linux-kernel - Re: [RFC PATCH] perf/stat: Add --disable-hwdt

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Message-ID: <20170207072512.GB18584@gmail.com>
Date:   Tue, 7 Feb 2017 08:25:12 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Robert Richter <rric@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Vince Weaver <vince@...ter.net>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] perf/stat: Add --disable-hwdt

* Borislav Petkov <bp@...en8.de> wrote:

> On Mon, Feb 06, 2017 at 02:18:32PM +0100, Robert Richter wrote:
>
> > Isn't there the danger the previous watchdog state is never restored if for 
> > some reason perf got killed? So maybe have some other task running that 
> > restores it once perf is gone.
> 
> Currently, I'm restoring it in the atexit() sighandler. Isn't that always 
> called?

Normally yes, but it's not guaranteed as atexit() is all user-space, SIGKILL (or 
OOM) or a straight exit (or a crash in the exit handler itself) will cause it to 
not run.

But there's only so much we can do about that, the /proc/sys API is fundamentally 
lossy in that regard. We'd have to add much more involved kernel support to 
guarantee that the watchdog state is restored.

A way to do it would be create a new /proc/sys/kernel/watchdog_disable_file that 
disables that watchdog while it's _open_. When a task exits and the kernel 
automatically closes the file, the watchdog is re-enabled again. (Or the process 
itself can close the file too.)

This method would also nest properly and would handle multi-processes races 
correctly: for example if a script runs perf as root, and root uses 'perf top', 
the two should not race and the hardware watchdog should not end up being 
disabled...

Thanks,

	Ingo