Messages in this thread Patch in this message | | | Date | Fri, 25 Jul 2014 09:33:56 -0400 | From | Rik van Riel <> | Subject | Re: [PATCH] sched: make update_sd_pick_busiest return true on a busier sd |
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On 07/23/2014 03:41 AM, Vincent Guittot wrote: > On 22 July 2014 20:45, Rik van Riel <riel@redhat.com> wrote: >> Currently update_sd_pick_busiest only returns true when an sd is >> overloaded, or for SD_ASYM_PACKING when a domain is busier than >> average and a higher numbered domain than the target. >> >> This breaks load balancing between domains that are not >> overloaded, in the !SD_ASYM_PACKING case. This patch makes >> update_sd_pick_busiest return true when the busiest sd yet is >> encountered. >> >> On a 4 node system, this seems to result in the load balancer >> finally putting 1 thread of a 4 thread test run of "perf bench >> numa mem" on each node, where before the load was generally not >> spread across all nodes. >> >> Behaviour for SD_ASYM_PACKING does not seem to match the >> comment, in that groups with below average load average are >> ignored, but I have no hardware to test that so I have left the >> behaviour of that code unchanged. >> >> Cc: mikey@neuling.org Cc: peterz@infradead.org Signed-off-by: Rik >> van Riel <riel@redhat.com> --- kernel/sched/fair.c | 18 >> +++++++++++------- 1 file changed, 11 insertions(+), 7 >> deletions(-) >> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index >> fea7d33..ff4ddba 100644 --- a/kernel/sched/fair.c +++ >> b/kernel/sched/fair.c @@ -5942,16 +5942,20 @@ static bool >> update_sd_pick_busiest(struct lb_env *env, * numbered CPUs in the >> group, therefore mark all groups * higher than ourself as busy. >> */ - if ((env->sd->flags & SD_ASYM_PACKING) && >> sgs->sum_nr_running && - env->dst_cpu < >> group_first_cpu(sg)) { - if (!sds->busiest) - >> return true; + if (env->sd->flags & SD_ASYM_PACKING) { + >> if (sgs->sum_nr_running && env->dst_cpu < group_first_cpu(sg)) { >> + if (!sds->busiest) + >> return true; >> >> - if (group_first_cpu(sds->busiest) > >> group_first_cpu(sg)) - return true; + >> if (group_first_cpu(sds->busiest) > group_first_cpu(sg)) + >> return true; + } + + return false; } >> >> - return false; + /* See above: sgs->avg_load > >> sds->busiest_stat.avg_load */ + return true; > > Hi Rik, > > I can see one issue with a default return set to true. You > increase the number of time where we will not effectively migrate a > task because we don't ensure that we will take the overloaded group > if there is one. We can be in a situation where a group is > overloaded but the load_balance will select a not overloaded group > with an average load higher than sched_domain average value just > because it is checked after.
Look at the first line of update_sd_pick_busiest()
static bool update_sd_pick_busiest(struct lb_env *env, struct sd_lb_stats *sds, struct sched_group *sg, struct sg_lb_stats *sgs) { if (sgs->avg_load <= sds->busiest_stat.avg_load) return false;
If the group load is less than the busiest, we have already returned false, and will not get to the code that I changed.
> Regarding your issue with "perf bench numa mem" that is not spread > on all nodes, SD_PREFER_SIBLING flag (of DIE level) should do the > job by reducing the capacity of "not local DIE" group at NUMA > level to 1 task during the load balance computation. So you should > have 1 task per sched_group at NUMA level.
That did not actually happen in my tests. I almost always ended up having only 0 tasks on one node.
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