* Andy Lutomirski <l...@amacapital.net> wrote: > On Jan 31, 2016 11:42 PM, "Ingo Molnar" <mi...@kernel.org> wrote: > > > > > > * r...@redhat.com <r...@redhat.com> wrote: > > > > > (v3: address comments raised by Frederic) > > > > > > Running with nohz_full introduces a fair amount of overhead. > > > Specifically, various things that are usually done from the > > > timer interrupt are now done at syscall, irq, and guest > > > entry and exit times. > > > > > > However, some of the code that is called every single time > > > has only ever worked at jiffy resolution. The code in > > > __acct_update_integrals was also doing some unnecessary > > > calculations. > > > > > > Getting rid of the unnecessary calculations, without > > > changing any of the functionality in __acct_update_integrals > > > gets us about an 11% win. > > > > > > Not calling the time statistics updating code more than > > > once per jiffy, like is done on housekeeping CPUs and on > > > all the CPUs of a non-nohz_full system, shaves off a > > > further 30%. > > > > > > I tested this series with a microbenchmark calling > > > an invalid syscall number ten million times in a row, > > > on a nohz_full cpu. > > > > > > Run times for the microbenchmark: > > > > > > 4.4 3.8 seconds > > > 4.5-rc1 3.7 seconds > > > 4.5-rc1 + first patch 3.3 seconds > > > 4.5-rc1 + first 3 patches 3.1 seconds > > > 4.5-rc1 + all patches 2.3 seconds > > > > Another suggestion (beyond fixing the 32-bit build ;-), could you please > > stick > > your syscall microbenchmark into 'perf bench', so that we have a > > standardized way > > of checking such numbers? > > > > In fact I'd suggest we introduce an entirely new sub-tool for system call > > performance measurement - and this might be the first functionality of it. > > > > I've attached a quick patch that is basically a copy of 'perf bench numa' > > and > > which measures getppid() performance (simple syscall where the result is not > > cached by glibc). > > > > I kept the process, threading and memory allocation bits of numa.c, just in > > case > > we need them to measure more complex syscalls. Maybe we could keep the > > threading > > bits and remove the memory allocation parameters, to simplify the benchmark? > > > > Anyway, this could be a good base to start off on. > > So much code...
Arguably 90% of that should be factored out, as it's now a duplicate between bench/numa.c and bench/syscall.c. Technically, for a minimum benchmark, something like this would already be functional for tools/perf/bench/syscall.c: #include "../perf.h" #include "../util/util.h" #include "../builtin.h" #include "bench.h" static void run_syscall_benchmark(void) { [ .... your benchmark loop as-is ... ] } int bench_syscall(int argc __maybe_unused, const char **argv __maybe_unused, const char *prefix __maybe_unused) { run_syscall_benchmark(); switch (bench_format) { case BENCH_FORMAT_DEFAULT: printf("print results in human-readable format\n"); break; case BENCH_FORMAT_SIMPLE: printf("print results in machine-parseable format\n"); break; default: BUG_ON(1); } return 0; } Plus the small amount of glue for bench_sycall() I sent in the first patch. Completely untested. If the loop is long enough then even without any timing measurement this would be usable via: perf stat --null --repeat 10 perf bench syscall as the 'perf stat' will do the timing and statistics. > I'll try to take a look this week. It shouldn't be so hard to port my > rdpmc-based widget over to this. Sounds great to me! Thanks, Ingo