From: Davidlohr Bueso <dave@stgolabs.net>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
mingo@kernel.org, john.stultz@linaro.org, dimitrysh@google.com,
linux-kernel@vger.kernel.org, Davidlohr Bueso <dbueso@suse.de>
Subject: Re: [PATCH 3/3] locking/percpu-rwsem: Avoid unnecessary writer wakeups
Date: Mon, 21 Nov 2016 19:59:11 -0800 [thread overview]
Message-ID: <20161122035911.GA17027@linux-80c1.suse> (raw)
In-Reply-To: <20161121150722.GA7951@redhat.com>
On Mon, 21 Nov 2016, Oleg Nesterov wrote:
>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?
But the patch is really: if (!(ctr_0 + ctr_1)). wrt to stale values is this
like due to the data dependency we only see the real value of this_cpu ctr,
and no guarantee for the other cpus? If so I had not considered that scenario,
and yes we'd need stronger guarantees.
I'd have to wonder if other users of per_cpu_sum() would fall into a similar
trap. Hmm and each user seems to implement its own copy of the same thing.
>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).
Why do you think using per_cpu_sum() does not make sense? As mentioned in the
changelog it optimizes for incoming readers while the writer is doing sync_enter
and getting the regular rwsem. What am I missing?
>
>I need to re-check, but what do you think about the change below?
While optimizing for multiple writers (rcu_sync_enter) is certainly valid
(at least considering the cgroups rwsem you mention), I think that my
heuristic covers the otherwise more common case. Could both optimizations
not work together?
Of course, the window of where readers_block == 1 is quite large, so there
can be a lot of false positives.
Thanks,
Davidlohr
next prev parent reply other threads:[~2016-11-22 3:59 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-11-18 18:54 [PATCH -tip 0/3] locking/percpu-rwsem: writer-side optimizations Davidlohr Bueso
2016-11-18 18:54 ` [PATCH 1/3] locking/percpu-rwsem: Move text file into Documentation/locking/ Davidlohr Bueso
2016-11-18 18:54 ` [PATCH 2/3] locking/percpu-rwsem: Replace bulky wait-queues with swait Davidlohr Bueso
2016-11-21 12:55 ` Oleg Nesterov
2016-11-21 17:26 ` Davidlohr Bueso
2016-12-03 2:18 ` [PATCH v2 2/3] locking/percpu-rwsem: Rework writer block/wake to not use wait-queues Davidlohr Bueso
2016-12-05 8:36 ` Peter Zijlstra
2016-12-05 11:26 ` Oleg Nesterov
2016-12-05 11:32 ` Oleg Nesterov
2016-12-05 17:37 ` Davidlohr Bueso
2016-12-05 17:19 ` Oleg Nesterov
2016-12-05 17:13 ` Oleg Nesterov
2016-11-18 18:54 ` [PATCH 3/3] locking/percpu-rwsem: Avoid unnecessary writer wakeups Davidlohr Bueso
2016-11-21 12:23 ` Oleg Nesterov
2016-11-21 12:29 ` Peter Zijlstra
2016-11-21 12:47 ` Oleg Nesterov
2016-11-21 15:07 ` Oleg Nesterov
2016-11-22 3:59 ` Davidlohr Bueso [this message]
2016-11-23 14:43 ` Oleg Nesterov
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