On Thu, Jul 18, 2019 at 5:09 PM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> 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.. > > 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...) I meant here, at the benefit of making write-side cheap and avoiding synchronize_rcu...
