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

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Date:   Tue, 7 Feb 2017 11:54:12 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Ingo Molnar <mingo@...nel.org>
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

On Tue, Feb 07, 2017 at 08:25:12AM +0100, Ingo Molnar wrote:
> 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.

So I think doing all this is meh but I guess I probably should do it
just so that we're thorough.

> 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...

Hmm, so I don't like the aspect of adding a /proc file just for that.

Can we do something with sys_perf_event_open(..., flags) instead and
pass in a new flag that says:

PERF_FLAG_SHOO_COUNTER_USERS

or so which would go and turn off HW WDT (and possibly future things
using counters) while we're running a session?

The name should be generic enough so that we can use it for future
temporary disabling of things while a perf session runs.

Then on perf's exit path - I see there are a bunch of _destroy() things
being called when events are disappearing - we'd reenable stuff again
based on that flag.

This way we're clean in userspace and have the maximum control over
everything since we're in the kernel.

>From a quick staring, there's PERF_FLAG_FD_CLOEXEC which is a good
example for something like that. It doesn't do what I'd like to do but I
think I should model this in a similar fashion.

Thoughts?

-- 
Regards/Gruss,
    Boris.

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