Messages in this thread | | | Date | Wed, 06 May 2015 10:49:42 +0100 | From | Dietmar Eggemann <> | Subject | Re: [RFCv3 PATCH 12/48] sched: Make usage tracking cpu scale-invariant |
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On 03/05/15 07:27, pang.xunlei@zte.com.cn wrote: > Hi Dietmar, > > Dietmar Eggemann <dietmar.eggemann@arm.com> wrote 2015-03-24 AM 03:19:41: >> >> Re: [RFCv3 PATCH 12/48] sched: Make usage tracking cpu scale-invariant
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>> In the previous patch-set https://lkml.org/lkml/2014/12/2/332we >> cpu-scaled both (sched_avg::runnable_avg_sum (load) and >> sched_avg::running_avg_sum (utilization)) but during the review Vincent >> pointed out that a cpu-scaled invariant load signal messes up >> load-balancing based on s[dg]_lb_stats::avg_load in overload scenarios. >> >> avg_load = load/capacity and load can't be simply replaced here by >> 'cpu-scale invariant load' (which is load*capacity). > > I can't see why it shouldn't. > > For "avg_load = load/capacity", "avg_load" stands for how busy the cpu > works, > it is actually a value relative to its capacity. The system is seen > balanced > for the case that a task runs on a 512-capacity cpu contributing 50% usage, > and two the same tasks run on the 1024-capacity cpu contributing 50% usage. > "capacity" in this formula contains uarch capacity, "load" in this formula > must be an absolute real load, not relative. > > But with current kernel implementation, "load" computed without this patch > is a relative value. For example, one task (1024 weight) runs on a 1024 > capacity CPU, it gets 256 load contribution(25% on this CPU). When it runs > on a 512 capacity CPU, it will get the 512 load contribution(50% on ths > CPU). > See, currently runnable "load" is relative, so "avg_load" is actually wrong > and its value equals that of "load". So I think the runnable load should be > made cpu scale-invariant as well. > > Please point me out if I was wrong.
Cpu-scaled load leads to wrong lb decisions in overload scenarios:
(1) Overload example taken from email thread between Vincent and Morten: https://lkml.org/lkml/2014/12/30/114
7 always running tasks, 4 on cluster 0, 3 on cluster 1:
cluster 0 cluster 1 capacity 1024 (2*512) 1024 (1*1024) load 4096 3072 scale_load 2048 3072
Simply using cpu-scaled load in the existing lb code would declare cluster 1 busier than cluster 0, although the compute capacity budget for one task is higher on cluster 1 (1024/3 = 341) than on cluster 0 (2*512/4 = 256).
(2) A non-overload example does not show this problem:
7 12.5% (scaled to 1024) tasks, 4 on cluster 0, 3 on cluster 1:
cluster 0 cluster 1 capacity 1024 (2*512) 1024 (1*1024) load 1024 384 scale_load 512 384
Here cluster 0 is busier taking load or cpu-scaled load.
We should continue to use avg_load based on load (maybe calculated out of scaled load once introduced?) for overload scenarios and use scale_load for non-overload scenarios. Since this hasn't been implemented yet, we got rid of cpu-scaled load in this RFC.
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