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

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Message-ID: <20200124075235.GX14914@hirez.programming.kicks-ass.net>
Date:   Fri, 24 Jan 2020 08:52:35 +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 23, 2020 at 04:33:54PM -0500, Alex Kogan wrote:
> Let me put this question to you. What do you think the number should be?

I think it would be very good to keep the inter-node latency below 1ms.

But to realize that we need data on the lock hold times. Specifically
for the heavily contended locks that make CNA worth it in the first
place.

I don't see that data, so I don't see how we can argue about this let
alone call something reasonable.