Re: Severe performance regression w/ 4.4+ on Android due to cgroup locking changes

[Paul E. McKenney]

On Thu, Jul 14, 2016 at 06:45:47PM +0200, Peter Zijlstra wrote:
> 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.

Ah, got it...

> > 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.

OK, in that case I will hold off pending John Stultz's performance
checks.

							Thanx, Paul