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Sender: Ingo Molnar <mingo.kernel.org@gmail.com>
Date:   Tue, 19 Sep 2023 11:49:54 +0200
From:   Ingo Molnar <mingo@kernel.org>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Ankur Arora <ankur.a.arora@oracle.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
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        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED
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* Thomas Gleixner <tglx@linutronix.de> wrote:

> On Mon, Sep 18 2023 at 20:21, Andy Lutomirski wrote:
> > On Wed, Aug 30, 2023, at 11:49 AM, Ankur Arora wrote:
> 
> > Why do we support anything other than full preempt?  I can think of
> > two reasons, neither of which I think is very good:
> >
> > 1. Once upon a time, tracking preempt state was expensive.  But we fixed that.
> >
> > 2. Folklore suggests that there's a latency vs throughput tradeoff,
> >    and serious workloads, for some definition of serious, want
> >    throughput, so they should run without full preemption.
> 
> It's absolutely not folklore. Run to completion is has well known 
> benefits as it avoids contention and avoids the overhead of scheduling 
> for a large amount of scenarios.
> 
> We've seen that painfully in PREEMPT_RT before we came up with the 
> concept of lazy preemption for throughput oriented tasks.

Yeah, for a large majority of workloads reduction in preemption increases 
batching and improves cache locality. Most scalability-conscious enterprise 
users want longer timeslices & better cache locality, not shorter 
timeslices with spread out cache use.

There's microbenchmarks that fit mostly in cache that benefit if work is 
immediately processed by freshly woken tasks - but that's not true for most 
workloads with a substantial real-life cache footprint.

Thanks,

	Ingo