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). * * NOCB kthreads have their own way to deal with that... */ @@ -1993,6 +1998,12 @@ rcu_report_qs_rdp(struct rcu_data *rdp) /* * ...but NOCB kthreads may miss or delay callbacks acceleration * if in the middle of a (de-)offloading process. + * + * Such missed acceleration may cause the callbacks to + * be stranded until RCU is fully de-offloaded, as + * acceleration from rcu_core() and note_gp_changes() + * cannot happen for fully/partially offloaded mode due + * to ordering dependency between rnp lock and nocb_lock. */ needacc = true; } -- 2.39.1.519.gcb327c4b5f-goog
