linux-kernel - RE: [PATCH v4 3/4] locking/qspinlock: Add ARCH_USE_QUEUED_SPINLOCKS

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Date:   Tue, 30 Mar 2021 08:31:59 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Guo Ren' <guoren@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
CC:     linux-riscv <linux-riscv@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "linux-csky@...r.kernel.org" <linux-csky@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        "Guo Ren" <guoren@...ux.alibaba.com>,
        Will Deacon <will@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        Waiman Long <longman@...hat.com>,
        Arnd Bergmann <arnd@...db.de>, Anup Patel <anup@...infault.org>
Subject: RE: [PATCH v4 3/4] locking/qspinlock: Add
 ARCH_USE_QUEUED_SPINLOCKS_XCHG32

From: Guo Ren
> Sent: 30 March 2021 04:14
...
> > Step 1 would be to get your architecute fixed such that it can provide
> > fwd progress guarantees for LL/SC. Otherwise there's absolutely no point
> > in building complex systems with it.
> 
> Quote Waiman's comment [1] on xchg16 optimization:
> 
> "This optimization is needed to make the qspinlock achieve performance
> parity with ticket spinlock at light load."
> 
> [1] https://lore.kernel.org/kvm/1429901803-29771-6-git-send-email-Waiman.Long@hp.com/
> 
> So for a non-xhg16 machine:
>  - ticket-lock for small numbers of CPUs
>  - qspinlock for large numbers of CPUs
> 
> Okay, I'll put all of them into the next patch :P

Doesn't that also imply that you need ticket-locks for
lightly contended locks even with a lot of CPUs?

If you actually get a lot of CPUs waiting on the same spinlock
you probably ought to change the code to reduce lock contention.

	David

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