linux-kernel - Re: Final: Add 32 bit VDSO time function support

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Date:	Sun, 02 Mar 2014 09:01:12 +0100
From:	Stefani Seibold <stefani@...bold.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Greg KH <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	John Stultz <john.stultz@...aro.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	andriy.shevchenko@...ux.intel.com, Martin.Runge@...de-schwarz.com,
	Andreas.Brief@...de-schwarz.com
Subject: Re: Final: Add 32 bit VDSO time function support

Am Samstag, den 01.03.2014, 14:56 -0800 schrieb H. Peter Anvin:
> On 02/28/2014 06:00 PM, Andy Lutomirski wrote:
> > 
> > This leads to a potentially interesting question: is rdtsc_barrier()
> > actually necessary on UP?  IIRC the point is that, if an
> > rdtsc_barrier(); rdtsc in one thread is "before" (in the sense of
> > being synchronized by some memory operation) an rdtsc_barrier(); rdtsc
> > in another thread, then the first rdtsc needs to return an earlier or
> > equal time to the second one.
> > 
> > I assume that no UP CPU is silly enough to execute two rdtsc
> > instructions out of order relative to each other in the absence of
> > barriers.  So this is a nonissue on UP.
> > 
> > On the other hand, suppose that some code does:
> > 
> > volatile long x = *(something that's not in cache)
> > clock_gettime
> > 
> > I can imagine a modern CPU speculating far enough ahead that the rdtsc
> > happens *before* the cache miss.  This won't cause visible
> > non-monotonicity as far as I can see, but it might annoy people who
> > try to benchmark their code.
> > 
> > Note: actually making this change might be a bit tricky.  I don't know
> > if the alternatives code is smart enough.
> > 
> 
> Let's put it this way... this is at best a third-order optimization...
> let's not worry about it right now.
> 

IMHO it is the behaviour that most developer expect. It would a bad idea
to get a time value before the previous operations are not finished. In
some use case this will result in a fail.

Imagine a HW where two designated register can only consecutively
accessed after a given time period is elapsed. This would be normally
done by a busy loop for very short periods. It would be okay when the
time period to wait is exceeded, but will maybe fail when the wait time
is to short.

- Stefani


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