linux-kernel - Re: [PATCH 2/2] locking/pvqspinlock: Optionally store lock holder cpu into lock

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Message-ID: <20200714090126.GR10769@hirez.programming.kicks-ass.net>
Date:   Tue, 14 Jul 2020 11:01:26 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Nicholas Piggin <npiggin@...il.com>, Arnd Bergmann <arnd@...db.de>,
        Borislav Petkov <bp@...en8.de>,
        Davidlohr Bueso <dave@...olabs.net>,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will.deacon@....com>, x86@...nel.org
Subject: Re: [PATCH 2/2] locking/pvqspinlock: Optionally store lock holder
 cpu into lock

On Mon, Jul 13, 2020 at 10:48:00PM -0400, Waiman Long wrote:
> Storing the cpu number into the lock can be useful for other reason too. It
> is not totally related to PPC support.

Well, the thing you did only works for 'small' (<253 CPU) systems.
There's a number of Power systems that's distinctly larger than that. So
it simply cannot work as anything other than a suggestion/hint. It must
not be a correctness thing.