linux-kernel - Re: [tip:perf/core] perf/x86: Add generic Intel uncore PMU support

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Date:	Thu, 21 Jun 2012 15:51:59 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org,
	a.p.zijlstra@...llo.nl, torvalds@...ux-foundation.org,
	zheng.z.yan@...el.com, tglx@...utronix.de,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:perf/core] perf/x86: Add generic Intel uncore PMU support

On Thu, 21 Jun 2012 15:47:49 -0700
"H. Peter Anvin" <hpa@...or.com> wrote:

> On 06/21/2012 03:43 PM, Andrew Morton wrote:
> > 
> > Regardless of that, we have some head-scratching to do:
> > 
> > 
> > #define UNCORE_PMU_HRTIMER_INTERVAL       (60 * NSEC_PER_SEC)
> > 
> > and
> > 
> > #define NSEC_PER_SEC    1000000000L
> > 
> > and 60 billion doesn't fit in 32 bits.  So do we fix the
> > perf_event_intel_uncore.c callsites?  Or do we fix the
> > UNCORE_PMU_HRTIMER_INTERVAL definition?  Or do we fix the NSEC_PER_SEC
> > definition?
> > 
> > I'm thinking perhaps the latter.  What *is* the type of a nanosecond in
> > Linux?  include/linux/ktime.h is pretty insistent that it is u64.  If
> > so, NSEC_PER_SEC should logically have type ULL.  But changing both its
> > size and signedness is a pretty big change.
> 
> We could change the size only.  The range from 9223372036.854775808 to
> 18446744073.709551615 seconds (292-584 years) isn't really that significant.
> 

What *is* significant is the effect of a signedness change upon
arithmetic, conversions, warnings, etc.  And whether such a change
might actually introduce bugs.


Back away and ask the broader questions: why did ktime_t choose
unsigned?  Is time a signed concept?  What is the right thing to do
here, from a long-term design perspective?
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