Messages in this thread | | | Subject | Re: [PATCH 16/19] sched/numa: Detect if node actively handling migration | From | Rik van Riel <> | Date | Tue, 05 Jun 2018 09:07:54 -0400 |
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On Mon, 2018-06-04 at 20:56 -0700, Srikar Dronamraju wrote: > * 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.
Your check is called once for every invocation of task_numa_compare. It does not matter whether it is inside or outside, except on the outside the variable manipulation will be easier to read.
> > > + * 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.
However, all those racing tasks start searching the CPUs on a node from the same start position.
That means they may all get stuck on the same task/cpu A, and not select the better task/cpu B.
What am I missing?
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