linux-kernel - [tip: timers/core] clocksource: Reduce the default clocksource

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Date:   Mon, 10 Jan 2022 13:06:44 -0000
From:   "tip-bot2 for Waiman Long" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Waiman Long <longman@...hat.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: timers/core] clocksource: Reduce the default
 clocksource_watchdog() retries to 2

The following commit has been merged into the timers/core branch of tip:

Commit-ID:     1a5620671a1b6fd9cc08761677d050f1702f910c
Gitweb:        https://git.kernel.org/tip/1a5620671a1b6fd9cc08761677d050f1702f910c
Author:        Waiman Long <longman@...hat.com>
AuthorDate:    Thu, 18 Nov 2021 14:14:37 -05:00
Committer:     Paul E. McKenney <paulmck@...nel.org>
CommitterDate: Tue, 30 Nov 2021 17:22:29 -08:00

clocksource: Reduce the default clocksource_watchdog() retries to 2

With the previous patch, there is an extra watchdog read in each retry.
Now the total number of clocksource reads is increased to 4 per iteration.
In order to avoid increasing the clock skew check overhead, the default
maximum number of retries is reduced from 3 to 2 to maintain the same 12
clocksource reads in the worst case.

Suggested-by: Paul E. McKenney <paulmck@...nel.org>
Signed-off-by: Waiman Long <longman@...hat.com>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 Documentation/admin-guide/kernel-parameters.txt | 4 ++--
 kernel/time/clocksource.c                       | 2 +-
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 9725c54..3ea934b 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -603,8 +603,8 @@
 	clocksource.max_cswd_read_retries= [KNL]
 			Number of clocksource_watchdog() retries due to
 			external delays before the clock will be marked
-			unstable.  Defaults to three retries, that is,
-			four attempts to read the clock under test.
+			unstable.  Defaults to two retries, that is,
+			three attempts to read the clock under test.
 
 	clocksource.verify_n_cpus= [KNL]
 			Limit the number of CPUs checked for clocksources
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index bcad1a1..b7e52a6 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -199,7 +199,7 @@ void clocksource_mark_unstable(struct clocksource *cs)
 	spin_unlock_irqrestore(&watchdog_lock, flags);
 }
 
-ulong max_cswd_read_retries = 3;
+ulong max_cswd_read_retries = 2;
 module_param(max_cswd_read_retries, ulong, 0644);
 EXPORT_SYMBOL_GPL(max_cswd_read_retries);
 static int verify_n_cpus = 8;