linux-kernel - Re: [PATCH] clocksource: Skip watchdog check for large watchdog intervals

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Message-ID: <20240104163050.GC3303@incl>
Date: Thu, 4 Jan 2024 17:30:50 +0100
From: Jiri Wiesner <jwiesner@...e.de>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: linux-kernel@...r.kernel.org, John Stultz <jstultz@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Stephen Boyd <sboyd@...nel.org>, Feng Tang <feng.tang@...el.com>
Subject: Re: [PATCH] clocksource: Skip watchdog check for large watchdog
 intervals

On Wed, Jan 03, 2024 at 02:08:08PM -0800, Paul E. McKenney wrote:
> I believe that there were concerns about a similar approach in the case
> where the jiffies counter is the clocksource

I ran a few simple tests on a 2 NUMA node Intel machine and found nothing 
so far. I tried booting with clocksource=jiffies and I changed the 
"nr_online_nodes <= 4" check in tsc_clocksource_as_watchdog() to enable 
the watchdog on my machine. I have a debugging module that monitors 
clocksource and watchdog reads in clocksource_watchdog() with kprobes. I 
see the cs/wd reads executed roughly every 0.5 second, as expected. When 
the machine is idle the average watchdog interval is 501.61 milliseconds 
(+-15.57 ms, with a minimum of 477.07 ms and a maximum of 517.93 ms). The 
result is similar when the CPUs of the machine are fully saturated with 
netperf processes. I also tried booting with clocksource=jiffies and 
tsc=watchdog. The watchdog interval was similar to the previous test.

AFAIK, the jiffies clocksource does get checked by the watchdog itself. 
And with that, I have run out of ideas.
-- 
Jiri Wiesner
SUSE Labs