Re: This percpu_rwsem that always enters its reader slow path

[Joel Fernandes <joel@joelfernandes.org>]

On Fri, Jul 19, 2019 at 04:50:11PM +0900, Byungchul Park wrote:
> On Thu, Jul 18, 2019 at 05:09:45PM -0400, Joel Fernandes wrote:
> > Hello friends,
> > 
> > Just providing an update on my debugging of percpu_rwsem (related to
> > rcu-sync) for the day! which I pinged Byungchul about. Please ignore
> > this email if you are busy :) I am just archiving it in here..
> > 
> > As you may know, percpu_rwsem uses rcu-sync framework to reduce cost
> > of read-side by making it free of any serializing/atomic instructions
> > at all. However, there was one sempahore which broke the rules!
> > 
> > I spent a couple hours trying to figure out why
> > cgroup_threadgroup_rwsem always entered the reader-slow path on my
> > system (RCU-sync turns out to be non-idle for this rwsem). I really
> > thought it was a bug, because I felt what's the pointed of rcu-sync if
> > it never goes idle..
> 
> Yes, with the following patch, the cgroup rwsem cannot make use of
> rcu_sync any more, but it still gets benefit from percpu structure
> as you told me like avoiding cache bouncing and contention on a shared
> area even though every read lock keeps firing smp full barrier.

Yes. So it seems to me main benefit of RCU in percpu_rw_sempahore is to
completely avoid memory barriers in the read path, while also benefiting from
the percpu nature of the lock.

> What matters is which one is more expensive between (1) firing smp_mb
> and (2) accessing a shared data, sem->count, and acquiring/releasing
> sem->wait_lock. I think using percpu-rwsem involving the smp barrier is
> much better even with rcu_sync disabled.

Right. Fully agreed.

thanks,

 - Joel