Messages in this thread | | | From | Vincent Guittot <> | Date | Wed, 30 Oct 2019 17:03:24 +0100 | Subject | Re: [PATCH v4 01/11] sched/fair: clean up asym packing |
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On Wed, 30 Oct 2019 at 15:51, Mel Gorman <mgorman@techsingularity.net> wrote: > > 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
The checks of SD_ASYM_PACKING and CPU_NOT_IDLE are already above in the function but out of the patch. In fact this part of update_sd_pick_busiest is already dedicated to asym_packing.
What I'm doing is that instead of checking asym_packing in update_sd_pick_busiest and then rechecking the same thing in find_busiest_group, I save the check result and reuse it
Also patch 04 moves further this code
> 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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