linux-kernel - RE: MIPS TSC synchronization

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Date:	Wed, 25 Nov 2009 18:20:38 -0800
From:	"Ashwin Tanugula" <ashwin.tanugula@...adcom.com>
To:	"Mathieu Desnoyers" <mathieu.desnoyers@...ymtl.ca>
cc:	"ltt-dev@...ts.casi.polymtl.ca" <ltt-dev@...ts.casi.polymtl.ca>,
	"Ralf Baechle" <ralf@...ux-mips.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: MIPS TSC synchronization

Hi,

Here is the information you asked for.

- Number of CPUs

Two

- Threads per CPUs (seems to be one, given CONFIG_MIPS_MT_SMTC is not
  set in your configuration)

One

- your .config

Attached

Sorry for the late reply.

Thanks,
Ashwin


>
>
>
> > > However, I was only able to view two out of 10 traces generated.
> > >
>
> > These errors seem to be caused by the fact that you run a SMP mips system with frequency scaling (either dynamic freq. scaling., or halting the clock in idle).
>
> > You might wait to try to disable the freq. scaling and see if it works better.
>
>
> Hi Mathieu,
>
> 1)    With the patch on thop of 177, HAVE_UNSYNCHRONIZED_TSC was disabled and test_tsc_synchronization (arch/mips/kerne
> l/smp.c) was never called. So, we never knew if there was any TSC offset between the CPUs.

(adding Ralf and LKML in CC)

test_tsc_synchronization() is introduced by the LTTng tree. So in
mainline you would have never known that there was such a TSC offset
between your cores.

>
>
> 2)    With HAVE_UNSYNCHRONIZED_TSC enabled, TSC offset between the CPUs was  huge.
>
> Boot messages:
>
> checking TSC synchronization across all online CPUs:
> Measured 152179143 cycles TSC offset between CPUs, turning off TSC clock.
>
> I think this happened because of the stub function for synchronise_count_master in arch/mips/include/asm/r4k-timer.h
>
>
> 3)    With HAVE_UNSYNCHRONIZED_TSC and enabled SYNC_R4K, arch/mips/kernel/sync-r4k.c gets compiled and I don't see any offset.
>
>
> Boot Messages:
>
> checking TSC synchronization across all online CPUs: passed.
>
> And I can view my traces on lttv without any problem.
>

Great !

>
>
> I guess arch/mips/kernel/sync-r4k.c is buggy on SMTC.

I see that SYNC_R4K is only selected by MIPS_CMP usually. Maybe a select
for it should be added to your kind of board ?

>
>
> Can you please confirm if this is right or not?

Given I am not a MIPS expert, I added some people in CC. They might help
confirming this if you provide information about:

- Number of CPUs
- Threads per CPUs (seems to be one, given CONFIG_MIPS_MT_SMTC is not
  set in your configuration)
- your .config

Thanks,

Mathieu

>
> Thanks,
> Ashwin
>
>
>

--
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68


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