Re: [PATCH v2 3/3] X86: Add a thread cpu time implementation to vDSO

[Chris Mason <clm@fb.com>]

On Fri, Dec 19, 2014 at 11:48 AM, Andy Lutomirski <luto@amacapital.net> 
wrote:
> On Fri, Dec 19, 2014 at 3:23 AM, Peter Zijlstra 
> <peterz@infradead.org> wrote:
>>  On Thu, Dec 18, 2014 at 04:22:59PM -0800, Andy Lutomirski wrote:
>>>  Bad news: this patch is incorrect, I think.  Take a look at
>>>  update_rq_clock -- it does fancy things involving irq time and
>>>  paravirt steal time.  So this patch could result in extremely
>>>  non-monotonic results.
>> 
>>  Yeah, I'm not sure how (and if) we could make all that work :/
> 
> I obviously can't comment on what Facebook needs, but if I were
> rigging something up to profile my own code*, I'd want a count of
> elapsed time, including user, system, and probably interrupt as well.
> I would probably not want to count time during which I'm not
> scheduled, and I would also probably not want to count steal time.
> The latter makes any implementation kind of nasty.
> 
> The API presumably doesn't need to be any particular clock id for
> clock_gettime, and it may not even need to be clock_gettime at all.
> 
> Is perf self-monitoring good enough for this?  If not, can we make it
> good enough?
> 
> * I do this today using CLOCK_MONOTONIC

The clock_gettime calls are used for a wide variety of things, but 
usually they are trying to instrument how much CPU the application is 
using.  So for example with the HHVM interpreter they have a ratio of 
the number of hhvm instructions they were able to execute in N seconds 
of cputime.  This gets used to optimize the HHVM implementation and can 
be used as a push blocking counter (code can't go in if it makes it 
slower).

Wall time isn't a great representation of this because it includes 
factors that might be outside a given HHVM patch, but it sounds like 
we're saying almost the same thing.

I'm not familiar with the perf self monitoring?

-chris