linux-kernel - Re: [RFC PATCH] kernel: allow to configure PREEMPT_NONE, PREEMPT

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Date:   Wed, 7 Oct 2020 14:29:23 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Frederic Weisbecker <fweisbecker@...e.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...e.de>, Ingo Molnar <mingo@...hat.com>
Subject: Re: [RFC PATCH] kernel: allow to configure PREEMPT_NONE,
 PREEMPT_VOLUNTARY on kernel command line

On Wed 07-10-20 14:19:39, Peter Zijlstra wrote:
> On Wed, Oct 07, 2020 at 02:04:01PM +0200, Michal Hocko wrote:
> > From: Michal Hocko <mhocko@...e.com>
> > 
> > Many people are still relying on pre built distribution kernels and so
> > distributions have to provide mutliple kernel flavors to offer different
> > preemption models. Most of them are providing PREEMPT_NONE for typical
> > server deployments and PREEMPT_VOLUNTARY for desktop users.
> 
> Is there actually a benefit to NONE? We were recently talking about
> removing it.

I believe Mel can provide much better insight. We have been historically using
PREEMPT_NONE for our enterprise customers mostly for nice throughput
numbers. Many users are really targeting throughput much more than
latencies. My understanding is that even though VOLUNTARY preemption model
doesn't add too many preemption points on top of NONE it is still
something that is observable (IIRC 2-3% on hackbench).

> The much more interesting (runtime) switch (IMO) would be between
> VOLUNTARY and PREEMPT.

Absolutely and as said we are looking into that. This is meant to be a
first baby step in that direction. Still very useful in our current
situation when we want to provide both NONE and VOLUNTARY.

-- 
Michal Hocko
SUSE Labs