linux-kernel - [PATCH v5 02/18] watchdog/perf: More properly prevent false positives with turbo modes

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Message-ID: <20230519101840.v5.2.I843b0d1de3e096ba111a179f3adb16d576bef5c7@changeid>
Date:   Fri, 19 May 2023 10:18:26 -0700
From:   Douglas Anderson <dianders@...omium.org>
To:     Petr Mladek <pmladek@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Matthias Kaehlcke <mka@...omium.org>,
        kgdb-bugreport@...ts.sourceforge.net,
        Stephane Eranian <eranian@...gle.com>, mpe@...erman.id.au,
        Tzung-Bi Shih <tzungbi@...omium.org>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Mark Rutland <mark.rutland@....com>,
        linuxppc-dev@...ts.ozlabs.org, Sumit Garg <sumit.garg@...aro.org>,
        npiggin@...il.com, davem@...emloft.net,
        Marc Zyngier <maz@...nel.org>,
        Stephen Boyd <swboyd@...omium.org>, sparclinux@...r.kernel.org,
        christophe.leroy@...roup.eu,
        Catalin Marinas <catalin.marinas@....com>,
        ravi.v.shankar@...el.com, Randy Dunlap <rdunlap@...radead.org>,
        Pingfan Liu <kernelfans@...il.com>,
        Guenter Roeck <groeck@...omium.org>,
        Lecopzer Chen <lecopzer.chen@...iatek.com>,
        Ian Rogers <irogers@...gle.com>, ito-yuichi@...itsu.com,
        ricardo.neri@...el.com, linux-arm-kernel@...ts.infradead.org,
        linux-perf-users@...r.kernel.org, Will Deacon <will@...nel.org>,
        Chen-Yu Tsai <wens@...e.org>, linux-kernel@...r.kernel.org,
        Masayoshi Mizuma <msys.mizuma@...il.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Douglas Anderson <dianders@...omium.org>
Subject: [PATCH v5 02/18] watchdog/perf: More properly prevent false positives with turbo modes

Currently, in the watchdog_overflow_callback() we first check to see
if the watchdog had been touched and _then_ we handle the workaround
for turbo mode. This order should be reversed.

Specifically, "touching" the hardlockup detector's watchdog should
avoid lockups being detected for one period that should be roughly the
same regardless of whether we're running turbo or not. That means that
we should do the extra accounting for turbo _before_ we look at (and
clear) the global indicating that we've been touched.

NOTE: this fix is made based on code inspection. I am not aware of any
reports where the old code would have generated false positives. That
being said, this order seems more correct and also makes it easier
down the line to share code with the "buddy" hardlockup detector.

Fixes: 7edaeb6841df ("kernel/watchdog: Prevent false positives with turbo modes")
Signed-off-by: Douglas Anderson <dianders@...omium.org>
---

Changes in v5:
- ("More properly prevent false ...") promoted to its own patch for v5.

 kernel/watchdog_hld.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/watchdog_hld.c b/kernel/watchdog_hld.c
index 247bf0b1582c..1e8a49dc956e 100644
--- a/kernel/watchdog_hld.c
+++ b/kernel/watchdog_hld.c
@@ -114,14 +114,14 @@ static void watchdog_overflow_callback(struct perf_event *event,
 	/* Ensure the watchdog never gets throttled */
 	event->hw.interrupts = 0;
 
+	if (!watchdog_check_timestamp())
+		return;
+
 	if (__this_cpu_read(watchdog_nmi_touch) == true) {
 		__this_cpu_write(watchdog_nmi_touch, false);
 		return;
 	}
 
-	if (!watchdog_check_timestamp())
-		return;
-
 	/* check for a hardlockup
 	 * This is done by making sure our timer interrupt
 	 * is incrementing.  The timer interrupt should have
-- 
2.40.1.698.g37aff9b760-goog