On 23/11/2020 11:05, Vincent Guittot wrote: > On Mon, 23 Nov 2020 at 10:30, Dietmar Eggemann <[email protected]> > wrote: >> >> On 20/11/2020 09:56, Peter Zijlstra wrote: >>> On Fri, Nov 20, 2020 at 08:55:27AM +0100, Peter Zijlstra wrote: >>>> - In saturated scenarios task movement will cause some transient dips, >>>> suppose we have a CPU saturated with 4 tasks, then when we migrate a >>>> task >>>> to an idle CPU, the old CPU will have a 'running' value of 0.75 while >>>> the >>>> new CPU will gain 0.25. This is inevitable and time progression will >>>> correct this. XXX do we still guarantee f_max due to no idle-time? >>> >>> Do we want something like this? Is the 1.5 threshold sane? (it's been too >>> long since I looked at actual numbers here) >> >> Did some tests on a big.little system: >> >> (1) rt-app workload on big CPU: >> >> - task0-3 (runtime/period=4000us/16000us, started with >> 4000us delay to each other) run on CPU1 >> - then task3 migrates to CPU2 and runs there for 64ms >> - then task2 migrates to CPU2 too and both tasks run there >> for another 64ms >> >> ... >> task3-3-1684 [001] 3982.798729: sched_pelt_cfs: cpu=1 path=/ >> load=232890 runnable=3260 util=1011 >> migration/1-14 [001] 3982.798756: sched_migrate_task: comm=task3-3 >> pid=1684 prio=101 orig_cpu=1 dest_cpu=2* >> migration/1-14 [001] 3982.798767: sched_pelt_cfs: cpu=1 path=/ >> load=161374 runnable=2263 util=*700* <-- util dip !!! >> task1-1-1682 [001] 3982.799802: sched_pelt_cfs: cpu=1 path=/ >> load=160988 runnable=2257 util=706 >> ... >> task2-2-1683 [001] 3982.849123: sched_pelt_cfs: cpu=1 path=/ >> load=161124 runnable=2284 util=904 >> task2-2-1683 [001] 3982.851960: sched_pelt_cfs: cpu=1 path=/ >> load=160130 runnable=2271 util=911 >> migration/1-14 [001] 3982.851984: sched_migrate_task: comm=task2-2 >> pid=1683 prio=101 orig_cpu=1 dest_cpu=2** >> migration/1-14 [001] 3982.851995: sched_pelt_cfs: cpu=1 path=/ >> load=88672 runnable=*1257* util=512 <-- runnable below 1536 >> task1-1-1682 [001] 3982.852983: sched_pelt_cfs: cpu=1 path=/ >> load=88321 runnable=1252 util=521 >> ... >> >> >> * task1,2,3 remain on CPU1 and still have to catch up, no idle >> time on CPU1 >> >> ** task 1,2 remain on CPU1, there is idle time on CPU1! >> >> >> (2) rt-app workload on LITTLE CPU (orig cpu_capacity: 446) >> >> - task0-3 (runtime/period=1742us/16000us, started with >> 4000us delay to each other) run on CPU4 >> - then task3 migrates to CPU5 and runs there for 64ms >> - then task2 migrates to CPU5 too and both tasks run there >> for another 64ms >> >> ... >> task1-1-1777 [004] 789.443015: sched_pelt_cfs: cpu=4 path=/ >> load=234718 runnable=3018 util=976 >> migration/4-29 [004] 789.444718: sched_migrate_task: comm=task3-3 >> pid=1779 prio=101 orig_cpu=4 dest_cpu=5* >> migration/4-29 [004] 789.444739: sched_pelt_cfs: cpu=4 path=/ >> load=163543 runnable=2114 util=*778* <--util dip !!! >> task2-2-1778 [004] 789.447013: sched_pelt_cfs: cpu=4 path=/ >> load=163392 runnable=2120 util=777 >> ... >> task1-1-1777 [004] 789.507012: sched_pelt_cfs: cpu=4 path=/ >> load=164482 runnable=2223 util=879 >> migration/4-29 [004] 789.508023: sched_migrate_task: comm=task2-2 >> pid=1778 prio=101 orig_cpu=4 dest_cpu=5** >> migration/4-29 [004] 789.508044: sched_pelt_cfs: cpu=4 path=/ >> load=94099 runnable=*1264* util=611 <-- runnable below 1536 >> task0-0-1776 [004] 789.511011: sched_pelt_cfs: cpu=4 path=/ >> load=93898 runnable=1264 util=622 >> ... >> >> * task1,2,3 remain on CPU1 and still have to catch up, no idle >> time on CPU1 >> >> ** task 1,2 remain on CPU1, no idle time on CPU1 yet. >> >> So for the big CPU, there is idle time and for the LITTLE there >> isn't with runnable below the threshold. > > I'm not sure to catch what you want to highlight with your tests ?
I thought the question was whether 'runnable_avg = 1.5 x SCHED_CAPACITY_SCALE' is a good threshold to decide to drive frequency by runnable_avg or util_avg. [...]
