Re: [PATCH RFC] doc: rcu: remove obsolete (non-)requirement about disabling preemption

[Joel Fernandes]

On Sun, Oct 14, 2018 at 07:47:58PM -0700, 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();

By the way, just to mention, this needs to be changed to
synchronize_rcu_expedited() and then built. So with that I see the multi
millesecond delay.

thanks,

 - Joel