From: Peter Zijlstra <peterz@infradead.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Tejun Heo <tj@kernel.org>, John Stultz <john.stultz@linaro.org>,
Ingo Molnar <mingo@redhat.com>,
lkml <linux-kernel@vger.kernel.org>,
Dmitry Shmidt <dimitrysh@google.com>,
Rom Lemarchand <romlem@google.com>,
Colin Cross <ccross@google.com>, Todd Kjos <tkjos@google.com>,
Oleg Nesterov <oleg@redhat.com>
Subject: Re: Severe performance regression w/ 4.4+ on Android due to cgroup locking changes
Date: Thu, 14 Jul 2016 18:45:47 +0200 [thread overview]
Message-ID: <20160714164547.GG30154@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <20160714162355.GW7094@linux.vnet.ibm.com>
On Thu, Jul 14, 2016 at 09:23:55AM -0700, Paul E. McKenney wrote:
> Hmmm... How does this handle the following sequence of events for
> the case where we are not biased towards the reader?
>
> o The per-CPU rwsem is set up with RCU_NONE and readers_slow
> (as opposed to readers_block). The rcu_sync ->gp_state is
> GP_PENDING, which means that rcu_sync_is_idle() will always
> return true.
/false/, rcu_sync_is_idle() will always be false, to force us into the
slowpath.
> o Task A on CPU 0 runs percpu_down_read() to completion, and remains
> within the critical section. CPU 0's ->refcount is therefore 1.
>
> o Task B on CPU 1 does percpu_down_write(), which write-acquires
> ->rw_sem, does rcu_sync_enter() (which is a no-op due to
> RCU_NONE), sets ->state to readers_block, and is just now going
> to wait for readers, which first invokes readers_active_check(),
> which starts summing the ->refcount for CPUs 0, 1, and 2,
> obtaining the value 1 so far.
>
> o Task C CPU 2 enters percpu_down_read(), disables preemption,
> increments CPU 2's ->refcount, sees rcu_sync_is_idle() return
> true (so skips __percpu_down_read()), enables preemption, and
> enters its critical section.
false, so does __percpu_down_read()
>
> o Task C migrates to CPU 3 and invokes percpu_up_read(), which
> disables preemption, sees rcu_sync_is_idle() return true, calls
> __this_cpu_dec() on CPU 3's ->refcount, and enables preemption.
> The value of CPU 3's ->refcount is thus (unsigned int)-1.
__percpu_up_read()
>
> o Task B on CPU 1 continues execution in readers_active_check(), with
> the full sum being zero.
>
> So it looks to me like we have Task A as a writer at the same time that
> Task A is a reader, which would not be so good.
>
> So what am I missing here?
for RCU_NONE we init rsp->gp_state to !0, which makes:
static inline rcu_sync_is_idle()'s
return !rsp->gp_state (aka. rsp->gp_state == 0)
return false.
> And a couple of checkpatch nits below. Yes, I had to apply the patch to
> figure out what it was doing. ;-)
Yah, too much churn to read :-)
In any case, rest assured you've already gone over this part of the
patch several times. I repurposed an old percpu-rwsem optimization, Oleg
recognised it.
next prev parent reply other threads:[~2016-07-14 16:45 UTC|newest]
Thread overview: 67+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-07-13 0:00 Severe performance regression w/ 4.4+ on Android due to cgroup locking changes John Stultz
2016-07-13 8:21 ` Peter Zijlstra
2016-07-13 14:42 ` Paul E. McKenney
2016-07-13 18:13 ` Dmitry Shmidt
2016-07-13 18:32 ` Paul E. McKenney
2016-07-13 18:21 ` Tejun Heo
2016-07-13 18:33 ` Tejun Heo
2016-07-13 20:13 ` John Stultz
2016-07-13 20:18 ` Tejun Heo
2016-07-13 20:26 ` Peter Zijlstra
2016-07-13 20:39 ` Tejun Heo
2016-07-13 20:51 ` Peter Zijlstra
2016-07-13 21:01 ` Tejun Heo
2016-07-13 21:03 ` Paul E. McKenney
2016-07-13 21:05 ` Tejun Heo
2016-07-13 21:18 ` Paul E. McKenney
2016-07-13 21:42 ` Paul E. McKenney
2016-07-13 21:46 ` John Stultz
2016-07-13 22:17 ` Paul E. McKenney
2016-07-13 22:39 ` John Stultz
2016-07-13 23:02 ` Paul E. McKenney
2016-07-13 23:04 ` Paul E. McKenney
2016-07-14 11:35 ` Tejun Heo
2016-07-14 12:04 ` Peter Zijlstra
2016-07-14 12:08 ` Tejun Heo
2016-07-14 12:20 ` Peter Zijlstra
2016-07-14 15:07 ` Tejun Heo
2016-07-14 15:24 ` Tejun Heo
2016-07-14 16:32 ` Peter Zijlstra
2016-07-14 17:34 ` Oleg Nesterov
2016-07-14 16:54 ` John Stultz
2016-07-13 22:25 ` John Stultz
2016-07-13 22:01 ` Tejun Heo
2016-07-13 22:33 ` Paul E. McKenney
2016-07-14 6:49 ` Peter Zijlstra
2016-07-14 11:20 ` Tejun Heo
2016-07-14 12:11 ` Peter Zijlstra
2016-07-14 15:14 ` Tejun Heo
2016-07-14 13:18 ` Peter Zijlstra
2016-07-14 14:14 ` Peter Zijlstra
2016-07-14 14:58 ` Oleg Nesterov
2016-07-14 16:14 ` Peter Zijlstra
2016-07-14 16:37 ` Peter Zijlstra
2016-07-14 17:05 ` Oleg Nesterov
2016-07-14 16:23 ` Paul E. McKenney
2016-07-14 16:45 ` Peter Zijlstra [this message]
2016-07-14 17:15 ` Paul E. McKenney
2016-07-14 16:43 ` John Stultz
2016-07-14 16:49 ` Peter Zijlstra
2016-07-14 17:02 ` John Stultz
2016-07-14 17:13 ` Oleg Nesterov
2016-07-14 17:30 ` John Stultz
2016-07-14 17:41 ` Oleg Nesterov
2016-07-14 17:51 ` John Stultz
2016-07-14 18:09 ` Oleg Nesterov
2016-07-14 18:36 ` Peter Zijlstra
2016-07-14 19:35 ` Peter Zijlstra
2016-07-13 20:57 ` John Stultz
2016-07-13 20:52 ` Paul E. McKenney
2016-07-13 20:57 ` Peter Zijlstra
2016-07-13 21:08 ` Paul E. McKenney
2016-07-13 21:01 ` Dmitry Shmidt
2016-07-13 21:03 ` John Stultz
2016-07-13 21:05 ` Paul E. McKenney
2016-07-13 20:31 ` Dmitry Shmidt
2016-07-13 20:44 ` Colin Cross
2016-07-13 20:54 ` Tejun Heo
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