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

[Peter Zijlstra]

On Thu, Jul 14, 2016 at 04:58:44PM +0200, Oleg Nesterov wrote:
> 
> But note that we do not need RCU_NONE. All we need is the trivial change
> below.

Hurm, maybe. So having that unbalanced keeps us in GP_PASSED state and
since we'll never drop gp_count back to 0 nothing will ever happen.

Cute, yes.

> Damn, I am trying to find my old rcu-sync patches which I didn't
> send, but can't... OK, this almost off-topic right now, just this "enter"
> is ugly and we can't switch the slow/fast modes dynamically.
> 
> The rest of you patch is "optimize the slow path" and we already discussed
> it before, I personally like it. Perhaps you can redo it without RCU_NONE
> part?

Indeed, I rebased that patch on top of the current tree and had to add
support for down_trylock() but otherwise much the same thing.

I can send it out again.