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. 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. Or am I missing the point? Please let me know if so. Thanks, Byungchul > Then I landed on the commit below, and turns it was done for Android > and reported by John :) And the patch author was a certain guy named > Peter :) > > commit 3942a9bd7b5842a924e99ee6ec1350b8006c94ec > Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx> > Date: Thu Aug 11 18:54:13 2016 +0200 > > locking, rcu, cgroup: Avoid synchronize_sched() in __cgroup_procs_write() > ----------- > > Basically, this commit makes the read-side cost percpu_rwsem slightly > more expensive (one smp_load_acquire of readers_block, at the cost of > making write-side a bit more expensive...) > > So turns out it is weird, but it is certainly not a bug. > > Learned something new but wasted my time a bit :) > > Cheers, and see you later, > - Joel
