Subject: Re: [PATCH] gettimeofday stability

From: Karim Yaghmour <[email-masked]>
Date: Wed, 11 Apr 2001 16:09:11 -0400

Gabriel Paubert wrote:
>
> Finally, if you _really_ run into this problem, given the delay between
> the decrementer interrupt and the update of tb_last_stamp, it means that
> you likely introduce uncertainties of several microseconds. I forgot also
> to mention that, to complicate matters, you have to check CPU type before
> you touch the TB (601 versus all others).
>

While porting the Linux Trace Toolkit to PPC I noticed a problem
that may be explained by the symptoms described. The way it works
is that LTT uses do_gettimeofday() to stamp the events that occur.
Occasionnaly, a trace would contain entries where the timestamp
will jump (from one event to the next) of approximately 4295 seconds.
Later on, this would come back to a "normal" value. And the
4295 seconds are 2^32/1000000. Hence the underflow.

This has been noticed with both 2.2.x and 2.4.x kernels.

===================================================
                 Karim Yaghmour
               [email-masked]
      Embedded and Real-Time Linux Expert
===================================================

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