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
