On Sat, Feb 04, 2023 at 02:20:50AM +0000, Joel Fernandes (Google) wrote: > Recent discussion triggered due to a patch linked below, from Qiang, > shed light on the need to accelerate from QS reporting paths. > > Update the comments to capture this piece of knowledge. > > Link: https://lore.kernel.org/all/20230118073014.2020743-1-qiang1.zhang@xxxxxxxxx/ > Cc: Qiang Zhang <Qiang1.zhang@xxxxxxxxx> > Cc: Frederic Weisbecker <frederic@xxxxxxxxxx> > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> > > --- > kernel/rcu/tree.c | 13 ++++++++++++- > 1 file changed, 12 insertions(+), 1 deletion(-) > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 93eb03f8ed99..713eb6ca6902 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -1983,7 +1983,12 @@ rcu_report_qs_rdp(struct rcu_data *rdp) > } else { > /* > * This GP can't end until cpu checks in, so all of our > - * callbacks can be processed during the next GP. > + * callbacks can be processed during the next GP. Do > + * the acceleration from here otherwise there may be extra > + * grace period delays, as any accelerations from rcu_core() > + * or note_gp_changes() may happen only after the GP after the > + * current one has already started. Further, rcu_core() > + * only accelerates if RCU is idle (no GP in progress). Actually note_gp_changes() should take care of that. My gut feeling is that the acceleration in rcu_report_qs_rdp() only stands for: * callbacks that may be enqueued from an IRQ firing during the small window between the RNP unlock in note_gp_changes() and the RNP lock in rcu_report_qs_rdp() * __note_gp_changes() got called even before from the GP kthread, and callbacks got enqueued between that and rcu_core(). Thanks.
