Re: [PATCH 01/14] x86/cqm: Intel Resource Monitoring Documentation

[David Carrillo-Cisneros <davidcc@google.com>]

On Tue, Dec 27, 2016 at 12:00 PM, Andi Kleen <andi@firstfloor.org> wrote:
> Shivappa Vikas <vikas.shivappa@intel.com> writes:
>>
>> Ok , looks like the interface  is the problem. Will try to fix
>> this. We are just trying to have a light weight monitoring
>> option so that its reasonable to monitor for a
>> very long time (like lifetime of process etc). Mainly to not have all
>> the perf scheduling overhead.
>
> That seems like an odd reason to define a completely new user interface.
> This is to avoid one MSR write for a RMID change per context switch
> in/out cgroup or is it other code too?
>
> Is there some number you can put to the overhead?

I obtained some timing by manually instrumenting the kernel in a Haswell EP.

When using one intel_cmt/llc_occupancy/ cgroup perf_event in one CPU, the
avg time to do __perf_event_task_sched_out + __perf_event_task_sched_in is
~1170ns

most of the time is spend in cgroup ctx switch (~1120ns) .

When using continuous monitoring in CQM driver, the avg time to
find the rmid to write inside of pqr_context switch  is ~16ns

Note that this excludes the MSR write. It's only the overhead of
finding the RMID
to write in PQR_ASSOC. Both paths call the same routine to find the
RMID, so there are
about 1100 ns of overhead in perf_cgroup_switch. By inspection I assume most
of it comes from iterating over the pmu list.

> Or is there some other overhead other than the MSR write
> you're concerned about?

No, that problem is solved with the PQR software cache introduced in the series.


> Perhaps some optimization could be done in the code to make it faster,
> then the new interface wouldn't be needed.

There are some. One in my list is to create a list of pmus with at
least one cgroup event
and use it to iterate over in perf_cgroup_switch, instead of using the
"pmus" list.
The pmus list has grown a lot recently with the addition of all the uncore pmus.

Despite this optimization, it's unlikely that the whole sched_out +
sched_in gets that
close to the 15 ns of the non perf_event approach.

Please note that context switch time for llc_occupancy events has more
impact than for
other events because in order to obtain reliable measurements, the
RMID switch must
be active _all_ the time, not only while the event is read.

>
> FWIW there are some pending changes to context switch that will
> eliminate at least one common MSR write [1]. If that was fixed
> you could do the RMID MSR write "for free"

That may save the need for the PQR software cache in this series, but
won't speed up
the context switch.

Thanks,
David