On Thu, Aug 01, 2019 at 04:40:20PM +0200, Vincent Guittot wrote:
> The load_balance algorithm contains some heuristics which have becomes
> meaningless since the rework of metrics and the introduction of PELT.
> 
> Furthermore, it's sometimes difficult to fix wrong scheduling decisions
> because everything is based on load whereas some imbalances are not
> related to the load. The current algorithm ends up to create virtual and
> meaningless value like the avg_load_per_task or tweaks the state of a
> group to make it overloaded whereas it's not, in order to try to migrate
> tasks.
> 
> load_balance should better qualify the imbalance of the group and define
> cleary what has to be moved to fix this imbalance.
> 
> The type of sched_group has been extended to better reflect the type of
> imbalance. We now have :
>       group_has_spare
>       group_fully_busy
>       group_misfit_task
>       group_asym_capacity
>       group_imbalanced
>       group_overloaded
> 
> Based on the type of sched_group, load_balance now sets what it wants to
> move in order to fix the imnbalance. It can be some load as before but
> also some utilization, a number of task or a type of task:
>       migrate_task
>       migrate_util
>       migrate_load
>       migrate_misfit
> 
> This new load_balance algorithm fixes several pending wrong tasks
> placement:
> - the 1 task per CPU case with asymetrics system
> - the case of cfs task preempted by other class
> - the case of tasks not evenly spread on groups with spare capacity
> 
> The load balance decisions have been gathered in 3 functions:
> - update_sd_pick_busiest() select the busiest sched_group.
> - find_busiest_group() checks if there is an imabalance between local and
>   busiest group.
> - calculate_imbalance() decides what have to be moved.

I like this lots, very good stuff.

It is weird how misfit is still load, but istr later patches cure that.

Here's a whole pile of nits, some overlap with what Valentin already
noted. His suggestions for the changelog also make sense to me.

---
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8205,10 +8205,10 @@ static bool update_sd_pick_busiest(struc
 {
        struct sg_lb_stats *busiest = &sds->busiest_stat;
 
-
        /* Make sure that there is at least one task to pull */
        if (!sgs->sum_h_nr_running)
                return false;
+
        /*
         * Don't try to pull misfit tasks we can't help.
         * We can use max_capacity here as reduction in capacity on some
@@ -8239,11 +8239,11 @@ static bool update_sd_pick_busiest(struc
                break;
 
        case group_imbalanced:
-               /* Select the 1st imbalanced group as we don't have
-                * any way to choose one more than another
+               /*
+                * Select the 1st imbalanced group as we don't have any way to
+                * choose one more than another
                 */
                return false;
-               break;
 
        case group_asym_capacity:
                /* Prefer to move from lowest priority CPU's work */
@@ -8253,8 +8253,8 @@ static bool update_sd_pick_busiest(struc
 
        case group_misfit_task:
                /*
-                * If we have more than one misfit sg go with the
-                * biggest misfit.
+                * If we have more than one misfit sg go with the biggest
+                * misfit.
                 */
                if (sgs->group_misfit_task_load < 
busiest->group_misfit_task_load)
                        return false;
@@ -8262,14 +8262,14 @@ static bool update_sd_pick_busiest(struc
 
        case group_fully_busy:
                /*
-                * Select the fully busy group with highest avg_load.
-                * In theory, there is no need to pull task from such
-                * kind of group because tasks have all compute
-                * capacity that they need but we can still improve the
-                * overall throughput by reducing contention when
-                * accessing shared HW resources.
-                * XXX for now avg_load is not computed and always 0 so
-                * we select the 1st one.
+                * Select the fully busy group with highest avg_load.  In
+                * theory, there is no need to pull task from such kind of
+                * group because tasks have all compute capacity that they need
+                * but we can still improve the overall throughput by reducing
+                * contention when accessing shared HW resources.
+                *
+                * XXX for now avg_load is not computed and always 0 so we
+                * select the 1st one.
                 */
                if (sgs->avg_load <= busiest->avg_load)
                        return false;
@@ -8277,11 +8277,11 @@ static bool update_sd_pick_busiest(struc
 
        case group_has_spare:
                /*
-                * Select not overloaded group with lowest number of
-                * idle cpus. We could also compare the spare capacity
-                * which is more stable but it can end up that the
-                * group has less spare capacity but finally more idle
-                * cpus which means less opportunity to pull tasks.
+                * Select not overloaded group with lowest number of idle cpus.
+                * We could also compare the spare capacity which is more
+                * stable but it can end up that the group has less spare
+                * capacity but finally more idle cpus which means less
+                * opportunity to pull tasks.
                 */
                if (sgs->idle_cpus >= busiest->idle_cpus)
                        return false;
@@ -8289,10 +8289,10 @@ static bool update_sd_pick_busiest(struc
        }
 
        /*
-        * Candidate sg has no more than one task per CPU and
-        * has higher per-CPU capacity. Migrating tasks to less
-        * capable CPUs may harm throughput. Maximize throughput,
-        * power/energy consequences are not considered.
+        * Candidate sg has no more than one task per CPU and has higher
+        * per-CPU capacity. Migrating tasks to less capable CPUs may harm
+        * throughput. Maximize throughput, power/energy consequences are not
+        * considered.
         */
        if ((env->sd->flags & SD_ASYM_CPUCAPACITY) &&
            (sgs->group_type <= group_fully_busy) &&
@@ -8428,6 +8428,7 @@ static inline void calculate_imbalance(s
 
        local = &sds->local_stat;
        busiest = &sds->busiest_stat;
+
        if (busiest->group_type == group_imbalanced) {
                /*
                 * In the group_imb case we cannot rely on group-wide averages
@@ -8442,8 +8443,8 @@ static inline void calculate_imbalance(s
 
        if (busiest->group_type == group_asym_capacity) {
                /*
-                * In case of asym capacity, we will try to migrate all load
-                * to the preferred CPU
+                * In case of asym capacity, we will try to migrate all load to
+                * the preferred CPU
                 */
                env->balance_type = migrate_load;
                env->imbalance = busiest->group_load;
@@ -8483,7 +8484,8 @@ static inline void calculate_imbalance(s
                         * groups.
                         */
                        env->balance_type = migrate_task;
-                       env->imbalance = (busiest->sum_h_nr_running - 
local->sum_h_nr_running) >> 1;
+                       env->imbalance = (busiest->sum_h_nr_running -
+                                         local->sum_h_nr_running) >> 1;
                        return;
                }
 
@@ -8502,7 +8504,7 @@ static inline void calculate_imbalance(s
         */
        if (local->group_type < group_overloaded) {
                /*
-                * Local will become overvloaded so the avg_load metrics are
+                * Local will become overloaded so the avg_load metrics are
                 * finally needed
                 */
 
@@ -8514,12 +8516,12 @@ static inline void calculate_imbalance(s
        }
 
        /*
-        * Both group are or will become overloaded and we're trying to get
-        * all the CPUs to the average_load, so we don't want to push
-        * ourselves above the average load, nor do we wish to reduce the
-        * max loaded CPU below the average load. At the same time, we also
-        * don't want to reduce the group load below the group capacity.
-        * Thus we look for the minimum possible imbalance.
+        * Both group are or will become overloaded and we're trying to get all
+        * the CPUs to the average_load, so we don't want to push ourselves
+        * above the average load, nor do we wish to reduce the max loaded CPU
+        * below the average load. At the same time, we also don't want to
+        * reduce the group load below the group capacity.  Thus we look for
+        * the minimum possible imbalance.
         */
        env->balance_type = migrate_load;
        env->imbalance = min(
@@ -8641,7 +8643,6 @@ static struct sched_group *find_busiest_
                if (100 * busiest->avg_load <=
                                env->sd->imbalance_pct * local->avg_load)
                        goto out_balanced;
-
        }
 
        /* Try to move all excess tasks to child's sibling domain */
@@ -8651,7 +8652,7 @@ static struct sched_group *find_busiest_
 
        if (busiest->group_type != group_overloaded &&
             (env->idle == CPU_NOT_IDLE ||
-             local->idle_cpus <= (busiest->idle_cpus + 1)))
+             local->idle_cpus <= (busiest->idle_cpus + 1))) {
                /*
                 * If the busiest group is not overloaded
                 * and there is no imbalance between this and busiest group
@@ -8661,15 +8662,14 @@ static struct sched_group *find_busiest_
                 * group.
                 */
                goto out_balanced;
+       }
 
 force_balance:
        /* Looks like there is an imbalance. Compute it */
        calculate_imbalance(env, &sds);
-
        return env->imbalance ? sds.busiest : NULL;
 
 out_balanced:
-
        env->imbalance = 0;
        return NULL;
 }

Reply via email to