linux-kernel - Re: [PATCH v2] sched/core: Don't use dying mm as active

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Date:   Mon, 29 Jul 2019 18:26:53 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Rik van Riel <riel@...riel.com>
Cc:     Waiman Long <longman@...hat.com>, Ingo Molnar <mingo@...hat.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Phil Auld <pauld@...hat.com>, Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH v2] sched/core: Don't use dying mm as active_mm of
 kthreads

On Mon, Jul 29, 2019 at 12:10:12PM -0400, Rik van Riel wrote:
> On Mon, 2019-07-29 at 17:44 +0200, Peter Zijlstra wrote:
> > On Mon, Jul 29, 2019 at 11:28:04AM -0400, Rik van Riel wrote:
> > > On Mon, 2019-07-29 at 17:03 +0200, Peter Zijlstra wrote:
> > > 
> > > > The 'sad' part is that x86 already switches to init_mm on idle
> > > > and we
> > > > only keep the active_mm around for 'stupid'.
> > > 
> > > Wait, where do we do that?
> > 
> > drivers/idle/intel_idle.c:              leave_mm(cpu);
> > drivers/acpi/processor_idle.c:  acpi_unlazy_tlb(smp_processor_id());
> 
> This is only done for deeper c-states, isn't it?

Not C1 but I forever forget where it starts doing that. IIRC it isn't
too hard to hit it often, and I'm fairly sure we always do it when we
hit NOHZ.

> > > > Rik and Andy were working on getting that 'fixed' a while ago,
> > > > not
> > > > sure
> > > > where that went.
> > > 
> > > My lazy TLB stuff got merged last year. 
> > 
> > Yes, but we never got around to getting rid of active_mm for x86,
> > right?
> 
> True, we still use active_mm. Getting rid of the
> active_mm refcounting alltogether did not look
> entirely worthwhile the hassle.

OK, clearly I forgot some of the details ;-)