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Date: Sun, 8 May 2016 08:19:19 +0800 From: Wanpeng Li <kernellwp@...il.com> To: Tomasz Grabiec <tgrabiec@...lladb.com> Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org> Subject: Re: PROBLEM: Scheduling latency not respected in CFS with HRTICK enabled 2016-05-06 23:43 GMT+08:00 Tomasz Grabiec <tgrabiec@...lladb.com>: > Hi, > > System details: > > Kernel: 4.2.0-27-generic #32~14.04.1-Ubuntu SMP Fri Jan 22 15:32:26 > UTC 2016 x86_64 x86_64 x86_64 GNU/Linux > CPU: 2 sockets, 8 cores per socket, 2 threads per core > CPU Model: Intel(R) Xeon(R) CPU E5-2630 v3 @ 2.40GHz > > I was trying to reduce scheduling delays below 1/HZ, which is 4ms > for our kernel. I enabled HRTICK scheduler feature and set > scheduler tunables to: > > sched_latency_ns: 1000000 > sched_min_granularity_ns: 400000 > sched_wakeup_granularity_ns: 400000 > sched_autogroup_enabled: 0 > sched_tunable_scaling: 1 > sched_cfs_bandwidth_slice_us: 5000 > sched_child_runs_first: 0 > sched_migration_cost_ns: 500000 > sched_nr_migrate: 32 > sched_rr_timeslice_ms: 25 > sched_rt_period_us: 1000000 > sched_rt_runtime_us: 950000 > sched_shares_window_ns: 1000000 > sched_time_avg_ms: 1000 > > Note that scheduling latency is set to 1ms and min_granularity to 0.4ms. > > The test case I used was to run two processes competing for CPU > pinned to a single core, like this: > > python -c "import math; math.factorial(100000000)" & > pid1=$! > python -c "import math; math.factorial(100000000)" & > pid2=$! > sleep 1 > sudo taskset -cap 2 $pid1 > sudo taskset -cap 2 $pid2 > > Using perf I've found that scheduling delays do not stay around > 0.5ms as could be expected from scheduling latency, but switch > between modes of 0.5ms and 4ms back and forth. > > For example (perf script): > > python 147208 [002] 3272129.329011: sched:sched_stat_wait: > comm=python pid=147207 delay=504875 [ns] > python 147207 [002] 3272129.329515: sched:sched_stat_wait: > comm=python pid=147208 delay=504096 [ns] > python 147208 [002] 3272129.330019: sched:sched_stat_wait: > comm=python pid=147207 delay=503936 [ns] > python 147207 [002] 3272129.330523: sched:sched_stat_wait: > comm=python pid=147208 delay=503888 [ns] > python 147208 [002] 3272129.331067: sched:sched_stat_wait: > comm=python pid=147207 delay=543902 [ns] > python 147207 [002] 3272129.334047: sched:sched_stat_wait: > comm=python pid=147208 delay=2980114 [ns] > python 147208 [002] 3272129.338048: sched:sched_stat_wait: > comm=python pid=147207 delay=4000695 [ns] > python 147207 [002] 3272129.342051: sched:sched_stat_wait: > comm=python pid=147208 delay=4002851 [ns] > python 147208 [002] 3272129.346055: sched:sched_stat_wait: > comm=python pid=147207 delay=4004720 [ns] > python 147207 [002] 3272129.350058: sched:sched_stat_wait: > comm=python pid=147208 delay=4002403 [ns] > python 147208 [002] 3272129.354064: sched:sched_stat_wait: > comm=python pid=147207 delay=4006037 [ns] > python 147207 [002] 3272129.358064: sched:sched_stat_wait: > comm=python pid=147208 delay=3999730 [ns] > > More detailed trace around such transition point looks like this: > > perf record \ > -e sched:sched_stat_runtime \ > -e sched:sched_stat_wait \ > -e sched:sched_switch \ > -e probe:hrtick \ > -e probe:hrtick_start \ > -e probe:scheduler_tick -C 2 > > python 873 [002] 3273334.927836: probe:hrtick: (ffffffff8109b430) > python 873 [002] 3273334.927837: sched:sched_stat_runtime: > comm=python pid=873 runtime=502915 [ns] vruntime=1549788940847739 [ns] > python 873 [002] 3273334.927837: sched:sched_stat_runtime: > comm=python pid=873 runtime=880 [ns] vruntime=1549788940848619 [ns] > python 873 [002] 3273334.927838: sched:sched_stat_wait: > comm=python pid=874 delay=503795 [ns] > python 873 [002] 3273334.927838: probe:hrtick_start: (ffffffff8109f4f0) > python 873 [002] 3273334.927839: sched:sched_switch: > prev_comm=python prev_pid=873 prev_prio=120 prev_state=R ==> > next_comm=python next_pid=874 next_prio=120 > python 874 [002] 3273334.928233: probe:scheduler_tick: (ffffffff810a1f30) > python 874 [002] 3273334.928233: sched:sched_stat_runtime: > comm=python pid=874 runtime=395992 [ns] vruntime=1549788941220502 [ns] > python 874 [002] 3273334.928236: sched:sched_stat_runtime: > comm=python pid=874 runtime=2278 [ns] vruntime=1549788941222780 [ns] > python 874 [002] 3273334.928238: sched:sched_stat_wait: > comm=kworker/2:0 pid=93149 delay=0 [ns] > python 874 [002] 3273334.928238: probe:hrtick_start: (ffffffff8109f4f0) > python 874 [002] 3273334.928239: sched:sched_switch: > prev_comm=python prev_pid=874 prev_prio=120 prev_state=R ==> > next_comm=kworker/2:0 next_pid=93149 next_prio=120 > kworker/2:0 93149 [002] 3273334.928242: sched:sched_stat_runtime: > comm=kworker/2:0 pid=93149 runtime=5827 [ns] vruntime=1549788940354446 > [ns] > kworker/2:0 93149 [002] 3273334.928242: probe:hrtick_start: (ffffffff8109f4f0) > kworker/2:0 93149 [002] 3273334.928243: sched:sched_stat_wait: > comm=python pid=874 delay=5827 [ns] > kworker/2:0 93149 [002] 3273334.928244: probe:hrtick_start: (ffffffff8109f4f0) > kworker/2:0 93149 [002] 3273334.928244: sched:sched_switch: > prev_comm=kworker/2:0 prev_pid=93149 prev_prio=120 prev_state=S ==> > next_comm=python next_pid=874 next_prio=120 > python 874 [002] 3273334.928745: probe:hrtick: (ffffffff8109b430) > python 874 [002] 3273334.928745: sched:sched_stat_runtime: > comm=python pid=874 runtime=503888 [ns] vruntime=1549788941726668 [ns] > python 874 [002] 3273334.928746: sched:sched_stat_runtime: > comm=python pid=874 runtime=910 [ns] vruntime=1549788941727578 [ns] > python 874 [002] 3273334.928747: sched:sched_stat_wait: > comm=python pid=873 delay=908895 [ns] > python 874 [002] 3273334.928747: probe:hrtick_start: (ffffffff8109f4f0) > python 874 [002] 3273334.928748: sched:sched_switch: > prev_comm=python prev_pid=874 prev_prio=120 prev_state=R ==> > next_comm=python next_pid=873 next_prio=120 > python 873 [002] 3273334.929249: probe:hrtick: (ffffffff8109b430) > python 873 [002] 3273334.929249: sched:sched_stat_runtime: > comm=python pid=873 runtime=502850 [ns] vruntime=1549788941351469 [ns] > python 873 [002] 3273334.929250: sched:sched_stat_runtime: > comm=python pid=873 runtime=918 [ns] vruntime=1549788941352387 [ns] > python 873 [002] 3273334.932236: probe:scheduler_tick: (ffffffff810a1f30) > python 873 [002] 3273334.932237: sched:sched_stat_runtime: > comm=python pid=873 runtime=2986981 [ns] vruntime=1549788944339368 > [ns] > python 873 [002] 3273334.932239: sched:sched_stat_runtime: > comm=python pid=873 runtime=1685 [ns] vruntime=1549788944341053 [ns] > python 873 [002] 3273334.932239: sched:sched_stat_wait: > comm=python pid=874 delay=3492434 [ns] > python 873 [002] 3273334.932240: probe:hrtick_start: (ffffffff8109f4f0) > python 873 [002] 3273334.932240: sched:sched_switch: > prev_comm=python prev_pid=873 prev_prio=120 prev_state=R ==> > next_comm=python next_pid=874 next_prio=120 > python 874 [002] 3273334.932741: probe:hrtick: (ffffffff8109b430) > python 874 [002] 3273334.932742: sched:sched_stat_runtime: > comm=python pid=874 runtime=503174 [ns] vruntime=1549788942230752 [ns] > python 874 [002] 3273334.932743: sched:sched_stat_runtime: > comm=python pid=874 runtime=948 [ns] vruntime=1549788942231700 [ns] > python 874 [002] 3273334.936240: probe:scheduler_tick: (ffffffff810a1f30) > python 874 [002] 3273334.936241: sched:sched_stat_runtime: > comm=python pid=874 runtime=3497530 [ns] vruntime=1549788945729230 > [ns] > python 874 [002] 3273334.936249: sched:sched_stat_runtime: > comm=python pid=874 runtime=8553 [ns] vruntime=1549788945737783 [ns] > python 874 [002] 3273334.936250: sched:sched_stat_wait: > comm=python pid=873 delay=4010205 [ns] > python 874 [002] 3273334.936250: probe:hrtick_start: (ffffffff8109f4f0) > python 874 [002] 3273334.936251: sched:sched_switch: > prev_comm=python prev_pid=874 prev_prio=120 prev_state=R ==> > next_comm=python next_pid=873 next_prio=120 > > > Please advise, is this a bug? > Could you cat /sys/kernel/debug/sched_features? Regards, Wanpeng Li
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