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Date:   Thu, 20 Apr 2023 10:40:25 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Marcelo Tosatti <mtosatti@redhat.com>
Cc:     Frederic Weisbecker <frederic@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Christoph Lameter <cl@linux.com>,
        Aaron Tomlin <atomlin@atomlin.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Russell King <linux@armlinux.org.uk>,
        Huacai Chen <chenhuacai@kernel.org>,
        Heiko Carstens <hca@linux.ibm.com>, x86@kernel.org,
        Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [PATCH v7 00/13] fold per-CPU vmstats remotely
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On Wed 19-04-23 13:35:12, Marcelo Tosatti wrote:
[...]
> This is a burden for application writers and for system configuration.

Yes. And I find it reasonable to expect that burden put there as there
are non-trivial requirements for those workloads anyway. It is not
out-of-the-box thing, right?

> Or it could be done automatically (from outside of the application).
> Which is what is described and implemented here:
> 
> https://lore.kernel.org/lkml/20220204173537.429902988@fedora.localdomain/
> 
> "Task isolation is divided in two main steps: configuration and
> activation.
> 
> Each step can be performed by an external tool or the latency
> sensitive application itself. util-linux contains the "chisol" tool
> for this purpose."

I cannot say I would be a fan of prctl interfaces in general but I do
agree with the overal idea to forcing a quiescent state on a set of
CPUs.

> But not only that, the second thing is:
> 
> "> Another important point is this: if an application dirties                                                                          
> > its own per-CPU vmstat cache, while performing a system call,                                                                       
> 
> Or while handling a VM-exit from a vCPU.

Do you have any specific examples on this?

> This are, in my mind, sufficient reasons to discard the "flush per-cpu
> caches" idea. This is also why i chose to abandon the prctrl interface
> patchset.
> 
> > and a vmstat sync event is triggered on a different CPU, you'd have to:                                                             
> >                                                                                                                                     
> > 1) Wait for that CPU to return to userspace and sync its stats                                                                      
> > (unfeasible).                                                                                                                       
> >                                                                                                                                     
> > 2) Queue work to execute on that CPU (undesirable, as that causes                                                                   
> > an interruption).                                                                                                                   
> >                                                                                                                                     
> > 3) Remotely sync the vmstat for that CPU."
> 
> So the only option is to remotely sync vmstat for the CPU
> (unless you have a better suggestion).

`echo 1 > /proc/sys/vm/stat_refresh' achieves essentially the same
without any kernel changes.

But let me repeat, this is not just about vmstats. Just have a look at
other queue_work_on users. You do not want to handy pick each and every
one and do so in the future as well.
-- 
Michal Hocko
SUSE Labs