Re: [PATCH v3] sched/fair: Use all little CPUs for CPU-bound workload

[Pierre Gondois]

Hello Sasha,
Would it be possible to pick this patch for the 6.1 stable branch ?
Or is there something I should do for this purpose ?

Regards,
Pierre

On 6/25/24 15:25, Pierre Gondois wrote:
> Hello stable folk,
>
> This patch was merged as:
>     commit 3af7524b1419 ("sched/fair: Use all little CPUs for CPU-bound workloads")
> into 6.7, improving the following:
>     commit 0b0695f2b34a ("sched/fair: Rework load_balance()")
>
> Would it be possible to port it to the 6.1 stable branch ?
> The patch should apply cleanly by cherry-picking onto v6.1.94,
>
> Regards,
> Pierre
>
>
> On 12/6/23 10:00, Pierre Gondois wrote:
> > Running n CPU-bound tasks on an n CPUs platform:
> > - with asymmetric CPU capacity
> > - not being a DynamIq system (i.e. having a PKG level sched domain
> >     without the SD_SHARE_PKG_RESOURCES flag set)
> > might result in a task placement where two tasks run on a big CPU
> > and none on a little CPU. This placement could be more optimal by
> > using all CPUs.
> >
> > Testing platform:
> > Juno-r2:
> > - 2 big CPUs (1-2), maximum capacity of 1024
> > - 4 little CPUs (0,3-5), maximum capacity of 383
> >
> > Testing workload ([1]):
> > Spawn 6 CPU-bound tasks. During the first 100ms (step 1), each tasks
> > is affine to a CPU, except for:
> > - one little CPU which is left idle.
> > - one big CPU which has 2 tasks affine.
> > After the 100ms (step 2), remove the cpumask affinity.
> >
> > Before patch:
> > During step 2, the load balancer running from the idle CPU tags sched
> > domains as:
> > - little CPUs: 'group_has_spare'. Cf. group_has_capacity() and
> >     group_is_overloaded(), 3 CPU-bound tasks run on a 4 CPUs
> >     sched-domain, and the idle CPU provides enough spare capacity
> >     regarding the imbalance_pct
> > - big CPUs: 'group_overloaded'. Indeed, 3 tasks run on a 2 CPUs
> >     sched-domain, so the following path is used:
> >     group_is_overloaded()
> >     \-if (sgs->sum_nr_running <= sgs->group_weight) return true;
> >
> >     The following path which would change the migration type to
> >     'migrate_task' is not taken:
> >     calculate_imbalance()
> >     \-if (env->idle != CPU_NOT_IDLE && env->imbalance == 0)
> >     as the local group has some spare capacity, so the imbalance
> >     is not 0.
> >
> > The migration type requested is 'migrate_util' and the busiest
> > runqueue is the big CPU's runqueue having 2 tasks (each having a
> > utilization of 512). The idle little CPU cannot pull one of these
> > task as its capacity is too small for the task. The following path
> > is used:
> > detach_tasks()
> > \-case migrate_util:
> >     \-if (util > env->imbalance) goto next;
> >
> > After patch:
> > As the number of failed balancing attempts grows (with
> > 'nr_balance_failed'), progressively make it easier to migrate
> > a big task to the idling little CPU. A similar mechanism is
> > used for the 'migrate_load' migration type.
> >
> > Improvement:
> > Running the testing workload [1] with the step 2 representing
> > a ~10s load for a big CPU:
> > Before patch: ~19.3s
> > After patch: ~18s (-6.7%)
> >
> > Similar issue reported at:
> > https://lore.kernel.org/lkml/20230716014125.139577-1-qyousef@xxxxxxxxxxx/
> >
> > v1:
> > https://lore.kernel.org/all/20231110125902.2152380-1-pierre.gondois@xxxxxxx/
> > v2:
> > https://lore.kernel.org/all/20231124153323.3202444-1-pierre.gondois@xxxxxxx/
> >
> > Suggested-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> > Signed-off-by: Pierre Gondois <pierre.gondois@xxxxxxx>
> > Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> > Reviewed-by: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
> > ---
> >
> > Notes:
> >       v2:
> >       - Used Vincent's approach.
> >       v3:
> >       - Updated commit message.
> >       - Added Reviewed-by tags
> >
> >    kernel/sched/fair.c | 2 +-
> >    1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index d7a3c63a2171..9481b8cff31b 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -9060,7 +9060,7 @@ static int detach_tasks(struct lb_env *env)
> >    		case migrate_util:
> >    			util = task_util_est(p);
> >    
> > -			if (util > env->imbalance)
> > +			if (shr_bound(util, env->sd->nr_balance_failed) > env->imbalance)
> >    				goto next;
> >    
> >    			env->imbalance -= util;