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

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Message-Id: <20120621161842.9319cb99.akpm@linux-foundation.org>
Date:	Thu, 21 Jun 2012 16:18:42 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"H. Peter Anvin" <hpa@...or.com>, 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 16:15:58 -0700
Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Thu, 21 Jun 2012 16:10:23 -0700
> "H. Peter Anvin" <hpa@...or.com> wrote:
> 
> > On 06/21/2012 03:51 PM, Andrew Morton wrote:
> > > 
> > > 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?
> > 
> > Time is definitely a signed concept -- it has no beginning or end (well,
> > the Big Bang, but the __110 Myr or so uncertainty of the exact timing of
> > the Big Bang makes it a horridly awkward choice for epoch.)
> > 
> > Now, for some users of time you can inherently guarantee there will
> > never be any references to time before a particular event, e.g. system
> > boot, in which case an unsigned number might make sense, but as a whole
> > I think using a signed type as time_t in nearly all Unix implementation
> > was The Right Thing.
> > 
> 
> So why is ktime_t unsigned?

err, actually, it isn't.  But lots of the APIs to manipulate ktime_t
use u64.  Reason?

I do agree that time quantities should be signed.
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