On Tue, Feb 7, 2023 at 8:24 AM Frederic Weisbecker <frederic@xxxxxxxxxx> wrote: > > 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. You are referring to rcu_core() -> rcu_check_quiescent_state() -> note_gp_changes() doing the acceleration prior to the rcu_core() -> rcu_report_qs_rdp() call, correct? Ah, but note_gp_changes() has an early return which triggers if either: 1. The rnp spinlock trylock failed. 2. The start of a new grace period was already detected before, so rdp->gp_seq == rnp->gp_seq. So I think it is possible that we are in the middle of a GP, and rcu_core() is called because QS reporting is required for the CPU, and say the current GP started we are in the middle off occurs from the same CPU so rdp->gp_seq == rnp->gp_seq. Now, rcu_core()'s call to note_gp_changes() should return early but its later call to report_qs_rdp() will not accelerate the callback without the code we are commenting here. > 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() Sure, this also seems like a valid reason. > * __note_gp_changes() got called even before from the GP kthread, and callbacks > got enqueued between that and rcu_core(). Agreed. In this case we will take the early return in note_gp_changes() when called from the rcu_core(). So yeah, that was kind of my point as well but slightly different reasoning. Let me know if you disagree with anything I mentioned, though. - Joel
