Re: [RFC PATCH] tick/sched: Ensure quiet_vmstat() is called when the idle tick was stopped too

From: Marcelo Tosatti <mtosatti@redhat.com>
To: Aaron Tomlin <atomlin@atomlin.com>
Cc: Frederic Weisbecker <frederic@kernel.org>,
	Christoph Lameter <cl@linux.com>,
	tglx@linutronix.de, mingo@kernel.org,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	Phil Auld <pauld@redhat.com>
Subject: Re: [RFC PATCH] tick/sched: Ensure quiet_vmstat() is called when the idle tick was stopped too
Date: Thu, 24 Feb 2022 09:30:11 -0300	[thread overview]
Message-ID: <Yhd6UxWH3FQHg+eF@fuller.cnet> (raw)
In-Reply-To: <Yhd5olg9CjXSAf2s@fuller.cnet>

On Thu, Feb 24, 2022 at 09:27:14AM -0300, Marcelo Tosatti wrote:
> On Sat, Feb 19, 2022 at 03:46:16PM +0000, Aaron Tomlin wrote:
> > On Fri 2022-02-18 12:54 +0000, Aaron Tomlin wrote:
> > > On Thu 2022-02-17 17:32 +0100, Frederic Weisbecker wrote:
> > > > > If I understand correctly, in the context of nohz_full, since such work is
> > > > > deferred, it will only be handled in a scenario when the periodic/or
> > > > > scheduling-clock tick is enabled i.e. the timer was reprogrammed on exit
> > > > > from idle.
> > > > 
> > > > Oh I see, it's a deferrable delayed work...
> > > > Then I can see two other issues:
> > > > 
> > > > 1) Can an interrupt in idle modify the vmstat and thus trigger the need to
> > > >    flush it? 
> 
> Yes. Page allocation and page freeing for example.
> 
>    6   3730  ../mm/page_alloc.c <<rmqueue>>
>              __mod_zone_freepage_state(zone, -(1 << order),
>    4   1096  ../mm/page_alloc.c <<__free_one_page>>
>              __mod_zone_freepage_state(zone, -(1 << order),
> 
> > > >    I believe it's the case and then the problem goes beyond nohz_full
> > > >    because if the idle interrupt fired while the tick is stopped and didn't set
> > > >    TIF_RESCHED, we go back to sleep without calling quiet_vmstat().
> > > 
> > > Yes: e.g. a nohz_full CPU, in idle code, could indeed receive a reschedule
> > > IPI; re-enable local IRQs and generic idle code sees the TIF_NEED_RESCHED
> > > flag against the idle task. Additionally, the selected task could
> > > indirectly released a few pages [to satisfy a low-memory condition] and
> > > modify CPU-specific vmstat data i.e. vm_stat_diff[NR_FREE_PAGES].
> > > 
> > > 
> > > > 2) What if we are running task A in kernel mode while the tick is stopped
> > > >    (nohz_full). Task A modifies the vmstat and goes to userspace for a long
> > > >    while.
> > > > Your patch fixes case 1) but not case 2). The problem is that TIMER_DEFERRABLE
> > > > should really be about dynticks-idle only and not dynticks-full. I've always
> > > > been afraid about enforcing that rule though because that would break old
> > > > noise-free setups. But perhaps I should...

Can't grasp the sentence above "The problem is that ...".
What rule?