Re: [PATCH rcu 3/3] rcu: Force quiescent states only for ongoing grace period

[Joel Fernandes]

On Tue, Dec 12, 2023 at 12:40 PM Neeraj Upadhyay (AMD)
<neeraj.iitr10@xxxxxxxxx> wrote:
>
> From: Zqiang <qiang.zhang1211@xxxxxxxxx>
>
> If an rcutorture test scenario creates an fqs_task kthread, it will
> periodically invoke rcu_force_quiescent_state() in order to start
> force-quiescent-state (FQS) operations.  However, an FQS operation
> will be started even if there is no RCU grace period in progress.
> Although testing FQS operations startup when there is no grace period in
> progress is necessary, it need not happen all that often.  This commit
> therefore causes rcu_force_quiescent_state() to take an early exit
> if there is no grace period in progress.
>
> Note that there will still be attempts to start an FQS scan in the
> absence of a grace period because the grace period might end right
> after the rcu_force_quiescent_state() function's check.  In actual
> testing, this happens about once every ten minutes, which should
> provide adequate testing.
>
> Signed-off-by: Zqiang <qiang.zhang1211@xxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.iitr10@xxxxxxxxx>
> ---
>  kernel/rcu/tree.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 3ac3c846105f..1ae851777806 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2338,6 +2338,8 @@ void rcu_force_quiescent_state(void)
>         struct rcu_node *rnp;
>         struct rcu_node *rnp_old = NULL;
>
> +       if (!rcu_gp_in_progress())
> +               return;

Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>

(Probably no need to resend with the tag, just update the patch in the PR).

Thanks.