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SubjectRe: [PATCH v2 2/5]: trace_event: export HZ in timer's tracepoint format

* Xiao Guangrong <xiaoguangrong@cn.fujitsu.com> wrote:

> Export HZ in timer's tracepoint, we can use it to get
> TIMER/ITIMER_VIRTUAL/ITIMER_PROF exact latency and it's
> suggested by Ingo
>
> Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
> ---
> include/trace/events/timer.h | 5 ++++-
> 1 files changed, 4 insertions(+), 1 deletions(-)
>
> diff --git a/include/trace/events/timer.h b/include/trace/events/timer.h
> index 13ec15a..7749ae5 100644
> --- a/include/trace/events/timer.h
> +++ b/include/trace/events/timer.h
> @@ -74,14 +74,17 @@ TRACE_EVENT(timer_expire_entry,
> TP_STRUCT__entry(
> __field( void *, timer )
> __field( unsigned long, now )
> + __field( int, hz )
> ),
>
> TP_fast_assign(
> __entry->timer = timer;
> __entry->now = jiffies;
> + __entry->hz = HZ;
> ),
>
> - TP_printk("timer=%p now=%lu", __entry->timer, __entry->now)
> + TP_printk("timer=%p now=%lu HZ=%d", __entry->timer, __entry->now,
> + __entry->hz)
> );

I think we can do something slightly different and more efficient: just create
a new timer event to report the value of HZ.

That way we dont clutter the timer_expire_entry record format with a
repetitive HZ field. It's an extra 4 bytes overhead: that has to be written,
passed along, copied and thrown away in 99.9999% of the cases - such overhead
should be avoided.

If you created a special timer_params event, which would produce precisely one
event when triggered via say a new perf ioctl. I.e. add something like this to
perf_event.h:

#define PERF_EVENT_IOC_INJECT _IOW('$', 7, __u64)

and add code to kernel/perf_event.c's perf_ioctl() function that takes that
__u64 parameter as an event ID and injects an 'artificial' event.

Such a new feature would be useful for other things as well: backtesting rare
events, injecting other types of 'parameter/query events', etc.

There might be more details to this, but it would be a useful scheme IMO - and
it would still integrate nicely with the whole ftrace event enumeration scheme
so tooling support would be easier.

What do you think?

Ingo


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