Messages in this thread | | | Date | Thu, 14 Jul 2016 20:36:09 +0200 | From | Peter Zijlstra <> | 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.
| |