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sched: Improve sysbench performance by fixing spurious active migration
Since commit caeb178c ("sched/fair: Make update_sd_pick_busiest() ...") sd_pick_busiest returns a group that can be neither imbalanced nor overloaded but is only more loaded than others. This change has been introduced to ensure a better load balance in system that are not overloaded but as a side effect, it can also generate useless active migration between groups. Let take the example of 3 tasks on a quad cores system. We will always have an idle core so the load balance will find a busiest group (core) whenever an ILB is triggered and it will force an active migration (once above nr_balance_failed threshold) so the idle core becomes busy but another core will become idle. With the next ILB, the freshly idle core will try to pull the task of a busy CPU. The number of spurious active migration is not so huge in quad core system because the ILB is not triggered so much. But it becomes significant as soon as you have more than one sched_domain level like on a dual cluster of quad cores where the ILB is triggered every tick when you have more than 1 busy_cpu We need to ensure that the migration generate a real improveùent and will not only move the avg_load imbalance on another CPU. Before caeb178c, the filtering of such use case was ensured by the following test in f_b_g: if ((local->idle_cpus < busiest->idle_cpus) && busiest->sum_nr_running <= busiest->group_weight) This patch modified the condition to take into account situation where busiest group is not overloaded: If the diff between the number of idle cpus in 2 groups is less than or equal to 1 and the busiest group is not overloaded, moving a task will not improve the load balance but just move it. A test with sysbench on a dual clusters of quad cores gives the following results: command: sysbench --test=cpu --num-threads=5 --max-time=5 run The HZ is 200 which means that 1000 ticks has fired during the test. With Mainline, perf gives the following figures: Samples: 727 of event 'sched:sched_migrate_task' Event count (approx.): 727 Overhead Command Shared Object Symbol ........ ............... ............. .............. 12.52% migration/1 [unknown] [.] 00000000 12.52% migration/5 [unknown] [.] 00000000 12.52% migration/7 [unknown] [.] 00000000 12.10% migration/6 [unknown] [.] 00000000 11.83% migration/0 [unknown] [.] 00000000 11.83% migration/3 [unknown] [.] 00000000 11.14% migration/4 [unknown] [.] 00000000 10.87% migration/2 [unknown] [.] 00000000 2.75% sysbench [unknown] [.] 00000000 0.83% swapper [unknown] [.] 00000000 0.55% ktps65090charge [unknown] [.] 00000000 0.41% mmcqd/1 [unknown] [.] 00000000 0.14% perf [unknown] [.] 00000000 With this patch, perf gives the following figures Samples: 20 of event 'sched:sched_migrate_task' Event count (approx.): 20 Overhead Command Shared Object Symbol ........ ............... ............. .............. 80.00% sysbench [unknown] [.] 00000000 10.00% swapper [unknown] [.] 00000000 5.00% ktps65090charge [unknown] [.] 00000000 5.00% migration/1 [unknown] [.] 00000000 Signed-off-by:Vincent Guittot <vincent.guittot@linaro.org> Reviewed-by: