Re: [PATCH] mm: introduce sysctl file to flush per-cpu vmstat statistics

From: Christoph Lameter <cl@linux.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Marcelo Tosatti <mtosatti@redhat.com>,
	 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>,
	 Frederic Weisbecker <frederic@kernel.org>,
	 Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH] mm: introduce sysctl file to flush per-cpu vmstat statistics
Date: Wed, 2 Dec 2020 17:43:48 +0000 (UTC)	[thread overview]
Message-ID: <alpine.DEB.2.22.394.2012021737090.20713@www.lameter.com> (raw)
In-Reply-To: <87h7p4dwus.fsf@nanos.tec.linutronix.de>

On Wed, 2 Dec 2020, Thomas Gleixner wrote:

> prctl() is the right thing to do.

Ok great consensus on that one.

> >> 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.
> >
> > Sounds good but what would this do? Give a warning like the isolation
> > patchset?
>
> This all needs a lot more thought about the overall picture. We already
> have too many knobs and ad hoc hooks which fiddle with isolation.
>
> The current CPU isolation is a best effort approach and I agree that for
> more strict isolation modes we need to be able to enforce that and hunt
> down offenders and think about them one by one.

There are two apprahces actually to make the OS quiet. One is the best
effort approach which is more like the current NOHZ one with additional
actions to flush things. The other is the strict approach were one wants a
guarantee that the OS does not do anything at all.

 >
> >> To be called before real time loop, one would have:
>
> Can we please agree in the first place, that "real time" is absolutely
> the wrong term here?

Sorry I once swore that I would not use real time since that term is so
damaged by multiple meanings and marketing endeavor.

"to be called before a loop without any disturbance by the operating
system"

> It's about running undisturbed CPU bound computations whatever nature
> they are. It does not matter whether that loop does busy polling ala
> DPDK, whether it runs a huge math computation on a data set or
> whatever people come up with.

Right.

> > Often code can tolerate a few interruptions (in some code branches
> > regular syscalls may be needed) but one wants the thread to be
> > as quiet as possible.
>
> So you say some code can tolerate a few interrupts, then comes Alex and
> says 'no disturbance' at all.

Yes that is the current NOHZ approach.  You switch it on and after the OS
detects are polling loop it will quiet things down. Instead of detecting
it we are actively telling the OS to quiet down now.

> We really need to stop to look at particular workloads and defining
> adhoc solutions tailored to their particular itch if we don't want to
> end up with an uncoordinated and unmaintainable zoo of interfaces, hooks
> and knobs.

True....Multiple subsystem needs to be hooked into this to quiet things
down.

> That allows Marcelo to start tackling the vmstat side and Alex can
> utilize that and build the other parts into it piece by piece.

Yup. I agree.