Messages in this thread | | | From | Vincent Guittot <> | Date | Mon, 2 Dec 2019 14:51:43 +0100 | Subject | Re: [PATCH] sched/cfs: fix spurious active migration |
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On Mon, 2 Dec 2019 at 14:22, Peter Zijlstra <peterz@infradead.org> wrote: > > On Fri, Nov 29, 2019 at 03:04:47PM +0100, Vincent Guittot wrote: > > The load balance can fail to find a suitable task during the periodic check > > because the imbalance is smaller than half of the load of the waiting > > tasks. This results in the increase of the number of failed load balance, > > which can end up to start an active migration. This active migration is > > useless because the current running task is not a better choice than the > > waiting ones. In fact, the current task was probably not running but > > waiting for the CPU during one of the previous attempts and it had already > > not been selected. > > > > When load balance fails too many times to migrate a task, we should relax > > the contraint on the maximum load of the tasks that can be migrated > > similarly to what is done with cache hotness. > > > > Before the rework, load balance used to set the imbalance to the average > > load_per_task in order to mitigate such situation. This increased the > > likelihood of migrating a task but also of selecting a larger task than > > needed while more appropriate ones were in the list. > > > > Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> > > --- > > > > I haven't seen any noticable performance changes on the benchmarks that I > > usually run but the problem can be easily highlight with a simple test > > with 9 always running tasks on 8 cores. > > > > kernel/sched/fair.c | 9 ++++++++- > > 1 file changed, 8 insertions(+), 1 deletion(-) > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > index e0d662a..d1b4fa7 100644 > > --- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -7433,7 +7433,14 @@ static int detach_tasks(struct lb_env *env) > > load < 16 && !env->sd->nr_balance_failed) > > goto next; > > > > - if (load/2 > env->imbalance) > > + /* > > + * Make sure that we don't migrate too much load. > > + * Nevertheless, let relax the constraint if > > + * scheduler fails to find a good waiting task to > > + * migrate. > > + */ > > + if (load/2 > env->imbalance && > > + env->sd->nr_balance_failed <= env->sd->cache_nice_tries) > > goto next; > > > > env->imbalance -= load; > > The alternative is carrying a flag that inhibits incrementing > nr_balance_failed. > > Not migrating anything when doing so would make the imbalance worse is > not a failure after all.
Yeah I thought about this possibility but this behavior will make a big difference compared to legacy load balance and i'm not sure about the impact on performance because we can generate significant unfairness with 2 tasks sharing a CPU while others have a full CPU in the example that I mentioned above.
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