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

[Andi Kleen]

On Tue, Dec 27, 2016 at 01:33:46PM -0800, David Carrillo-Cisneros wrote:
> 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.

Do Kan's pmu list patches help? 

https://patchwork.kernel.org/patch/9420035/

> 
> > 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.

So it's already fixed?

How much is the cost with your cache?

> 
> 
> > 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.

Kan's patches above already do that I believe.

> 
> 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.

It would be good to see how close we can get. I assume
there is more potential for optimizations and fast pathing.

-Andi