Messages in this thread | | | From | Vincent Guittot <> | Date | Thu, 13 Dec 2018 16:14:46 +0100 | Subject | Re: [PATCH v2] sched/fair: fix 1 task per CPU |
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On Thu, 13 Dec 2018 at 14:12, Peter Zijlstra <peterz@infradead.org> wrote: > > On Thu, Dec 13, 2018 at 12:04:20PM +0100, Vincent Guittot wrote: > > On Thu, 13 Dec 2018 at 11:44, Peter Zijlstra <peterz@infradead.org> wrote: > > > > > > On Mon, Sep 10, 2018 at 04:43:09PM +0200, Vincent Guittot wrote: > > > > When CPUs have different capacity because of RT/DL tasks or > > > > micro-architecture or max frequency differences, there are situation where > > > > the imbalance is not correctly set to migrate waiting task on the idle CPU. > > > > > > > > The UC uses the force_balance case: > > > > > > > > if (env->idle != CPU_NOT_IDLE && group_has_capacity(env, local) && > > > > busiest->group_no_capacity) > > > > goto force_balance; > > > > > > > > But calculate_imbalance fails to set the right amount of load to migrate > > > > a task because of the special condition: > > > > > > > > busiest->avg_load <= sds->avg_load || local->avg_load >= sds->avg_load) > > > > > > > > Add in fix_small_imbalance, this special case that triggered the force > > > > balance in order to make sure that the amount of load to migrate will be > > > > enough. > > > > > > So I think this patch is going in the wrong direction for a number of > > > reasons: > > > > > > - we'd like to get rid of fix_small_imbalance(), and this adds to it; > > > > > > - the whole load_per_task stuff is terminally broken, it _cannot_ work > > > right. > > > > > > > > > What I've suggested in the past is parameterizing the load balancer and > > > picking different criteria to balance on: > > > > This patch is clearly a fix of the current implementation. > > What you suggest below makes sense but implies a significant rework in > > the calculate_imbalance and the load_balancer in general and will need > > more time to reach a stable state. > > Nevertheless, I will have a look at that > > > > I imagine that your feedback for https://lkml.org/lkml/2018/10/2/283 > > will be the same ? > > No; those actually look ok. It is mostly that I really don't think > load_per_task makes any kind of sense. > > It sorta works when all tasks are of the same weight, but if you start > using nice -- or way worse, cgroups -- then the number is complete > bollocks.
Yes. As soon as we goes out of a simple load balance UC, load_per_task becomes a kind of default random choice
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