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From: Christopher Lameter <cl@linux.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: Marcelo Tosatti <mtosatti@redhat.com>,
	linux-mm@kvack.org,  Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH] mm: introduce sysctl file to flush per-cpu vmstat statistics
Date: Fri, 20 Nov 2020 18:04:35 +0000 (UTC)	[thread overview]
Message-ID: <alpine.DEB.2.22.394.2011201800390.248138@www.lameter.com> (raw)
In-Reply-To: <20201117190926.GU29991@casper.infradead.org>

On Tue, 17 Nov 2020, Matthew Wilcox wrote:

> > So what we would need would be something like a sysctl that puts the
> > system into a quiet state by completing all workqueue items. Idle all
> > subsystems that need it and put the cpu into NOHZ mode.
>
> Why does it need to be something userspace does?  It would seem reasonable
> that the kernel could figure out for itself that it's about to start
> this CPU operating in isolated mode and flush all the work first?

The kernel holds objects for various purposes in caches just in case. And
these objects are expired when they are considered cache cold. Similarly
vmstat does not fold counters into the global ones just in case more
things need to be locally counted in order to prevent contention on the
global counters.

The kernel cannot know that user space will not be using system calls but
wants a quiet operating environment without interruptions for cache
expiration and other things. The kernel can do all sorts of preventitive
work to avoid deferred actions and create a OS noise free environment for
the application.



  reply	other threads:[~2020-11-20 18:04 UTC|newest]

Thread overview: 42+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-17 16:28 [PATCH] mm: introduce sysctl file to flush per-cpu vmstat statistics Marcelo Tosatti
2020-11-17 18:03 ` Matthew Wilcox
2020-11-17 19:06   ` Christopher Lameter
2020-11-17 19:09     ` Matthew Wilcox
2020-11-20 18:04       ` Christopher Lameter [this message]
2020-11-17 20:23     ` Marcelo Tosatti
2020-11-20 18:02       ` Marcelo Tosatti
2020-11-20 18:20       ` Christopher Lameter
2020-11-23 18:02         ` Marcelo Tosatti
2020-11-24 17:12         ` Vlastimil Babka
2020-11-24 19:52           ` Marcelo Tosatti
2020-11-27 15:48         ` Marcelo Tosatti
2020-11-28  3:49           ` [EXT] " Alex Belits
2020-11-30 18:18             ` Marcelo Tosatti
2020-11-30 18:29               ` Marcelo Tosatti
2020-12-03 22:47                 ` Alex Belits
2020-12-03 22:21               ` Alex Belits
2020-11-30  9:31           ` Christoph Lameter
2020-12-02 12:43             ` Marcelo Tosatti
2020-12-02 15:57             ` Thomas Gleixner
2020-12-02 17:43               ` Christoph Lameter
2020-12-03  3:17                 ` Thomas Gleixner
2020-12-07  8:08                   ` Christoph Lameter
2020-12-07 16:09                     ` Thomas Gleixner
2020-12-07 19:01                       ` Thomas Gleixner
2020-12-02 18:38               ` Marcelo Tosatti
2020-12-04  0:20                 ` Frederic Weisbecker
2020-12-04 13:31                   ` Marcelo Tosatti
2020-12-04  1:43               ` [EXT] " Alex Belits
2021-01-13 12:15                 ` [RFC] tentative prctl task isolation interface Marcelo Tosatti
2021-01-14  9:22                   ` Christoph Lameter
2021-01-14 19:34                     ` Marcelo Tosatti
2021-01-15 13:24                       ` Christoph Lameter
2021-01-15 18:35                         ` Alex Belits
2021-01-21 15:51                           ` Marcelo Tosatti
2021-01-21 16:20                             ` Marcelo Tosatti
2021-01-22 13:05                               ` Marcelo Tosatti
2021-02-01 10:48                             ` Christoph Lameter
2021-02-01 12:47                               ` Alex Belits
2021-02-01 18:20                               ` Marcelo Tosatti
2021-01-18 15:18                         ` Marcelo Tosatti
2020-11-24  5:02 ` [mm] e655d17ffa: BUG:using_smp_processor_id()in_preemptible kernel test robot

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