On 15.10.2018 05:47, Joel Fernandes wrote: > On Sun, Oct 14, 2018 at 07:33:28PM -0700, Paul E. McKenney wrote: >> 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.) > > The CONFIG_PREEMPT config I am using 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. > > The experiment is mentioned at: > https://www.mail-archive.com/linux-kernel@xxxxxxxxxxxxxxx/msg912055.html > > If you apply the below diff, it applies cleanly on rcu/dev. And then run: > taskset 2 perl -e 'syscall 157, 666, 5000' & > taskset 1 perl -e 'syscall 157, 777' > > diff --git a/kernel/sys.c b/kernel/sys.c > index cf5c67533ff1..b654b7566ca3 100644 > --- a/kernel/sys.c > +++ b/kernel/sys.c > @@ -2261,6 +2261,9 @@ int __weak arch_prctl_spec_ctrl_set(struct task_struct *t, unsigned long which, > return -EINVAL; > } > > +#include <linux/delay.h> > + > + > SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3, > unsigned long, arg4, unsigned long, arg5) > { > @@ -2274,6 +2277,19 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3, > > error = 0; > switch (option) { > + case 666: > + preempt_disable(); > + pr_crit("SPIN start\n"); > + while (arg2--) > + mdelay(1); > + pr_crit("SPIN done!\n"); > + preempt_enable(); > + break; > + case 777: > + pr_crit("SYNC start\n"); > + synchronize_rcu(); > + pr_crit("SYNC done!\n"); But you are using the console printing infrastructure which is rather heavyweight. Try replacing pr_* calls with trace_printk so that you write to the lock-free ring buffer, this will reduce the noise from the heavy console printing infrastructure. > + break; > case PR_SET_PDEATHSIG: > if (!valid_signal(arg2)) { > error = -EINVAL; >
