This patch kit optimizes perf stat for a large number of events on systems with many CPUs and PMUs.
Some profiling shows that the most overhead is doing IPIs to all the target CPUs. We can optimize this by using sched_setaffinity to set the affinity to a target CPU once and then doing the perf operation for all events on that CPU. This requires some restructuring, but cuts the set up time quite a bit. In theory we could go further by parallelizing these setups too, but that would be much more complicated and for now just batching it per CPU seems to be sufficient. At some point with many more cores parallelization or a better bulk perf setup API might be needed though. In addition perf does a lot of redundant /sys accesses with many PMUs, which can be also expensve. This is also optimized. On a large test case (>700 events with many weak groups) on a 94 CPU system I go from real 0m8.607s user 0m0.550s sys 0m8.041s to real 0m3.269s user 0m0.760s sys 0m1.694s so shaving ~6 seconds of system time, at slightly more cost in perf stat itself. On a 4 socket system with the savings are more dramatic: real 0m15.641s user 0m0.873s sys 0m14.729s to real 0m4.493s user 0m1.578s sys 0m2.444s so 11s difference in the user visible set up time. Also available in git://git.kernel.org/pub/scm/linux/kernel/git/ak/linux-misc perf/stat-scale-3 v1: Initial post. -Andi
