linux-kernel - Re: [PATCH RESEND] sched/nohz: Affine unpinned timers to housekeepers

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Message-ID: <20150828083259.GA19320@gmail.com>
Date:	Fri, 28 Aug 2015 10:32:59 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Vatika Harlalka <vatikaharlalka@...il.com>,
	Chris Metcalf <cmetcalf@...hip.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Christoph Lameter <cl@...ux.com>
Subject: Re: [PATCH RESEND] sched/nohz: Affine unpinned timers to housekeepers


* Frederic Weisbecker <fweisbec@...il.com> wrote:

> On Tue, Aug 25, 2015 at 10:29:04AM +0200, Ingo Molnar wrote:
> > 
> > * Frederic Weisbecker <fweisbec@...il.com> wrote:
> > 
> > > On Mon, Aug 24, 2015 at 08:44:12AM +0200, Ingo Molnar wrote:
> > > > 2)
> > > > 
> > > > What happens if the boot CPU is offlined? (under CONFIG_BOOTPARAM_HOTPLUG_CPU0=y)
> > > > 
> > > > I don't see CPU hotplug callbacks fixing up the housekeeping_mask if the boot CPU 
> > > > is offlined.
> > > 
> > > We have tick_nohz_cpu_down_callback() which makes sure that the timekeeper, which
> > > is the boot CPU in nohz full, never gets offlined.
> > 
> > That solution really sucks - it essentially regresses a feature the user 
> > explicitly asked for! I also see no way for the user to migrate the timekeeping 
> > functionality over to another CPU without rebooting.
> > 
> > If this is the last timekeeping CPU then it should migrate the timekeeping 
> > functionality to another CPU, and perhaps printk a warning if all other CPUs are 
> > nohz-full and we have to mark one of them as the timekeeper.
> > 
> > Also, the nohz-full and timekeeper functionality should not be a boot parameter 
> > only thing, but should be runtime configurable.
> 
> When I tried to allow moving the timekeeping duty over all housekeeping CPUs, 
> Thomas got angry because it broke the KISS current nohz full code. Indeed, there 
> must be at least one running all the time on behalf of nohz full CPUs that can 
> run anytime. Thus balancing the timekeeping duty over housekeepers is a bit more 
> complicated than in normal configurations.
> 
> Now surely we can do that using an IPI from CPU_DOWN_PREPARE to a housekeeper if 
> any remains or to a nohz full one. Then we must make sure the new timekeeper 
> never goes to idle.
> 
> But nohz_full is a corner usecase and I'm not sure it's worth the complexity. If 
> a nohz full user came and complained about CPU0 hotplog not working, I would 
> definetly retry it but I haven't heard about that yet. Besides, hotplug is very 
> isolation-unfriendly in general due to stop machine.

Ok, I guess we can live with this.

Mind sending an updated series with all patches?

Thanks,

	Ingo
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