Hi, We try to find an idle CPU to run the next task, but in case we don't find an idle CPU it is better to pick a CPU which will run the task the soonest, for performance reason.
A CPU which isn't idle but has only SCHED_IDLE activity queued on it should be a good target based on this criteria as any normal fair task will most likely preempt the currently running SCHED_IDLE task immediately. In fact, choosing a SCHED_IDLE CPU over a fully idle one shall give better results as it should be able to run the task sooner than an idle CPU (which requires to be woken up from an idle state). This patchset updates both fast and slow paths with this optimization. Testing is done with the help of rt-app currently and here are the details: - Tested on Octacore Hikey platform (all CPUs change frequency together). - rt-app json [1] creates few tasks and we monitor the scheduling latency for them by looking at "wu_lat" field (usec). - The histograms are created using https://github.com/adkein/textogram: textogram -a 0 -z 1000 -n 10 - the stats are accumulated using: https://github.com/nferraz/st - NOTE: The % values shown don't add up, just look at total numbers instead Test 1: Create 8 CFS tasks (no SCHED_IDLE tasks) without this patchset: 0 - 100 : ################################################## 72% (3688) 100 - 200 : ################ 24% (1253) 200 - 300 : ## 2% (149) 300 - 400 : 0% (22) 400 - 500 : 0% (1) 500 - 600 : 0% (3) 600 - 700 : 0% (1) 700 - 800 : 0% (1) 800 - 900 : 900 - 1000 : 0% (1) >1000 : 0% (17) N min max sum mean stddev 5136 0 2452 535985 104.358 104.585 Test 2: Create 8 CFS tasks and 5 SCHED_IDLE tasks: A. Without sched-idle patchset: 0 - 100 : ################################################## 88% (3102) 100 - 200 : ## 4% (148) 200 - 300 : 1% (41) 300 - 400 : 0% (27) 400 - 500 : 0% (33) 500 - 600 : 0% (32) 600 - 700 : 1% (36) 700 - 800 : 0% (27) 800 - 900 : 0% (19) 900 - 1000 : 0% (26) >1000 : 34% (1218) N min max sum mean stddev 4710 0 67664 5.25956e+06 1116.68 2315.09 B. With sched-idle patchset: 0 - 100 : ################################################## 99% (5042) 100 - 200 : 0% (8) 200 - 300 : 300 - 400 : 400 - 500 : 0% (2) 500 - 600 : 0% (1) 600 - 700 : 700 - 800 : 0% (1) 800 - 900 : 0% (1) 900 - 1000 : >1000 : 0% (40) N min max sum mean stddev 5095 0 7773 523170 102.683 475.482 The mean latency dropped to 10% and the stddev to around 25% with this patchset. @Song: Can you please see if the slowpath changes bring any further improvements in your test case ? V2->V3: - Removed a pointless branch from 1/2 (PeterZ). - Removed the RFC tags as patches are getting in better shape now. - Updated the slow path as well, earlier versions only supported fast paths. - Rebased over latest tip/master, fixed rebase conflicts. - Improved commit logs. -- viresh [1] https://pastebin.com/TMHGGBxD Viresh Kumar (2): sched: Start tracking SCHED_IDLE tasks count in cfs_rq sched/fair: Fallback to sched-idle CPU if idle CPU isn't found kernel/sched/fair.c | 57 ++++++++++++++++++++++++++++++++++---------- kernel/sched/sched.h | 2 ++ 2 files changed, 47 insertions(+), 12 deletions(-) -- 2.21.0.rc0.269.g1a574e7a288b
