Messages in this thread | | | Date | Wed, 30 Oct 2019 14:51:33 +0000 | From | Mel Gorman <> | Subject | Re: [PATCH v4 01/11] sched/fair: clean up asym packing |
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On Fri, Oct 18, 2019 at 03:26:28PM +0200, Vincent Guittot wrote: > Clean up asym packing to follow the default load balance behavior: > - classify the group by creating a group_asym_packing field. > - calculate the imbalance in calculate_imbalance() instead of bypassing it. > > We don't need to test twice same conditions anymore to detect asym packing > and we consolidate the calculation of imbalance in calculate_imbalance(). > > There is no functional changes. > > Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> > Acked-by: Rik van Riel <riel@surriel.com> > --- > kernel/sched/fair.c | 63 ++++++++++++++--------------------------------------- > 1 file changed, 16 insertions(+), 47 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 1f0a5e1..617145c 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -7675,6 +7675,7 @@ struct sg_lb_stats { > unsigned int group_weight; > enum group_type group_type; > int group_no_capacity; > + unsigned int group_asym_packing; /* Tasks should be moved to preferred CPU */ > unsigned long group_misfit_task_load; /* A CPU has a task too big for its capacity */ > #ifdef CONFIG_NUMA_BALANCING > unsigned int nr_numa_running; > @@ -8129,9 +8130,17 @@ static bool update_sd_pick_busiest(struct lb_env *env, > * ASYM_PACKING needs to move all the work to the highest > * prority CPUs in the group, therefore mark all groups > * of lower priority than ourself as busy. > + * > + * This is primarily intended to used at the sibling level. Some > + * cores like POWER7 prefer to use lower numbered SMT threads. In the > + * case of POWER7, it can move to lower SMT modes only when higher > + * threads are idle. When in lower SMT modes, the threads will > + * perform better since they share less core resources. Hence when we > + * have idle threads, we want them to be the higher ones. > */ > if (sgs->sum_nr_running && > sched_asym_prefer(env->dst_cpu, sg->asym_prefer_cpu)) { > + sgs->group_asym_packing = 1; > if (!sds->busiest) > return true; >
(I did not read any of the earlier implementations of this series, maybe this was discussed already in which case, sorry for the noise)
Are you *sure* this is not a functional change?
Asym packing is a twisty maze of headaches and I'm not familiar enough with it to be 100% certain without spending a lot of time on this. Even spotting how Power7 ends up using asym packing with lower-numbered SMT threads is a bit of a challenge. Specifically, it relies on the scheduler domain SD_ASYM_PACKING flag for SMT domains to use the weak implementation of arch_asym_cpu_priority which by defaults favours the lower-numbered CPU.
The check_asym_packing implementation checks that flag but I can't see the equiavlent type of check here. update_sd_pick_busiest could be called for domains that span NUMA or basically any domain that does not specify SD_ASYM_PACKING and end up favouring a lower-numbered CPU (or whatever arch_asym_cpu_priority returns in the case of x86 which has a different idea for favoured CPUs).
sched_asym_prefer appears to be a function that is very easy to use incorrectly. Should it take env and check the SD flags first?
-- Mel Gorman SUSE Labs
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