Re: [PATCH v8 5/5] tick/sched: Ensure quiet_vmstat() is called when the idle tick was stopped too

From: Aaron Tomlin <atomlin@redhat.com>
To: Hillf Danton <hdanton@sina.com>
Cc: frederic@kernel.org, mtosatti@redhat.com,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH v8 5/5] tick/sched: Ensure quiet_vmstat() is called when the idle tick was stopped too
Date: Mon, 26 Sep 2022 10:20:04 +0100	[thread overview]
Message-ID: <20220926092004.to2zrcxjz5oj3lid@ava.usersys.com> (raw)
In-Reply-To: <20220925010511.1482-1-hdanton@sina.com>

On Sun 2022-09-25 09:05 +0800, Hillf Danton wrote:
> On 24 Sep 2022 16:24:41 +0100 Aaron Tomlin <atomlin@redhat.com> wrote:
> > 
> > In the context of the idle task and an adaptive-tick mode/or a nohz_full
> > CPU, quiet_vmstat() can be called: before stopping the idle tick,
> > entering an idle state and on exit. In particular, for the latter case,
> > when the idle task is required to reschedule, the idle tick can remain
> > stopped and the timer expiration time endless i.e., KTIME_MAX. Now,
> > indeed before a nohz_full CPU enters an idle state, CPU-specific vmstat
> > counters should be processed to ensure the respective values have been
> > reset and folded into the zone specific 'vm_stat[]'. That being said, it
> > can only occur when: the idle tick was previously stopped, and
> > reprogramming of the timer is not required.
> > 
> > A customer provided some evidence which indicates that the idle tick was
> > stopped; albeit, CPU-specific vmstat counters still remained populated.
> > Thus one can only assume quiet_vmstat() was not invoked on return to the
> > idle loop.
> 
> Why did housekeeping CPUs fail to do their works, with this assumption
> put aside?

Hi Hillf,

I'm not sure I understand your question.

In this context, when tick processing is stopped, delayed work is not going
to be handled until the CPU exits idle.

Kind regards,

-- 
Aaron Tomlin