linux-kernel - RE: [PATCH] rcu/tree: Improve comments in rcu_report_qs

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Message-ID: <PH0PR11MB58800C6FD1C0DDF8EC67DB5DDADA9@PH0PR11MB5880.namprd11.prod.outlook.com>
Date:   Mon, 6 Feb 2023 03:09:35 +0000
From:   "Zhang, Qiang1" <qiang1.zhang@...el.com>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     Frederic Weisbecker <frederic@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        "Mathieu Desnoyers" <mathieu.desnoyers@...icios.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
        "Steven Rostedt" <rostedt@...dmis.org>,
        Boqun Feng <boqun.feng@...il.com>
Subject: RE: [PATCH] rcu/tree: Improve comments in rcu_report_qs_rdp()


>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@intel.com/
>Cc: Qiang Zhang <Qiang1.zhang@...el.com>
>Cc: Frederic Weisbecker <frederic@...nel.org>
>Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
>
>---
> 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()


Does the extra grace period delays means that if not accelerate callback, 
the grace period will take more time to end ? or refers to a delay in the
start time of a new grace period?

Thanks
Zqiang

>+		 * 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
>