On Wed, 12 Oct 2016 11:21:14 -0500
Julia Cartwright <julia@xxxxxx> wrote:
> On Wed, Oct 12, 2016 at 11:12:51AM -0400, Luiz Capitulino wrote:
> > Hi,
> >
> > We have the following patch applied to the RT tree:
> >
> > commit a9d3cc781a3306bfa332fa7cb6134b70696058d5
> > Author: Josh Cartwright <joshc@xxxxxx>
> > Date: Tue Oct 27 07:31:53 2015 -0500
> >
> > net: Make synchronize_rcu_expedited() conditional on !RT_FULL
> >
> > However, as noted by Michael, making rcu_normal=1 default in the
> > RT kernel should have the same effect (ie. not calling
> > synchronize_sched_expedited()).
> >
> > So, honest question, is there a reason not to:
> >
> > 1. Drop the patch above
> > 2. Make rcu_normal=1 default?
>
> I think this is probably a cleaner way to fix the problems which
> motivated this patch in the first place. In my defense, the above patch
> predates rcu_normal :).
No need to defend yourself! We debugged this very spike in one of
our kernels that don't have rcu_normal. We decided to do exactly
what you're doing before looking at upstream. Your patch helped
us confirm that we were in the right track.
> Something like this, perhaps?
Looks good, but (honest question) what does it buy us using
rcu_normal_after_boot vs rcu_normal? Is the boot process
improved someway?
As long as we're rcu_normal=1 before launching user-space,
this should be fine.
>
> Julia
>
> -- 8< --
> Subject: [PATCH] rcu: enable rcu_normal_after_boot by default for RT
>
> The forcing of an expedited grace period is an expensive and very
> RT-application unfriendly operation, as it forcibly preempts all running
> tasks on CPUs which are preventing the gp from expiring.
>
> By default, as a policy decision, disable the expediting of grace
> periods (after boot) on configurations which enable PREEMPT_RT_FULL.
>
> Suggested-by: Luiz Capitulino <lcapitulino@xxxxxxxxxx>
> Signed-off-by: Julia Cartwright <julia@xxxxxx>
> ---
> kernel/rcu/update.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> index b40d346..4975917 100644
> --- a/kernel/rcu/update.c
> +++ b/kernel/rcu/update.c
> @@ -63,7 +63,7 @@ MODULE_ALIAS("rcupdate");
> #ifndef CONFIG_TINY_RCU
> module_param(rcu_expedited, int, 0);
> module_param(rcu_normal, int, 0);
> -static int rcu_normal_after_boot;
> +static int rcu_normal_after_boot = IS_ENABLED(CONFIG_PREEMPT_RT_FULL);
> module_param(rcu_normal_after_boot, int, 0);
> #endif /* #ifndef CONFIG_TINY_RCU */
>
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