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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>,
	Alex Belits <abelits@marvell.com>, Phil Auld <pauld@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Frederic Weisbecker <frederic@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH] mm: introduce sysctl file to flush per-cpu vmstat statistics
Date: Fri, 27 Nov 2020 12:48:45 -0300	[thread overview]
Message-ID: <20201127154845.GA9100@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.
> 
> > 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.

Christopher,

Decided to switch to prctl interface, and then it starts
to become similar to "task mode isolation" patchset API.

In addition to quiescing pending activities on the CPU, it would 
also be useful to assign a per-task attribute (which is then assigned
to a per-CPU attribute), indicating whether that CPU is running
an isolated task or not.

This per-CPU attribute can be used to, for example, return -EBUSY 
from ring_buffer_resize() (or any other IPI generating activity 
which can return an error to userspace).

So rather than:

	prctl(PR_QUIESCE_CPU)	(current interface, similar to
	initial message on the thread but with prctl rather than
	sysfs)

To be called before real time loop, one would have:

	prctl(PR_SET_TASK_ISOLATION, ISOLATION_ENABLE) [1]
	real time loop
	prctl(PR_SET_TASK_ISOLATION, ISOLATION_DISABLE)

(with the attribute also being cleared on task exit).

The general description would be:

"Set task isolated mode for a given task, returning an error
if the task is not pinned to a single CPU.

In this mode, the kernel will avoid interruptions to isolated 
CPUs when possible."

Any objections against such an interface ?


[1] perhaps a name that does not conflict with "task mode" patchset
is a better idea.



  parent reply	other threads:[~2020-11-27 15:49 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
2020-11-24 17:12         ` Vlastimil Babka
2020-11-24 19:52           ` Marcelo Tosatti
2020-11-27 15:48         ` Marcelo Tosatti [this message]
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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