From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <longman@redhat.com>
Cc: Ingo Molnar <mingo@redhat.com>, Will Deacon <will.deacon@arm.com>,
Thomas Gleixner <tglx@linutronix.de>,
Borislav Petkov <bp@alien8.de>, Arnd Bergmann <arnd@arndb.de>,
linux-kernel@vger.kernel.org, x86@kernel.org,
linux-arch@vger.kernel.org, Nicholas Piggin <npiggin@gmail.com>,
Davidlohr Bueso <dave@stgolabs.net>
Subject: Re: [PATCH 1/2] locking/qspinlock: Store lock holder cpu in lock if feasible
Date: Sun, 12 Jul 2020 19:33:48 +0200 [thread overview]
Message-ID: <20200712173348.GA10769@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20200711182128.29130-2-longman@redhat.com>
On Sat, Jul 11, 2020 at 02:21:27PM -0400, Waiman Long wrote:
> +static DEFINE_PER_CPU_READ_MOSTLY(u8, pcpu_lockval) = _Q_LOCKED_VAL;
>
> /*
> * We must be able to distinguish between no-tail and the tail at 0:0,
> @@ -138,6 +139,19 @@ struct mcs_spinlock *grab_mcs_node(struct mcs_spinlock *base, int idx)
>
> #define _Q_LOCKED_PENDING_MASK (_Q_LOCKED_MASK | _Q_PENDING_MASK)
>
> +static __init int __init_pcpu_lockval(void)
> +{
> + int cpu;
> +
> + for_each_possible_cpu(cpu) {
> + u8 lockval = (cpu + 2 < _Q_LOCKED_MASK - 1) ? cpu + 2
> + : _Q_LOCKED_VAL;
> + per_cpu(pcpu_lockval, cpu) = lockval;
> + }
> + return 0;
> +}
> +early_initcall(__init_pcpu_lockval);
> + u8 lockval = this_cpu_read(pcpu_lockval);
Urgh... so you'd rather read a guaranteed cold line than to use
smp_processor_id(), which we already use anyway?
I'm skeptical this helps anything, and it certainly makes the code more
horrible :-(
next prev parent reply other threads:[~2020-07-12 17:34 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-11 18:21 [PATCH 0/2] locking/qspinlock: Allow lock to store lock holder cpu number Waiman Long
2020-07-11 18:21 ` [PATCH 1/2] locking/qspinlock: Store lock holder cpu in lock if feasible Waiman Long
2020-07-12 17:33 ` Peter Zijlstra [this message]
2020-07-11 18:21 ` [PATCH 2/2] locking/pvqspinlock: Optionally store lock holder cpu into lock Waiman Long
2020-07-12 17:34 ` Peter Zijlstra
2020-07-12 23:05 ` Waiman Long
2020-07-13 4:17 ` Nicholas Piggin
2020-07-14 2:48 ` Waiman Long
2020-07-14 9:01 ` Peter Zijlstra
2020-07-15 16:33 ` Waiman Long
2020-07-13 9:21 ` Peter Zijlstra
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