On Sun, Oct 14, 2018 at 07:13:49PM -0700, Joel Fernandes wrote: > On Sun, Oct 14, 2018 at 07:08:27PM -0700, Joel Fernandes wrote: > > On Sun, Oct 14, 2018 at 04:17:31PM -0700, Paul E. McKenney wrote: > > > On Sun, Oct 14, 2018 at 02:29:55PM -0700, Joel Fernandes (Google) wrote: > > > > The Requirements.html document says "Disabling Preemption Does Not Block > > > > Grace Periods". However this is no longer true with the RCU > > > > consolidation. Lets remove the obsolete (non-)requirement entirely. > > > > > > > > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> > > > > > > Good catch, queued, thank you! > > > > Thanks! By the way after I sent the patch, I also tried Oleg's experiment to > > confirm that this is indeed obsolete. :) > > > > One thing interesting came up when I tried synchronize_rcu_expedited() > > instead of synchronize_rcu() in Oleg's experiment, I still saw a multiple > > millisecond delay between when the rcu read section completely and the > > synchronize_rcu_expedited returns: > > > > For example, with synchronize_rcu_expedited, the 'SPIN done' and the 'SYNC > > done' are about 3 millisecond apart: > > [ 77.599142] SPIN start > > [ 77.601595] SYNC start > > [ 82.604950] SPIN done! > > [ 82.607836] SYNC done! > > I saw anywhere from 2-6 milliseconds. > > > > The reason I bring this up is according to Requirements.html: In some cases, > > the multi-millisecond synchronize_rcu() latencies are unacceptable. In these > > cases, synchronize_rcu_expedited() may be used instead,.. so either I messed > > something up in the experiment, or I need to update this part of the document ;-) In normal testing, 2-6 milliseconds is indeed excessive. Could you please point me at Oleg's experiment? Also, what CONFIG_PREEMPT setting were you using? (My guess is CONFIG_PREEMPT=y.) > So I realized I'm running in Qemu so it could also be a scheduling delay of > the vcpu thread. So apologies about the noise if the experiment works fine > for you. I used rcuperf, which might not be doing the same thing as Oleg's experiment. Thanx, Paul
