On 2022-02-01 16:29:35 [+0100], Michal Hocko wrote: > > > Sorry, I know that this all is not really related to your work but if > > > the original optimization is solely based on artificial benchmarks then > > > I would rather drop it and also make your RT patchset easier. > > > > Do you have any real-world benchmark in mind? Like something that is > > already used for testing/ benchmarking and would fit here? > > Anything that even remotely resembles a real allocation heavy workload. So I figured out that build the kernel as user triggers the allocation path in_task() and in_interrupt(). I booted a PREEMPT_NONE kernel and run "perf stat -r 5 b.sh" where b.sh unpacks a kernel and runs a allmodconfig build on /dev/shm. The slow disk should not be a problem. With the optimisation: | Performance counter stats for './b.sh' (5 runs): | | 43.367.405,59 msec task-clock # 30,901 CPUs utilized ( +- 0,01% ) | 7.393.238 context-switches # 170,499 /sec ( +- 0,13% ) | 832.364 cpu-migrations # 19,196 /sec ( +- 0,15% ) | 625.235.644 page-faults # 14,419 K/sec ( +- 0,00% ) | 103.822.081.026.160 cycles # 2,394 GHz ( +- 0,01% ) | 75.392.684.840.822 stalled-cycles-frontend # 72,63% frontend cycles idle ( +- 0,02% ) | 54.971.177.787.990 stalled-cycles-backend # 52,95% backend cycles idle ( +- 0,02% ) | 69.543.893.308.966 instructions # 0,67 insn per cycle | # 1,08 stalled cycles per insn ( +- 0,00% ) | 14.585.269.354.314 branches # 336,357 M/sec ( +- 0,00% ) | 558.029.270.966 branch-misses # 3,83% of all branches ( +- 0,01% ) | | 1403,441 +- 0,466 seconds time elapsed ( +- 0,03% ) With the optimisation disabled: | Performance counter stats for './b.sh' (5 runs): | | 43.354.742,31 msec task-clock # 30,869 CPUs utilized ( +- 0,01% ) | 7.394.210 context-switches # 170,601 /sec ( +- 0,06% ) | 842.835 cpu-migrations # 19,446 /sec ( +- 0,63% ) | 625.242.341 page-faults # 14,426 K/sec ( +- 0,00% ) | 103.791.714.272.978 cycles # 2,395 GHz ( +- 0,01% ) | 75.369.652.256.425 stalled-cycles-frontend # 72,64% frontend cycles idle ( +- 0,01% ) | 54.947.610.706.450 stalled-cycles-backend # 52,96% backend cycles idle ( +- 0,01% ) | 69.529.388.440.691 instructions # 0,67 insn per cycle | # 1,08 stalled cycles per insn ( +- 0,01% ) | 14.584.515.016.870 branches # 336,497 M/sec ( +- 0,00% ) | 557.716.885.609 branch-misses # 3,82% of all branches ( +- 0,02% ) | | 1404,47 +- 1,05 seconds time elapsed ( +- 0,08% ) I'm still open to a more specific test ;) Sebastian
