Re: More weird latency trace output (was Re: 2.6.17-rt1)

[Lee Revell]

On Sun, 2006-06-25 at 00:12 +0200, Ingo Molnar wrote:
> * Lee Revell <rlrevell@xxxxxxxxxxx> wrote:
> 
> > On Thu, 2006-06-22 at 21:24 -0400, Lee Revell wrote:
> > > On Wed, 2006-06-21 at 22:51 -0400, Lee Revell wrote:
> > > > How can the latency tracer be reporting 1898us max latency while the
> > > > trace is of a 12us latency?  This must be a bug, correct?
> > > 
> > > I've found the bug.  The latency tracer uses get_cycles() for
> > > timestamping, which uses rdtsc, which is unusable for timing on dual
> > > core AMD64 machines due to the well known "unsynced TSCs" hardware bug.
> > > 
> > > Would a patch to convert the latency tracer to use gettimeofday() be
> > > acceptable?
> > 
> > OK, I tried that and it oopses on boot - presumably the latency tracer 
> > runs before clocksource infrastructure is initialized.
> > 
> > Does anyone have any suggestions at all as to what a proper solution 
> > would look like?  Is no one interested in this problem?
> 
> does it get better if you boot with idle=poll? [that could work around 
> the rdtsc drifting problem] Calling gettimeofday() from within the 
> tracer is close to impossible - way too many opportunities for 
> recursion. It's also pretty slow that way.

Probably, but that seems like an extremely heavy solution.  Won't it
turn my box into a space heater?

I'm thinking of replacing get_cycles() with low level code to just read
the PM timer.  I can deal with the slowness, this is just a debug
feature after all.

It seems that there might be a solution involving per CPU data, but I'm
not nearly a good enough kernel hacker to attempt that.

(I hope AMD fixes this issue in a future generation of CPUs.  At least,
they seem to acknowledge that it's a bug...)

Lee

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