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

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Date:   Fri, 5 Feb 2021 11:22:19 +0000
From:   Qais Yousef <qais.yousef@....com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Alexey Klimov <aklimov@...hat.com>
Cc:     linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        yury.norov@...il.com, daniel.m.jordan@...cle.com,
        tglx@...utronix.de, jobaker@...hat.com, audralmitchel@...il.com,
        arnd@...db.de, gregkh@...uxfoundation.org, rafael@...nel.org,
        tj@...nel.org, lizefan@...wei.com, hannes@...xchg.org,
        klimov.linux@...il.com
Subject: Re: [PATCH] cpu/hotplug: wait for cpuset_hotplug_work to finish on
 cpu onlining

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.

Thanks

--
Qais Yousef