linux-kernel - Re: [PATCH 3/3] locking/percpu-rwsem: Avoid unnecessary writer wakeups

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Message-ID: <20161121150722.GA7951@redhat.com>
Date:   Mon, 21 Nov 2016 16:07:23 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Davidlohr Bueso <dave@...olabs.net>, mingo@...nel.org,
        john.stultz@...aro.org, dimitrysh@...gle.com,
        linux-kernel@...r.kernel.org, Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH 3/3] locking/percpu-rwsem: Avoid unnecessary writer
        wakeups

On 11/21, Oleg Nesterov wrote:
>
> No, no, I meant that afaics both readers can see per_cpu_sum() != 0 and
> thus the writer won't be woken up. Till the next down_read/up_read.
>
> Suppose that we have 2 CPU's, both counters == 1, both readers decrement.
> its counter at the same time.
>
> 	READER_ON_CPU_0			READER_ON_CPU_1
>
> 	--ctr_0;			--ctr_1;
>
> 	if (ctr_0 + ctr_1)		if (ctr_0 + ctr_1)
> 		wakeup();			wakeup();
>
> Why we can't miss a wakeup?
>
> This patch doesn't even add a barrier, but I think wmb() won't be enough
> anyway.

And in fact I am not sure this optimization makes sense... But it would be
nice to avoid wake_up() when the writer sleeps in rcu_sync_enter(). Or this
is the "slow mode" sem (cgroup_threadgroup_rwsem).

I need to re-check, but what do you think about the change below?

Oleg.

--- x/kernel/locking/percpu-rwsem.c
+++ x/kernel/locking/percpu-rwsem.c
@@ -103,7 +103,9 @@ void __percpu_up_read(struct percpu_rw_s
 	__this_cpu_dec(*sem->read_count);
 
 	/* Prod writer to recheck readers_active */
-	wake_up(&sem->writer);
+	smp_mb();
+	if (sem->readers_block)
+		wake_up(&sem->writer);
 }
 EXPORT_SYMBOL_GPL(__percpu_up_read);