Messages in this thread | | | Date | Mon, 29 Sep 2014 14:39:21 +0100 | From | Dietmar Eggemann <> | Subject | Re: [PATCH v6 5/6] sched: replace capacity_factor by usage |
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On 23/09/14 17:08, Vincent Guittot wrote: > The scheduler tries to compute how many tasks a group of CPUs can handle by > assuming that a task's load is SCHED_LOAD_SCALE and a CPU capacity is > SCHED_CAPACITY_SCALE but the capacity_factor is hardly working for SMT system, > it sometimes works for big cores but fails to do the right thing for little > cores. > > Below are two examples to illustrate the problem that this patch solves: > > 1 - capacity_factor makes the assumption that max capacity of a CPU is > SCHED_CAPACITY_SCALE and the load of a thread is always is > SCHED_LOAD_SCALE. It compares the output of these figures with the sum > of nr_running to decide if a group is overloaded or not. > > But if the default capacity of a CPU is less than SCHED_CAPACITY_SCALE > (640 as an example), a group of 3 CPUS will have a max capacity_factor > of 2 ( div_round_closest(3x640/1024) = 2) which means that it will be > seen as overloaded if we have only one task per CPU. > > 2 - Then, if the default capacity of a CPU is greater than > SCHED_CAPACITY_SCALE (1512 as an example), a group of 4 CPUs will have > a capacity_factor of 4 (at max and thanks to the fix[0] for SMT system > that prevent the apparition of ghost CPUs) but if one CPU is fully > used by a rt task (and its capacity is reduced to nearly nothing), the > capacity factor of the group will still be 4 > (div_round_closest(3*1512/1024) = 5). > > So, this patch tries to solve this issue by removing capacity_factor > and replacing it with the 2 following metrics : > -The available CPU's capacity for CFS tasks which is the already used by > load_balance. > -The usage of the CPU by the CFS tasks. For the latter, I have > re-introduced the utilization_avg_contrib which is in the range > [0..SCHED_CPU_LOAD] whatever the capacity of the CPU is.
IMHO, this last sentence is misleading. The usage of a cpu can be temporally unbounded (in case a lot of tasks have just been spawned on this cpu, testcase: hackbench) but it converges very quickly towards a value between [0..1024]. Your implementation is already handling this case by capping usage to cpu_rq(cpu)->capacity_orig + 1 . BTW, couldn't find the definition of SCHED_CPU_LOAD.
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