linux-kernel - Re: [PATCH 2/2] mm: drop hotplug lock from lru_add_drain

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Date:   Thu, 2 Nov 2017 14:42:14 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Michal Hocko <mhocko@...nel.org>
cc:     linux-mm@...ck.org, Peter Zijlstra <peterz@...radead.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Mel Gorman <mgorman@...e.de>, Tejun Heo <tj@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] mm: drop hotplug lock from lru_add_drain_all

On Thu, 2 Nov 2017, Michal Hocko wrote:
> On Thu 02-11-17 14:02:53, Thomas Gleixner wrote:
> > On Thu, 2 Nov 2017, Michal Hocko wrote:
> > > On Thu 02-11-17 10:36:13, Michal Hocko wrote:
> > > [...]
> > > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > > > index 67330a438525..8c6e9c6d194c 100644
> > > > --- a/mm/page_alloc.c
> > > > +++ b/mm/page_alloc.c
> > > > @@ -6830,8 +6830,12 @@ void __init free_area_init(unsigned long *zones_size)
> > > >  
> > > >  static int page_alloc_cpu_dead(unsigned int cpu)
> > > >  {
> > > > +	unsigned long flags;
> > > >  
> > > > +	local_irq_save(flags);
> > > >  	lru_add_drain_cpu(cpu);
> > > > +	local_irq_restore(flags);
> > > > +
> > > >  	drain_pages(cpu);
> > >   
> > > I was staring into the hotplug code and tried to understand the context
> > > this callback runs in and AFAIU IRQ disabling is not needed at all
> > > because cpuhp_thread_fun runs with IRQ disabled when offlining an online
> > > cpu. I have a bit hard time to follow the code due to all the
> > > indirection so please correct me if I am wrong.
> > 
> > No. That function does neither run from the cpu hotplug thread of the
> > outgoing CPU nor its called with interrupts disabled.
> > 
> > The callback is in the DEAD section, i.e. its called on the controlling CPU
> > _after_ the hotplugged CPU vanished completely.
> 
> OK, so IIUC there is no race possible because kworkders simply do not
> run on that cpu anymore.

The CPU is gone and the kworker should have been drained and
stopped. Tejun?

Thanks,

	tglx