linux-kernel - Re: [PATCH] hangcheck-timer is broken on x86

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Message-ID: <20100326214648.GF9984@mail.oracle.com>
Date:	Fri, 26 Mar 2010 14:46:49 -0700
From:	Joel Becker <Joel.Becker@...cle.com>
To:	Yury Polyanskiy <ypolyans@...nceton.edu>
Cc:	linux-kernel@...r.kernel.org, Andrew Morton <akpm@...l.org>,
	john stultz <johnstul@...ibm.com>,
	Jan Glauber <jan.glauber@...ibm.com>
Subject: Re: [PATCH] hangcheck-timer is broken on x86

On Tue, Mar 23, 2010 at 11:36:11PM -0400, Yury Polyanskiy wrote:
> Second, and more importantly, loops_per_jiffy has little to do with the conversion from the
> the time scale of get_cycles() (aka rdtsc) to the time scale of jiffies.

	It used to!  Fundamentally, of course, we didn't have a
monotonic clock everywhere that satisfied hangcheck-timer's needs.  So
we had to use different approaches on different architectures.

> @@ -130,7 +129,9 @@ extern unsigned long long monotonic_clock(void);
>  #else
>  static inline unsigned long long monotonic_clock(void)
>  {
> -	return get_cycles();
> +	struct timespec ts;
> +	getrawmonotonic(&ts);
> +	return timespec_to_ns(&ts);
>  }
>  #endif  /* HAVE_MONOTONIC */

	I have two questions:

1) Does getrawmonotonic() satisfy hangcheck-timer?  What I mean is, will
it always return the wallclock nanoseconds even in the face of CPU speed
changes, suspend, udelay, or any other suspension of kernel operation?
Yes, I know this is a tougher standard than rdtsc(), but this is what
hangcheck-timer wants.  rdtsc() at least satisfied udelay and PCI hangs.

2) If it does satisfy, why not use it for all hangcheck usage instead of
any ifdefs?

Joel

-- 

"The cynics are right nine times out of ten."  
        - H. L. Mencken

Joel Becker
Principal Software Developer
Oracle
E-mail: joel.becker@...cle.com
Phone: (650) 506-8127
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