linux-kernel - Re: [Problem] Cache line starvation

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Message-ID: <20180921122058.GB24124@hirez.programming.kicks-ass.net>
Date:   Fri, 21 Sep 2018 14:20:58 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org,
        Daniel Wagner <daniel.wagner@...mens.com>,
        Will Deacon <will.deacon@....com>, x86@...nel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Boqun Feng <boqun.feng@...il.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [Problem] Cache line starvation

On Fri, Sep 21, 2018 at 02:02:26PM +0200, Sebastian Andrzej Siewior wrote:
> We reproducibly observe cache line starvation on a Core2Duo E6850 (2
> cores), a i5-6400 SKL (4 cores) and on a NXP LS2044A ARM Cortex-A72 (4
> cores).
> 
> The problem can be triggered with a v4.9-RT kernel by starting

> Daniel reported that disabling ticket locks on 4.4 makes the problem go
> away, but he hasn't run a long time test yet and as we saw with 4.14 it can
> take quite a while.

On 4.4 and 4.9 ARM64 still uses ticket locks. So I'm very interested to
know if the ticket locks on x86 really fix or just make it harder.

I've been looking at qspinlock in the light of this and there is indeed
room for improvement. The ticket lock certainly is much simpler.