Re: [PATCH v2 1/2] perf: Add sampling of the raw monotonic clock

On Fri, 2014-09-26 at 20:25 +0100, David Ahern wrote:

On 9/26/14, 9:05 AM, Pawel Moll wrote:

quoted

To do the correlation you need both timestamps to be "taken"
simultaneously:

         perf event     user event
        -----O--------------+-------------O------> t_mono
             :              |             :
             :              V             :
        -----O----------------------------O------> t_perf

Of course it's not possible get both values literally at the same time,
but placing them in a atomic context a couple of instructions from each
other still gives pretty good results. The larger this distance is, the

An early patchset on this topic added the realtime clock as an event and 
an ioctl was used to push a sample into the event stream. 

Yeah, I remember. If I remember correctly correctly the pushback was on
a custom event type, right?

Generally speaking I don't mind any solution that we'll get us to the
place both you and I want to be (just being able to time stamp some
performance data in userspace, how difficult can this be! ;-) but I like
the flexibility of an extra sample - one can pick and mix events and
samples at one's leisure.

In that case 
you have wall clock and perf-clock samples taken in the same kernel 
context and about as close together as you can get.

Yep, that's what I was saying - we can't quite get two timestamps at the
*same*, but getting them within a single atomic block of instructions
gives reasonable accuracy.

Thanks!

Pawel