RE: [PATCH] rcu: Direct boosting gp_tasks for strict grace periods

[Zhang, Qiang1]

On Fri, May 13, 2022 at 01:17:16PM +0000, Zhang, Qiang1 wrote:
> 
> On Fri, May 13, 2022 at 08:42:55AM +0800, Zqiang wrote:
> > If the CONFIG_RCU_STRICT_GRACE_PERIOD option is enabled, the normal 
> > grace period will be treated as expedited grace period, the gp_tasks 
> > that block current grace period needs to be boosted unconditionally, 
> > therefore this commit adds Kconfig check in rcu_initiate_boost().
> > 
> > Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
> 
> >Good eyes!  I have queued this for further review and testing, thank you!
> >
> >What sorts of tests did you run on it?
> 
> Hi Paul
> 
> I didn't think of  suitable test method, Can you provide me with a 
> test method to test it, I will be happy to test.
>
>Here is one possibility:
>
>tools/testing/selftests/rcutorture/bin/kvm.sh --allcpus --duration 60 --configs "TREE01 TREE02 TREE03 TREE04 TREE05 TREE07 TREE09 TREE10" --kconfig "CONFIG_NR_CPUS=4 CONFIG_RCU_STRICT_GRACE_PERIOD=y" --trust-make
>
>On a 16-CPU system, this will do eight kernel builds and run about two hours of testing.

Hi Paul

I tested according to the above command, and the results are as follows:

tools/testing/selftests/rcutorture/bin/kvm.sh --allcpus --duration 60 --configs TREE01 TREE02 TREE03 TREE04 TREE05 TREE07 TREE09 TREE10 --kconfig CONFIG_NR_CPUS=4 CONFIG_RCU_STRICT_GRACE_PERIOD=y --trust-make
TREE01 ------- 154236 GPs (42.8433/s) [rcu: g1986553 f0x0 total-gps=496930]
:CONFIG_NR_CPUS=4: improperly set
:CONFIG_RCU_STRICT_GRACE_PERIOD=y: improperly set
TREE02 ------- 410546 GPs (114.041/s) [rcu: g196373325 f0x2 total-gps=49093612] n_max_cbs: 805081
TREE03 ------- 160648 GPs (44.6244/s) [rcu: g128673793 f0x0 total-gps=32168735] n_max_cbs: 646284
TREE04 ------- 347059 GPs (96.4053/s) [rcu: g539425233 f0x2 total-gps=134856594] n_max_cbs: 205907
TREE05 ------- 360973 GPs (100.27/s) [rcu: g77594645 f0x0 total-gps=19398951] n_max_cbs: 31033
:CONFIG_RCU_FANOUT_LEAF=6: improperly set
TREE07 ------- 355903 GPs (98.8619/s) [rcu: g639182469 f0x0 total-gps=159795908] n_max_cbs: 259737
TREE09 ------- 305700 GPs (84.9167/s) [rcu: g9668841 f0x0 total-gps=2417506] n_max_cbs: 2498623
:CONFIG_NR_CPUS=4: improperly set
:CONFIG_RCU_STRICT_GRACE_PERIOD=y: improperly set
TREE10 ------- 220566 GPs (61.2683/s) [rcu: g3456465 f0x0 total-gps=864409] n_max_cbs: 507348
:CONFIG_RCU_STRICT_GRACE_PERIOD=y: improperly set

Thanks
Zqiang

>
>							Thanx, Paul

> Thanks,
> Zqiang
> 
> >
> >As usual, I could not resist the urge to wordsmith, so could you please check the version shown below?
> >
> >							Thanx, Paul
> 
> ----------------------------------------------------------------------
> --
> 
> commit 079e0f894c5d887c678f94332c1fa7287abfd6bc
> Author: Zqiang <qiang1.zhang@xxxxxxxxx>
> Date:   Fri May 13 08:42:55 2022 +0800
> 
>     rcu: Immediately boost preempted readers for strict grace periods
>     
>     The intent of the CONFIG_RCU_STRICT_GRACE_PERIOD Konfig option is to
>     cause normal grace periods to complete quickly in order to better catch
>     errors resulting from improperly leaking pointers from RCU read-side
>     critical sections.  However, kernels built with this option enabled still
>     wait for some hundreds of milliseconds before boosting RCU readers that
>     have been preempted within their current critical section.  The value
>     of this delay is set by the CONFIG_RCU_BOOST_DELAY Kconfig option,
>     which defaults to 500 milliseconds.
>     
>     This commit therefore causes kernels build with strict grace periods
>     to ignore CONFIG_RCU_BOOST_DELAY.  This causes rcu_initiate_boost()
>     to start boosting immediately after all CPUs on a given leaf rcu_node
>     structure have passed through their quiescent states.
>     
>     Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
>     Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> 
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index 
> 99cde4c947699..b4ab952f04ea6 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -1159,7 +1159,8 @@ static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
>  	    (rnp->gp_tasks != NULL &&
>  	     rnp->boost_tasks == NULL &&
>  	     rnp->qsmask == 0 &&
> -	     (!time_after(rnp->boost_time, jiffies) || rcu_state.cbovld))) {
> +	     (!time_after(rnp->boost_time, jiffies) || rcu_state.cbovld ||
> +	      IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD)))) {
>  		if (rnp->exp_tasks == NULL)
>  			WRITE_ONCE(rnp->boost_tasks, rnp->gp_tasks);
>  		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);