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

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