Messages in this thread | | | Date | Mon, 4 Jun 2018 20:56:16 -0700 | From | Srikar Dronamraju <> | Subject | Re: [PATCH 16/19] sched/numa: Detect if node actively handling migration |
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* Rik van Riel <riel@surriel.com> [2018-06-04 16:05:55]:
> On Mon, 2018-06-04 at 15:30 +0530, Srikar Dronamraju wrote: > > > @@ -1554,6 +1562,9 @@ static void task_numa_compare(struct > > task_numa_env *env, > > if (READ_ONCE(dst_rq->numa_migrate_on)) > > return; > > > > + if (*move && READ_ONCE(pgdat->active_node_migrate)) > > + *move = false; > > Why not do this check in task_numa_find_cpu? > > That way you won't have to pass this in as a > pointer, and you also will not have to recalculate > NODE_DATA(cpu_to_node(env->dst_cpu)) for every CPU. >
I thought about this. Lets say we evaluated that destination node can allow movement. While we iterate through the list of cpus trying to find the best cpu node, we find a idle cpu towards the end of the list. However if another task as already raced with us to move a task to this node, then we should bail out. Keeping the check in task_numa_compare will allow us to do this.
> > /* > > + * If the numa importance is less than SMALLIMP, > > ^^^ numa improvement >
okay
> > + * task migration might only result in ping pong > > + * of tasks and also hurt performance due to cache > > + * misses. > > + */ > > + if (imp < SMALLIMP || imp <= env->best_imp + SMALLIMP / 2) > > + goto unlock; > > I can see a use for the first test, but why limit the > search for the best score once you are past the > threshold? > > I don't understand the use for that second test. >
Lets say few threads are racing with each other to find a cpu on the node. The first thread has already found a task/cpu 'A' to swap and finds another task/cpu 'B' thats slightly better than the current best_cpu which is 'A'. Currently we allow the second task/cpu 'B' to be set as best_cpu. However the second or subsequent threads cannot find the first task/cpu A because its suppose to be in active migration. By the time it reaches task/cpu B even that may look to be in active migration. It may never know that task/cpu A was cleared. In this way, the second and subsequent threads may not get a task/cpu in the preferred node to swap just because the first task kept hopping task/cpu as its choice of migration.
While we can't complete avoid this, the second check will try to make sure we don't hop on/hop off just for small incremental numa improvement.
> What workload benefits from it?
> > -- > All Rights Reversed.
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