linux-kernel - Re: [PATCH 16/19] sched/numa: Detect if node actively handling migration

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Date:   Tue, 05 Jun 2018 09:07:54 -0400
From:   Rik van Riel <riel@...riel.com>
To:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 16/19] sched/numa: Detect if node actively handling
 migration

On Mon, 2018-06-04 at 20:56 -0700, Srikar Dronamraju wrote:
> * Rik van Riel <riel@...riel.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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