Re: [PATCH v4 01/11] sched/fair: clean up asym packing

[Vincent Guittot]

On Wed, 30 Oct 2019 at 15:51, Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxx>
> > Acked-by: Rik van Riel <riel@xxxxxxxxxxx>
> > ---
> >  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