Messages in this thread | | | From | Vincent Guittot <> | Date | Fri, 3 Oct 2014 09:24:23 +0200 | Subject | Re: [PATCH v6 5/6] sched: replace capacity_factor by usage |
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On 2 October 2014 18:57, Morten Rasmussen <morten.rasmussen@arm.com> wrote: > On Tue, Sep 23, 2014 at 05:08:04PM +0100, Vincent Guittot wrote: >> 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. > > I did some testing on TC2 which has the setup you describe above on the > A7 cluster when the clock-frequency property is set in DT. The two A15s > have max capacities above 1024. When using sysbench with five threads I > still get three tasks on the two A15s and two tasks on the three A7 > leaving one cpu idle (on average). > > Using cpu utilization (usage) does correctly identify the A15 cluster as > overloaded and it even gets as far as selecting the A15 running two > tasks as the source cpu in load_balance(). However, load_balance() bails > out without pulling the task due to calculate_imbalance() returning a > zero imbalance. calculate_imbalance() bails out just before the hunk you > changed due to comparison of the sched_group avg_loads. sgs->avg_load is > basically the sum of load in the group divided by its capacity. Since > load isn't scaled the avg_load of the overloaded A15 group is slightly > _lower_ than the partially idle A7 group. Hence calculate_imbalance() > bails out, which isn't what we want. > > I think we need to have a closer look at the imbalance calculation code > and any other users of sgs->avg_load to get rid of all code making > assumptions about cpu capacity.
We already had this discussion during the review of a previous version https://lkml.org/lkml/2014/6/3/422 and my answer has not changed; This patch is necessary to solve the 1 task per CPU issue of the HMP system but is not enough. I have a patch for solving the imbalance calculation issue and i have planned to send it once this patch will be in a good shape for being accepted by Peter.
I don't want to mix this patch and the next one because there are addressing different problem: this one is how evaluate the capacity of a system and detect when a group is overloaded and the next one will handle the case when the balance of the system can't rely on the average load figures of the group because we have a significant capacity difference between groups. Not that i haven't specifically mentioned HMP for the last patch because SMP system can also take advantage of it
Regards, Vincent
> > Morten
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