linux-kernel - Re: [RFC PATCH v3] tick/sched: Ensure quiet_vmstat() is called when the idle tick was stopped too

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Date:   Wed, 4 May 2022 10:32:30 +0100
From:   Aaron Tomlin <atomlin@...hat.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     frederic@...nel.org, cl@...ux.com, tglx@...utronix.de,
        mingo@...nel.org, peterz@...radead.org, pauld@...hat.com,
        neelx@...hat.com, oleksandr@...alenko.name,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC PATCH v3] tick/sched: Ensure quiet_vmstat() is called when
 the idle tick was stopped too

On Thu 2022-04-28 15:10 -0300, Marcelo Tosatti wrote:
> So you are syncing the vmstats on every system call return:

Hi Marcelo,

Sorry about the delay!

No - indeed, that would be too expensive. If I understand correctly,
Peter Zijlstra's feedback was in response to a previous suggestion made:

	"Could we *always* fold the vmstat counters when entering
	 idle mode? ..."

I think an exception should be made for the adaptive-tick mode/or a
nohz_full CPU case when the scheduling-clock tick is stopped. Also, I feel
correctness is key, as previously indicated since a significant divergence
can impact memory reclaim code.

> Have you measured performance of any system call heavy application
> with this change?

Unfortunately not. That being said, the aforementioned test and work will
only take place under a nohz_full CPU and if the tick is stopped.
So this should be somewhat limited, no?

> Then the comment on why its so slow:
> 
> "This loop is quite heavy. Maybe reducing the data necessary to be read
> to a couple of cachelines would improve it considerably."
> 
> The comment:
> 
> "Is there anything that prevents a nohz full CPU from running an
> application with short and frequent idling?"
> 
> Is confusing and can be ignored.

Understood.

Kind regards,

-- 
Aaron Tomlin