From: Will Deacon <will@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: "Boqun Feng" <boqun.feng@gmail.com>,
"Ingo Molnar" <mingo@kernel.org>,
"Waiman Long" <longman@redhat.com>,
"Arnd Bergmann" <arnd@arndb.de>,
linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org,
"Guo Ren" <guoren@kernel.org>,
"Palmer Dabbelt" <palmerdabbelt@google.com>,
"Anup Patel" <anup@brainfault.org>,
linux-riscv <linux-riscv@lists.infradead.org>,
"Christoph Müllner" <christophm30@gmail.com>,
"Stafford Horne" <shorne@gmail.com>
Subject: Re: [PATCH] locking: Generic ticket lock
Date: Tue, 14 Dec 2021 15:40:57 +0000 [thread overview]
Message-ID: <20211214154057.GB15416@willie-the-truck> (raw)
In-Reply-To: <YXFli3mzMishRpEq@hirez.programming.kicks-ass.net>
On Thu, Oct 21, 2021 at 03:05:15PM +0200, Peter Zijlstra wrote:
>
> There's currently a number of architectures that want/have graduated
> from test-and-set locks and are looking at qspinlock.
>
> *HOWEVER* qspinlock is very complicated and requires a lot of an
> architecture to actually work correctly. Specifically it requires
> forward progress between a fair number of atomic primitives, including
> an xchg16 operation, which I've seen a fair number of fundamentally
> broken implementations of in the tree (specifically for qspinlock no
> less).
>
> The benefit of qspinlock over ticket lock is also non-obvious, esp.
> at low contention (the vast majority of cases in the kernel), and it
> takes a fairly large number of CPUs (typically also NUMA) to make
> qspinlock beat ticket locks.
>
> Esp. things like ARM64's WFE can move the balance a lot in favour of
> simpler locks by reducing the cacheline pressure due to waiters (see
> their smp_cond_load_acquire() implementation for details).
>
> Unless you've audited qspinlock for your architecture and found it
> sound *and* can show actual benefit, simpler is better.
>
> Therefore provide ticket locks, which depend on a single atomic
> operation (fetch_add) while still providing fairness.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
> include/asm-generic/qspinlock.h | 30 +++++++++
> include/asm-generic/ticket_lock_types.h | 11 +++
> include/asm-generic/ticket_lock.h | 97 ++++++++++++++++++++++++++++++++
> 3 files changed, 138 insertions(+)
Huh. I looked quite closely at this a while back but seems like I forgot to
actually reply here. So, given that it doesn't seem to be in linux-next yet:
Acked-by: Will Deacon <will@kernel.org>
Will
prev parent reply other threads:[~2021-12-14 15:41 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-10-21 13:05 [PATCH] locking: Generic ticket lock Peter Zijlstra
2021-10-21 13:12 ` Peter Zijlstra
2021-10-21 13:50 ` Stafford Horne
2021-10-21 13:49 ` Arnd Bergmann
2021-10-21 15:14 ` Peter Zijlstra
2021-10-21 15:31 ` Arnd Bergmann
2021-10-21 16:28 ` Peter Zijlstra
2021-10-21 18:04 ` Waiman Long
2021-10-22 15:19 ` Boqun Feng
2021-10-22 2:04 ` Guo Ren
2021-10-22 9:23 ` Mark Rutland
2021-10-22 12:31 ` Peter Zijlstra
2021-12-14 15:40 ` Will Deacon [this message]
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