(mask);

Daniel,
thanks for your comments. I'll update the patch and resend.

Best regards,
Alexey