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From:   Alexey Klimov <aklimov@redhat.com>
Date:   Thu, 11 Feb 2021 13:38:51 +0000
Message-ID: <CAFBcO+81bFBUuR=MrLttBEs8gKh0hx+EcJA2MCbqMzu=CUoybg@mail.gmail.com>
Subject: Re: [PATCH] cpu/hotplug: wait for cpuset_hotplug_work to finish on
 cpu onlining
To:     Qais Yousef <qais.yousef@arm.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
        yury.norov@gmail.com, Daniel Jordan <daniel.m.jordan@oracle.com>,
        tglx@linutronix.de, Joshua Baker <jobaker@redhat.com>,
        audralmitchel@gmail.com, arnd@arndb.de, gregkh@linuxfoundation.org,
        rafael@kernel.org, tj@kernel.org, lizefan@huawei.com,
        hannes@cmpxchg.org, Alexey Klimov <klimov.linux@gmail.com>
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On Fri, Feb 5, 2021 at 11:22 AM Qais Yousef <qais.yousef@arm.com> wrote:
>
> On 02/04/21 10:46, Peter Zijlstra 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.
>
> And the resume latency will incur 5-30ms * nr_cpu_ids.
>
> Since you just care about device_online(), isn't cpu_device_up() a better place
> for the wait? This function is special helper for device_online(), leaving
> suspend/resume and kexec paths free from having to do this unnecessary wait.

Yup, the same idea here once Peter mentioned bringup_nonboot_cpus()
and bringup_hibernate_cpu().

Best regards,
Alexey