linux-kernel - Re: [PATCH 2/3] clear_warn_once: bind a timer to written reset value

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Date:   Mon, 30 Nov 2020 12:17:59 -0500
From:   Paul Gortmaker <paul.gortmaker@...driver.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        John Ogness <john.ogness@...utronix.de>
Subject: Re: [PATCH 2/3] clear_warn_once: bind a timer to written reset value

[Re: [PATCH 2/3] clear_warn_once: bind a timer to written reset value] On 30/11/2020 (Mon 11:20) Steven Rostedt wrote:

> On Thu, 26 Nov 2020 01:30:28 -0500
> Paul Gortmaker <paul.gortmaker@...driver.com> wrote:
> 
> > +++ b/Documentation/admin-guide/clearing-warn-once.rst
> > @@ -7,3 +7,12 @@ echo 1 > /sys/kernel/debug/clear_warn_once
> >  
> >  clears the state and allows the warnings to print once again.
> >  This can be useful after test suite runs to reproduce problems.
> > +
> > +Values greater than one set a timer for a periodic state reset; e.g.
> > +
> > +echo 3600 > /sys/kernel/debug/clear_warn_once
> 
> I wonder if the value should be in minutes and not seconds, otherwise, a
> wrong value could possibly DoS the machine, if you were to write 2 into it.
> If there were a lot of warnings in high frequency events.
> 
> Or is dumping out a bunch of warnings every 2 seconds not be a problem?

It doesn't seem to be a problem - at least in that running a defconfig
build on an otherwise out of the box common distro doesn't seem to trip
any WARN or printk_once events in my testing.  Of course there may be a
use case out there that is doing lots of them, however.

> Anyway, would there ever be a need to have it cleared in less than 1 minute
> intervals?

I don't think so - as I said in another follow up from last week:

https://lore.kernel.org/lkml/20201127174316.GA11748@windriver.com/

I'd also indicated in the above that I'd be fine with adding a minimum
of 1m if people feel better about that.  Also maybe moving the units to
minutes instead of seconds helps implicitly convey the intended use
better -- i.e. "don't be smashing on this every second" -- maybe that
was your point as well - and I'd agree with that.

Paul.
--

> 
> -- Steve
> 
> 
> > +
> > +will establish an hourly state reset, effectively turning WARN_ONCE
> > +into a long period rate-limited warning.