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SubjectRe: [PATCH 1/2] ftrace: single CPU tracers use CPU clock

* Steven Rostedt <rostedt@goodmis.org> wrote:

> > other tracers care too - for example to have the right chronology of
> > trace events. The scheduler cares too. What kind of worst-case
> > cross-CPU effects have you observed?
>
> 1: bash-3498 [01] 5459.824565: 0:140:R + 7971:120:R
> 2: <idle>-0 [00] 5459.824836: 0:140:R ==> 7971:120:R
> 3: bash-3498 [01] 5459.824984: 3498:120:S ==> 0:140:R
> 4: <idle>-0 [01] 5459.825342: 0:140:R ==> 7971:120:R
> 5: ls-7971 [00] 5459.825380: 7971:120:R + 3: 0:S
> 6: ls-7971 [00] 5459.825384: 7971:120:R ==> 3: 0:R
> 7: migration/0-3 [00] 5459.825401: 3: 0:S ==> 0:140:R
> 8: ls-7971 [01] 5459.825565: 7971:120:R + 598:115:S
>
> The above was from my tutorial at OLS. I added the annotated numbers
> at the beginning of the line. This is the sched_switch tracer.

so it's off by 60 usecs. That matters to the chronology of SMP events,
and to the irqsoff and preemptoff tracers - if we update the clock
incorrectly - i.e. if we modify the clock across CPUs so that a running
CPU can see an 'involuntary' jump in local time.

that should be fixed then - and all tracers (and the scheduler) will
improve - instead of having this special-case.

Ingo


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