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

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Message-ID: <ZGyIxvKhVjr-L8-I@alley>
Date:   Tue, 23 May 2023 11:35:02 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Douglas Anderson <dianders@...omium.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        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>
Subject: Re: [PATCH v5 02/18] watchdog/perf: More properly prevent false
 positives with turbo modes

On Fri 2023-05-19 10:18:26, Douglas Anderson wrote:
> 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.

The ideal solution would be to reset the turbo-mode-related
variables when the watchdog is touched. And keep checking
watchdog_nmi_touch first.

But this ordering change should be good enough. It causes that
we always check watchdog_nmi_touch when the turbo-more-related
variables are already reset.

> 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>

Reviewed-by: Petr Mladek <pmladek@...e.com>

Best Regards,
Petr