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

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Date:   Thu, 4 Feb 2021 14:42:00 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexey Klimov <aklimov@...hat.com>
Cc:     linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        yury.norov@...il.com, Daniel Jordan <daniel.m.jordan@...cle.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@....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 Thu, Feb 04, 2021 at 12:50:34PM +0000, Alexey Klimov wrote:
> On Thu, Feb 4, 2021 at 9:46 AM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > On Thu, Feb 04, 2021 at 01:01:57AM +0000, Alexey Klimov wrote:
> > > @@ -1281,6 +1282,11 @@ static int cpu_up(unsigned int cpu, enum cpuhp_state target)
> > >       err = _cpu_up(cpu, 0, target);
> > >  out:
> > >       cpu_maps_update_done();
> > > +
> > > +     /* To avoid out of line uevent */
> > > +     if (!err)
> > > +             cpuset_wait_for_hotplug();
> > > +
> > >       return err;
> > >  }
> > >
> >
> > > @@ -2071,14 +2075,18 @@ static void cpuhp_online_cpu_device(unsigned int cpu)
> > >       struct device *dev = get_cpu_device(cpu);
> > >
> > >       dev->offline = false;
> > > -     /* Tell user space about the state change */
> > > -     kobject_uevent(&dev->kobj, KOBJ_ONLINE);
> > >  }
> > >
> >
> > 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().

Also, looking again, you don't seem to be reinstating the OFFLINE event
you took out.