linux-kernel - Re: Unified tracing buffer

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Date:	Tue, 23 Sep 2008 10:49:34 -0400
From:	Masami Hiramatsu <mhiramat@...hat.com>
To:	Martin Bligh <mbligh@...gle.com>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	Steven Rostedt <rostedt@...dmis.org>, darren@...art.com,
	"Frank Ch. Eigler" <fche@...hat.com>,
	systemtap-ml <systemtap@...rces.redhat.com>
Subject: Re: Unified tracing buffer

Hi Martin,

Martin Bligh wrote:
>>> But perhaps it would be better if we started with a discussion of which
>>> platforms can't do global timestamps, and why not? I know some of them
>>> are fixable, but perhaps not all.
>> For example, my laptop (this machine/Core2Duo) doesn't return correct TSC. :-(
> 
> Can you define incorrect for me (in this case)?

On my laptop, TSC is disabled at boot time.

$ dmesg | grep TSC
checking TSC synchronization [CPU#0 -> CPU#1]:
Measured 4246549092 cycles TSC warp between CPUs, turning off TSC clock.
Marking TSC unstable due to: check_tsc_sync_source failed.

$ cat /proc/cpuinfo
processor	: 0
vendor_id	: GenuineIntel
cpu family	: 6
model		: 15
model name	: Intel(R) Core(TM)2 CPU         T7200  @ 2.00GHz
stepping	: 6
cpu MHz		: 1000.000
cache size	: 4096 KB
physical id	: 0
siblings	: 2
core id		: 0
cpu cores	: 2
apicid		: 0
initial apicid	: 0
fdiv_bug	: no
hlt_bug		: no
f00f_bug	: no
coma_bug	: no
fpu		: yes
fpu_exception	: yes
cpuid level	: 10
wp		: yes
flags		: fpu vme de pse tsc msr pae mce cx8 apic mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe nx lm constant_tsc arch_perfmon pebs bts pni
monitor ds_cpl vmx est tm2 ssse3 cx16 xtpr lahf_lm
bogomips	: 3998.45
clflush size	: 64
power management:

processor	: 1
vendor_id	: GenuineIntel
cpu family	: 6
model		: 15
model name	: Intel(R) Core(TM)2 CPU         T7200  @ 2.00GHz
stepping	: 6
cpu MHz		: 1000.000
cache size	: 4096 KB
physical id	: 0
siblings	: 2
core id		: 1
cpu cores	: 2
apicid		: 1
initial apicid	: 1
fdiv_bug	: no
hlt_bug		: no
f00f_bug	: no
coma_bug	: no
fpu		: yes
fpu_exception	: yes
cpuid level	: 10
wp		: yes
flags		: fpu vme de pse tsc msr pae mce cx8 apic mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe nx lm constant_tsc arch_perfmon pebs bts pni
monitor ds_cpl vmx est tm2 ssse3 cx16 xtpr lahf_lm
bogomips	: 3994.43
clflush size	: 64
power management:

Actually, I had measured TSC drifting and reported to systemtap-bugzilla
http://sources.redhat.com/bugzilla/show_bug.cgi?id=3916#c19

Curiously, I've tested on another Core2Duo laptop, which cpu is same model and
same stepping, but on that laptop I couldn't see TSC drifting.
So I think this might be a product level issue and a rare case...


> We had similar problems with some AMD platforms that we can fix by syncing
> the TSCs on exit_idle, etc.

Hmm, very interesting. :-)

Thank you,

-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America) Inc.
Software Solutions Division

e-mail: mhiramat@...hat.com


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