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.
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.
-Andi
