Messages in this thread | | | Date | Mon, 9 Nov 2020 12:48:13 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2 0/4] perf: Fix perf_event_attr::exclusive rotation |
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On Mon, Nov 02, 2020 at 06:41:43PM -0800, Andi Kleen wrote: > On Mon, Nov 02, 2020 at 03:16:25PM +0100, Peter Zijlstra wrote: > > On Sun, Nov 01, 2020 at 07:52:38PM -0800, Andi Kleen wrote: > > > The main motivation is actually that the "multiple groups" algorithm > > > in perf doesn't work all that great: it has quite a few cases where it > > > starves groups or makes the wrong decisions. That is because it is very > > > difficult (likely NP complete) problem and the kernel takes a lot > > > of short cuts to avoid spending too much time on it. > > > > The event scheduling should be starvation free, except in the presence > > of pinned events. > > > > If you can show starvation without pinned events, it's a bug. > > > > It will also always do equal or better than exclusive mode wrt PMU > > utilization. Again, if it doesn't it's a bug. > > Simple example (I think we've shown that one before): > > (on skylake) > $ cat /proc/sys/kernel/nmi_watchdog > 0 > $ perf stat -e instructions,cycles,frontend_retired.latency_ge_2,frontend_retired.latency_ge_16 -a sleep 2 > > Performance counter stats for 'system wide': > > 654,514,990 instructions # 0.34 insn per cycle (50.67%) > 1,924,297,028 cycles (74.28%) > 21,708,935 frontend_retired.latency_ge_2 (75.01%) > 1,769,952 frontend_retired.latency_ge_16 (24.99%) > > 2.002426541 seconds time elapsed > > The second frontend_retired should be both getting 50% and the fixed events should be getting > 100%. So several events are starved.
*should* how? Also, nothing is 0% so nothing is getting starved.
> Another similar example is trying to schedule the topdown events on Icelake in parallel to other > groups. It works with one extra group, but breaks with two. > > (on icelake) > $ cat /proc/sys/kernel/nmi_watchdog > 0 > $ perf stat -e '{slots,topdown-bad-spec,topdown-be-bound,topdown-fe-bound,topdown-retiring},{branches,branches,branches,branches,branches,branches,branches,branches},{branches,branches,branches,branches,branches,branches,branches,branches}' -a sleep 1 > > Performance counter stats for 'system wide': > > 71,229,087 slots (60.65%) > 5,066,320 topdown-bad-spec # 7.1% bad speculation (60.65%) > 35,080,387 topdown-be-bound # 49.2% backend bound (60.65%) > 22,769,750 topdown-fe-bound # 32.0% frontend bound (60.65%) > 8,336,760 topdown-retiring # 11.7% retiring (60.65%) > 424,584 branches (70.00%) > 424,584 branches (70.00%) > 424,584 branches (70.00%) > 424,584 branches (70.00%) > 424,584 branches (70.00%) > 424,584 branches (70.00%) > 424,584 branches (70.00%) > 424,584 branches (70.00%) > 3,634,075 branches (30.00%) > 3,634,075 branches (30.00%) > 3,634,075 branches (30.00%) > 3,634,075 branches (30.00%) > 3,634,075 branches (30.00%) > 3,634,075 branches (30.00%) > 3,634,075 branches (30.00%) > 3,634,075 branches (30.00%) > > 1.001312511 seconds time elapsed > > A tool using exclusive hopefully will be able to do better than this.
I don't see how, exclusive will always result in equal or worse PMU utilization, never better.
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