From: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
To: Rik van Riel <riel@surriel.com>
Cc: Ingo Molnar <mingo@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
LKML <linux-kernel@vger.kernel.org>,
Mel Gorman <mgorman@techsingularity.net>,
Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH 16/19] sched/numa: Detect if node actively handling migration
Date: Mon, 4 Jun 2018 20:56:16 -0700 [thread overview]
Message-ID: <20180605035616.GD30328@linux.vnet.ibm.com> (raw)
In-Reply-To: <1528142755.7898.122.camel@surriel.com>
* 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.
next prev parent reply other threads:[~2018-06-05 3:56 UTC|newest]
Thread overview: 66+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-06-04 10:00 [PATCH 00/19] Fixes for sched/numa_balancing Srikar Dronamraju
2018-06-04 10:00 ` [PATCH 01/19] sched/numa: Remove redundant field Srikar Dronamraju
2018-06-04 14:53 ` Rik van Riel
2018-06-05 8:41 ` Mel Gorman
2018-06-04 10:00 ` [PATCH 02/19] sched/numa: Evaluate move once per node Srikar Dronamraju
2018-06-04 14:51 ` Rik van Riel
2018-06-04 15:45 ` Srikar Dronamraju
2018-06-04 10:00 ` [PATCH 03/19] sched/numa: Simplify load_too_imbalanced Srikar Dronamraju
2018-06-04 14:57 ` Rik van Riel
2018-06-05 8:46 ` Mel Gorman
2018-06-04 10:00 ` [PATCH 04/19] sched/numa: Set preferred_node based on best_cpu Srikar Dronamraju
2018-06-04 12:18 ` Peter Zijlstra
2018-06-04 12:53 ` Srikar Dronamraju
2018-06-04 12:23 ` Peter Zijlstra
2018-06-04 12:59 ` Srikar Dronamraju
2018-06-04 13:39 ` Peter Zijlstra
2018-06-04 13:48 ` Srikar Dronamraju
2018-06-04 14:37 ` Rik van Riel
2018-06-04 15:56 ` Srikar Dronamraju
2018-06-04 10:00 ` [PATCH 05/19] sched/numa: Use task faults only if numa_group is not yet setup Srikar Dronamraju
2018-06-04 12:24 ` Peter Zijlstra
2018-06-04 13:09 ` Srikar Dronamraju
2018-06-04 10:00 ` [PATCH 06/19] sched/debug: Reverse the order of printing faults Srikar Dronamraju
2018-06-04 16:28 ` Rik van Riel
2018-06-05 8:50 ` Mel Gorman
2018-06-04 10:00 ` [PATCH 07/19] sched/numa: Skip nodes that are at hoplimit Srikar Dronamraju
2018-06-04 16:27 ` Rik van Riel
2018-06-05 8:50 ` Mel Gorman
2018-06-04 10:00 ` [PATCH 08/19] sched/numa: Remove unused task_capacity from numa_stats Srikar Dronamraju
2018-06-04 16:28 ` Rik van Riel
2018-06-05 8:57 ` Mel Gorman
2018-06-04 10:00 ` [PATCH 09/19] sched/numa: Modify migrate_swap to accept additional params Srikar Dronamraju
2018-06-04 17:00 ` Rik van Riel
2018-06-05 8:58 ` Mel Gorman
2018-06-04 10:00 ` [PATCH 10/19] sched/numa: Stop multiple tasks from moving to the cpu at the same time Srikar Dronamraju
2018-06-04 17:57 ` Rik van Riel
2018-06-05 9:51 ` Mel Gorman
2018-06-04 10:00 ` [PATCH 11/19] sched/numa: Restrict migrating in parallel to the same node Srikar Dronamraju
2018-06-04 17:59 ` Rik van Riel
2018-06-05 9:53 ` Mel Gorman
2018-06-06 12:58 ` Srikar Dronamraju
2018-06-04 10:00 ` [PATCH 12/19] sched:numa Remove numa_has_capacity Srikar Dronamraju
2018-06-04 18:07 ` Rik van Riel
2018-06-04 10:00 ` [PATCH 13/19] mm/migrate: Use xchg instead of spinlock Srikar Dronamraju
2018-06-04 18:22 ` Rik van Riel
2018-06-04 19:28 ` Peter Zijlstra
2018-06-05 7:24 ` Srikar Dronamraju
2018-06-05 8:16 ` Peter Zijlstra
2018-06-04 10:00 ` [PATCH 14/19] sched/numa: Updation of scan period need not be in lock Srikar Dronamraju
2018-06-04 18:24 ` Rik van Riel
2018-06-04 10:00 ` [PATCH 15/19] sched/numa: Use group_weights to identify if migration degrades locality Srikar Dronamraju
2018-06-04 18:56 ` Rik van Riel
2018-06-04 10:00 ` [PATCH 16/19] sched/numa: Detect if node actively handling migration Srikar Dronamraju
2018-06-04 20:05 ` Rik van Riel
2018-06-05 3:56 ` Srikar Dronamraju [this message]
2018-06-05 13:07 ` Rik van Riel
2018-06-06 12:55 ` Srikar Dronamraju
2018-06-06 13:55 ` Rik van Riel
2018-06-06 15:32 ` Srikar Dronamraju
2018-06-06 17:06 ` Rik van Riel
2018-06-04 10:00 ` [PATCH 17/19] sched/numa: Pass destination cpu as a parameter to migrate_task_rq Srikar Dronamraju
2018-06-04 10:00 ` [PATCH 18/19] sched/numa: Reset scan rate whenever task moves across nodes Srikar Dronamraju
2018-06-04 20:08 ` Rik van Riel
2018-06-05 9:58 ` Mel Gorman
2018-06-06 13:47 ` Srikar Dronamraju
2018-06-04 10:00 ` [PATCH 19/19] sched/numa: Move task_placement closer to numa_migrate_preferred Srikar Dronamraju
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