linux-kernel - Re: [PATCH v8 4/5] locking/qspinlock: Introduce starvation avoidance into CNA

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Date:   Mon, 3 Feb 2020 14:45:40 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alex Kogan <alex.kogan@...cle.com>
Cc:     Waiman Long <longman@...hat.com>, linux@...linux.org.uk,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Arnd Bergmann <arnd@...db.de>, linux-arch@...r.kernel.org,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, hpa@...or.com, x86@...nel.org,
        Hanjun Guo <guohanjun@...wei.com>,
        Jan Glauber <jglauber@...vell.com>,
        Steven Sistare <steven.sistare@...cle.com>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        dave.dice@...cle.com
Subject: Re: [PATCH v8 4/5] locking/qspinlock: Introduce starvation avoidance
 into CNA

On Thu, Jan 30, 2020 at 05:05:28PM -0500, Alex Kogan wrote:
> 
> > On Jan 25, 2020, at 6:19 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > 
> > On Fri, Jan 24, 2020 at 01:19:05PM -0500, Alex Kogan wrote:
> > 
> >> Is there a lightweight way to identify such a “prioritized” thread?
> > 
> > No; people might for instance care about tail latencies between their
> > identically spec'ed worker tasks.
> 
> I would argue that those users need to tune/reduce the intra-node handoff
> threshold for their needs. Or disable CNA altogether.

I really don't like boot time arguments (or tunables in generic) for a
machine to work as it should.

The default really should 'just work'.

> In general, Peter, seems like you are not on board with the way Longman
> suggested to handle prioritized threads. Am I right?

Right.

Presumably you have a workload where CNA is actually a win? That is,
what inspired you to go down this road? Which actual kernel lock is so
contended on NUMA machines that we need to do this?