Date:   Tue, 6 Jun 2017 14:30:11 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jeffrey Hugo <jhugo@...eaurora.org>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Austin Christ <austinwc@...eaurora.org>,
        Tyler Baicar <tbaicar@...eaurora.org>,
        Timur Tabi <timur@...eaurora.org>
Subject: Re: [PATCH V4 1/2] sched/fair: Fix load_balance() affinity redo path

On Fri, Jun 02, 2017 at 04:27:11PM -0600, Jeffrey Hugo wrote:
> If load_balance() fails to migrate any tasks because all tasks were
> affined, load_balance() removes the source cpu from consideration and
> attempts to redo and balance among the new subset of cpus.
> 
> There is a bug in this code path where the algorithm considers all active
> cpus in the system (minus the source that was just masked out).  This is
> not valid for two reasons: some active cpus may not be in the current
> scheduling domain and one of the active cpus is dst_cpu. These cpus should
> not be considered, as we cannot pull load from them.
> 
> Instead of failing out of load_balance(), we may end up redoing the search
> with no valid cpus and incorrectly concluding the domain is balanced.
> Additionally, if the group_imbalance flag was just set, it may also be
> incorrectly unset, thus the flag will not be seen by other cpus in future
> load_balance() runs as that algorithm intends.
> 
> Fix the check by removing cpus not in the current domain and the dst_cpu
> from considertation, thus limiting the evaluation to valid remaining cpus
> from which load might be migrated.
> 
> Co-authored-by: Austin Christ <austinwc@...eaurora.org>
> Co-authored-by: Dietmar Eggemann <dietmar.eggemann@....com>
> Signed-off-by: Jeffrey Hugo <jhugo@...eaurora.org>
> Tested-by: Tyler Baicar <tbaicar@...eaurora.org>
> ---
>  kernel/sched/fair.c | 22 ++++++++++------------
>  1 file changed, 10 insertions(+), 12 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index d711093..84255ab 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6737,10 +6737,10 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
>  		 * our sched_group. We may want to revisit it if we couldn't
>  		 * meet load balance goals by pulling other tasks on src_cpu.
>  		 *
> -		 * Also avoid computing new_dst_cpu if we have already computed
> -		 * one in current iteration.
> +		 * Avoid computing new_dst_cpu for NEWLY_IDLE or if we have
> +		 * already computed one in current iteration.
>  		 */
> -		if (!env->dst_grpmask || (env->flags & LBF_DST_PINNED))
> +		if (env->idle == CPU_NEWLY_IDLE || (env->flags & LBF_DST_PINNED))
>  			return 0;
>  
>  		/* Prevent to re-select dst_cpu via env's cpus */
> @@ -8091,14 +8091,7 @@ static int load_balance(int this_cpu, struct rq *this_rq,
>  		.tasks		= LIST_HEAD_INIT(env.tasks),
>  	};
>  
> -	/*
> -	 * For NEWLY_IDLE load_balancing, we don't need to consider
> -	 * other cpus in our group
> -	 */
> -	if (idle == CPU_NEWLY_IDLE)
> -		env.dst_grpmask = NULL;
> -
> -	cpumask_copy(cpus, cpu_active_mask);
> +	cpumask_and(cpus, sched_domain_span(sd), cpu_active_mask);
>  
>  	schedstat_inc(sd->lb_count[idle]);
>  
> @@ -8220,7 +8213,12 @@ static int load_balance(int this_cpu, struct rq *this_rq,
>  		/* All tasks on this runqueue were pinned by CPU affinity */
>  		if (unlikely(env.flags & LBF_ALL_PINNED)) {
>  			cpumask_clear_cpu(cpu_of(busiest), cpus);
> -			if (!cpumask_empty(cpus)) {
> +			/*
> +			 * Go back to "redo" iff the load-balance cpumask
> +			 * contains other potential busiest cpus for the
> +			 * current sched domain.
> +			 */
> +			if (!cpumask_subset(cpus, env.dst_grpmask)) {
>  				env.loop = 0;
>  				env.loop_break = sched_nr_migrate_break;
>  				goto redo;

So I was struggling with that subset condition. You want to ensure there
are CPUs outside of dst_grpmask left, otherwise balancing at this SD
level doesn't make sense anymore, right?

I think you might want to spell that out a little in that comment.
Currently the comment only explains what it does, which is something we
can read from the code. Comments should explain _why_ we do things and
its failing there.


So with that the problem is that active_load_balance_cpu_stop() calls
into can_migrate_task() with ->idle = CPU_IDLE and !dst_grpmask, which
then goes *bang*. Now active_load_balance_cpu_stop() doesn't need to
re-evaluate anything, so ideally it would just skip this entirely,
right?

So why not do #3:

---
 kernel/sched/fair.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 47a0c552c77b..fd639d32fa4c 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8523,6 +8523,13 @@ static int active_load_balance_cpu_stop(void *data)
 			.src_cpu	= busiest_rq->cpu,
 			.src_rq		= busiest_rq,
 			.idle		= CPU_IDLE,
+			/*
+			 * can_migrate_task() doesn't need to compute new_dst_cpu
+			 * for active balancing. Since we have CPU_IDLE, but no
+			 * @dst_grpmask we need to make that test go away with lying
+			 * about DST_PINNED.
+			 */
+			.flags		= LBF_DST_PINNED,
 		};
 
 		schedstat_inc(sd->alb_count);

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