linux-kernel - [PATCH v2 2/4] clocksource: Reduce the default clocksource

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Date:   Tue, 16 Nov 2021 18:44:24 -0500
From:   Waiman Long <longman@...hat.com>
To:     John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...nel.org>,
        Feng Tang <feng.tang@...el.com>,
        "Paul E. McKenney" <paulmck@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Cassio Neri <cassio.neri@...il.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Waiman Long <longman@...hat.com>
Subject: [PATCH v2 2/4] 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>
---
 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 9725c546a0d4..3ea934b034f7 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 a5c2b130adbf..a7814b543a9b 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;
-- 
2.27.0