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

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Date:	Thu, 14 Jul 2016 20:36:09 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Tejun Heo <tj@...nel.org>, John Stultz <john.stultz@...aro.org>,
	Ingo Molnar <mingo@...hat.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Dmitry Shmidt <dimitrysh@...gle.com>,
	Rom Lemarchand <romlem@...gle.com>,
	Colin Cross <ccross@...gle.com>, Todd Kjos <tkjos@...gle.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: Severe performance regression w/ 4.4+ on Android due to cgroup
 locking changes

On Thu, Jul 14, 2016 at 08:09:52PM +0200, Oleg Nesterov wrote:
> On 07/14, Peter Zijlstra wrote:
> >
> > The below is a compile tested only first draft so far. I'll go give it
> > some runtime next.
> 
> So I will wait for the new version, but at first glance this matches the
> code I already reviewed in the past (at least, tried hard to review ;)
> and it looks correct.
> 
> Just a couple of almost cosmetic nits, feel free to ignore.

Drat, I just mailed out the patches.. I can do a second version later.

> > --- a/include/linux/percpu-rwsem.h
> > +++ b/include/linux/percpu-rwsem.h
> > @@ -10,29 +10,107 @@
> >  
> >  struct percpu_rw_semaphore {
> >  	struct rcu_sync		rss;
> > -	unsigned int __percpu	*fast_read_ctr;
> > +	unsigned int __percpu	*refcount;
> >  	struct rw_semaphore	rw_sem;
> > -	atomic_t		slow_read_ctr;
> > -	wait_queue_head_t	write_waitq;
> > +	wait_queue_head_t	writer;
> > +	int			state;
>         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> 
> I think that this "int state" and "enum { readers_slow, readers_block }"
> just add a bit of complication/confusion.
> 
> All we need is the simple "bool readers_block" in percpu_rw_semaphore,
> no?

So I detest bool in structures because sizeof(bool) isn't defined.
Obviously an implementation needs to pick a size, but this is typically
architecture ABIs, so sizes can differ between architectures.

But yes, I suppose "int readers_block" will do just fine.

IIRC, earlier version had more states, but that all went away.

> > +void percpu_down_write(struct percpu_rw_semaphore *sem)
> >  {
> > +	down_write(&sem->rw_sem);
> > +
> > +	/* Notify readers to take the slow path. */
> > +	rcu_sync_enter(&sem->rss);
> 
> I'd suggest to call rcu_sync_enter() before down_write(). This can help
> when we wait for another writer which holds this lock.

Hurm, I think I figured that might have issues, but I cannot seem to
think of any just now :-), yes can do.