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

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Date:   Mon, 29 Mar 2021 14:54:23 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Anup Patel <Anup.Patel@....com>
Cc:     Guo Ren <guoren@...nel.org>,
        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

On Mon, Mar 29, 2021 at 12:13:10PM +0000, Anup Patel wrote:

> We had discussions in the RISC-V platforms group about this. Over there,
> We had evaluated all spin lock approaches (ticket, qspinlock, etc) tried
> in Linux till now. It was concluded in those discussions that eventually we
> have to move to qspinlock (even if we moved to ticket spinlock temporarily)
> because qspinlock avoids cache line bouncing. Also, moving to qspinlock
> will be aligned with other major architectures supported in Linux (such as
> x86, ARM64)
> 
> Some of the organizations working on high-end RISC-V systems (> 32
> CPUs) are interested in having an optimized spinlock implementation
> (just like other major architectures x86 and ARM64).
> 
> Based on above, Linux RISC-V should move to qspinlock.

That's all well and good, but first you need architectural forward
progress guarantees. Otherwise there's absolutely no point in building
complex locking primitives.

And unless you already have such big systems in silicon, where you
can benchmark against simpler locks (like ticket) there really is no
point in the complexity.