Re: [LTP] Re: LTP - gettimeofday02 FAIL

Re: [LTP] Re: LTP - gettimeofday02 FAIL

[Andi Kleen]

Paul Larson <plars@linuxtestproject.org> writes:

> This has been noticed, I've posted to lkml about it.  The only person
> who replied to me seems to be suggesting it is a hardware issue, but I
> can't believe it is impossible to work around.

It is very hard to solve properly and efficiently. When you search the
list archives you will find long threads about the problem
(search for "TSC" and gettimeofday and perhaps HPET or cyclone). Last one 
was one or two weeks ago.

The problem has been there always in some way in linux, now it is just
exposed in LTP because it tests for it.

-Andi

Re: [LTP] Re: LTP - gettimeofday02 FAIL

[Andi Kleen]

On Tue, Nov 19, 2002 at 03:27:31PM +0000, Alan Cox wrote:
> On Tue, 2002-11-19 at 14:24, Andi Kleen wrote:
> > It is very hard to solve properly and efficiently. When you search the
> > list archives you will find long threads about the problem
> > (search for "TSC" and gettimeofday and perhaps HPET or cyclone). Last one 
> > was one or two weeks ago.
> > 
> > The problem has been there always in some way in linux, now it is just
> > exposed in LTP because it tests for it.
> 
> Dual ppro boxes normally run with a locked synchronous TSC clock. That
> suggests the newer code broke stuff. It may also be due to the bug in

It is because of the HZ=1000. See Jim Houston's mail on the same topic,
he analyzed the failure.

Basically the current code cannot handle missing ticks properly on SMP and with
the new 1ms tick it is much more likely that a timer interrupt gets lost.

-Andi

Re: [LTP] Re: LTP - gettimeofday02 FAIL

[Alan Cox]

On Tue, 2002-11-19 at 14:24, Andi Kleen wrote:
> It is very hard to solve properly and efficiently. When you search the
> list archives you will find long threads about the problem
> (search for "TSC" and gettimeofday and perhaps HPET or cyclone). Last one 
> was one or two weeks ago.
> 
> The problem has been there always in some way in linux, now it is just
> exposed in LTP because it tests for it.

Dual ppro boxes normally run with a locked synchronous TSC clock. That
suggests the newer code broke stuff. It may also be due to the bug in
the 2.5 timer handling code (missing delays on timer reads, incorrect
assumption that the timer never reads its limit value during the timer 
switch back to zero)

Re: [LTP] Re: LTP - gettimeofday02 FAIL

[Andi Kleen]

On Tue, Nov 19, 2002 at 04:12:23PM +0000, Alan Cox wrote:
> On Tue, 2002-11-19 at 15:10, Andi Kleen wrote:
> > It is because of the HZ=1000. See Jim Houston's mail on the same topic,
> > he analyzed the failure.
> > 
> > Basically the current code cannot handle missing ticks properly on SMP and with
> > the new 1ms tick it is much more likely that a timer interrupt gets lost.
> 
> Ok so add that to the other existing 2.5 timer handling bugs 8(

2.4 has the same problem, it is just much less likely to hit because of HZ=100

-Andi

Re: [LTP] Re: LTP - gettimeofday02 FAIL

[Alan Cox]

On Tue, 2002-11-19 at 15:10, Andi Kleen wrote:
> It is because of the HZ=1000. See Jim Houston's mail on the same topic,
> he analyzed the failure.
> 
> Basically the current code cannot handle missing ticks properly on SMP and with
> the new 1ms tick it is much more likely that a timer interrupt gets lost.

Ok so add that to the other existing 2.5 timer handling bugs 8(

Re: [LTP] Re: LTP - gettimeofday02 FAIL

[Paul Larson]

[-- Attachment #1: Type: text/plain, Size: 622 bytes --]

On Tue, 2002-11-19 at 08:02, Dave Jones wrote:
> On Tue, Nov 19, 2002 at 07:37:23AM -0600, Paul Larson wrote:
>  > > I just tried gettimeofday02 on an old pentium-pro dual processor, and yes
>  > > the time goes backwards with a 2.5.48 kernel.
>  > This has been noticed, I've posted to lkml about it.  The only person
>  > who replied to me seems to be suggesting it is a hardware issue, but I
>  > can't believe it is impossible to work around.
> 
> Especially if earlier kernels got it right..
This is bug #100 in bugme if anyone wants to track it. 
http://bugme.osdl.org/show_bug.cgi?id=100

-Paul Larson

[-- Attachment #2: This is a digitally signed message part --]
[-- Type: application/pgp-signature, Size: 240 bytes --]

Re: [LTP] Re: LTP - gettimeofday02 FAIL

[Dave Jones]

On Tue, Nov 19, 2002 at 07:37:23AM -0600, Paul Larson wrote:
 > > I just tried gettimeofday02 on an old pentium-pro dual processor, and yes
 > > the time goes backwards with a 2.5.48 kernel.
 > This has been noticed, I've posted to lkml about it.  The only person
 > who replied to me seems to be suggesting it is a hardware issue, but I
 > can't believe it is impossible to work around.

Especially if earlier kernels got it right..

		Dave

-- 
| Dave Jones.        http://www.codemonkey.org.uk
| SuSE Labs

Re: [LTP] Re: LTP - gettimeofday02 FAIL

[Paul Larson]

[-- Attachment #1: Type: text/plain, Size: 406 bytes --]

On Mon, 2002-11-18 at 19:27, Jim Houston wrote:
> 
> Hi Everyone,
> 
> I just tried gettimeofday02 on an old pentium-pro dual processor, and yes
> the time goes backwards with a 2.5.48 kernel.
This has been noticed, I've posted to lkml about it.  The only person
who replied to me seems to be suggesting it is a hardware issue, but I
can't believe it is impossible to work around.

-Paul Larson

[-- Attachment #2: This is a digitally signed message part --]
[-- Type: application/pgp-signature, Size: 240 bytes --]