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From: Marcelo Tosatti <mtosatti@redhat.com>
To: Christopher Lameter <cl@linux.com>
Cc: Matthew Wilcox <willy@infradead.org>,
	linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH] mm: introduce sysctl file to flush per-cpu vmstat statistics
Date: Mon, 23 Nov 2020 15:02:23 -0300	[thread overview]
Message-ID: <20201123180223.GA19269@fuller.cnet> (raw)
In-Reply-To: <alpine.DEB.2.22.394.2011201817320.248402@www.lameter.com>

On Fri, Nov 20, 2020 at 06:20:06PM +0000, Christopher Lameter wrote:
> On Tue, 17 Nov 2020, Marcelo Tosatti 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.
> >
> > Are you suggesting that instead of a specific file to control vmstat
> > workqueue only, a more generic sysctl could be used?
> 
> Yes. Introduce a sysctl to quiet down the system. Clean caches that will
> trigger kernel threads and whatever else is pending on that processor.

OK, makes sense.

About sysctl, in particular:

NOTES         top

       Use of this system call was long discouraged: since Linux 2.6.24,
       uses of this system call result in warnings in the kernel log, and in
       Linux 5.5, the system call was finally removed.  Use the /proc/sys
       interface instead.

       Note that on older kernels where this system call still exists, it is
       available only if the kernel was configured with the
       CONFIG_SYSCTL_SYSCALL option.  Furthermore, glibc does not provide a
       wrapper for this system call, necessitating the use of syscall(2).

So will stick to the /proc/ suggestion.

> > About NOHZ mode: the CPU should enter NOHZ automatically as soon as
> > there is a single thread running, so unclear why that would be needed.
> 
> There are typically pending actions that still trigger interruptions.
> 
> If you would immediately quiet down the system if there is only one thread
> runnable then you would compromise system performance through frequent
> counter folding and cache cleaning etc.

Makes sense.



  reply	other threads:[~2020-11-24 18:29 UTC|newest]

Thread overview: 42+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-17 16:28 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
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 [this message]
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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