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

[Oleg Nesterov]

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.

> --- 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?

> +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.

Oleg.