Re: [PATCH 2/2] sched/fair: Fix use of NULL with find_idlest_group

[Josef Bacik <josef@toxicpanda.com>]

On Mon, Aug 21, 2017 at 04:21:28PM +0100, Brendan Jackman wrote:
> The current use of returning NULL from find_idlest_group is broken in
> two cases:
> 
> a1) The local group is not allowed.
> 
>    In this case, we currently do not change this_runnable_load or
>    this_avg_load from its initial value of 0, which means we return
>    NULL regardless of the load of the other, allowed groups. This
>    results in pointlessly continuing the find_idlest_group search
>    within the local group and then returning prev_cpu from
>    select_task_rq_fair.
> 
> a2) No CPUs in the sched_domain are allowed.
> 
>    In this case we also return NULL and again pointlessly continue
>    the search.
> 
> b) smp_processor_id() is the "idlest" and != prev_cpu.
> 
>    find_idlest_group also returns NULL when the local group is
>    allowed and is the idlest. The caller then continues the
>    find_idlest_group search at a lower level of the current CPU's
>    sched_domain hierarchy. However new_cpu is not updated. This means
>    the search is pointless and we return prev_cpu from
>    select_task_rq_fair.
> 
> This is fixed by:
> 
> 1. Returning NULL from find_idlest_group only when _no_ groups were
>    allowed in the current sched_domain. In this case, we now break
>    from the while(sd) loop and immediately return prev_cpu. This
>    fixes case a2).
> 
> 2. Initializing this_runnable_load and this_avg_load to ULONG_MAX
>    instead of 0. This means in case a1) we now return the idlest
>    non-local group.
> 
> 3. Explicitly updating new_cpu when find_idlest_group returns the
>    local group, fixing case b).
> 
> This patch also re-words the check for whether the group in
> consideration is local, under the assumption that the first group in
> the sched domain is always the local one.
> 
> Signed-off-by: Brendan Jackman <brendan.jackman@arm.com>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Morten Rasmussen <morten.rasmussen@arm.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
> Cc: Vincent Guittot <vincent.guittot@linaro.org>
> ---
>  kernel/sched/fair.c | 34 +++++++++++++++++++---------------
>  1 file changed, 19 insertions(+), 15 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 64618d768546..7cb5ed719cf9 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5382,26 +5382,29 @@ static unsigned long capacity_spare_wake(int cpu, struct task_struct *p)
>   * domain.
>   */
>  static struct sched_group *
> -find_idlest_group(struct sched_domain *sd, struct task_struct *p,
> -		  int this_cpu, int sd_flag)
> +find_idlest_group(struct sched_domain *sd, struct task_struct *p, int sd_flag)
>  {
>  	struct sched_group *idlest = NULL, *group = sd->groups;
> +	struct sched_group *local_group = sd->groups;
>  	struct sched_group *most_spare_sg = NULL;
> -	unsigned long min_runnable_load = ULONG_MAX, this_runnable_load = 0;
> -	unsigned long min_avg_load = ULONG_MAX, this_avg_load = 0;
> +	unsigned long min_runnable_load = ULONG_MAX, this_runnable_load = ULONG_MAX;
> +	unsigned long min_avg_load = ULONG_MAX, this_avg_load = ULONG_MAX;
>  	unsigned long most_spare = 0, this_spare = 0;
>  	int load_idx = sd->forkexec_idx;
>  	int imbalance_scale = 100 + (sd->imbalance_pct-100)/2;
>  	unsigned long imbalance = scale_load_down(NICE_0_LOAD) *
>  				(sd->imbalance_pct-100) / 100;
>  
> +	if (!cpumask_intersects(sched_domain_span(sd), &p->cpus_allowed))
> +		return NULL;
> +
>  	if (sd_flag & SD_BALANCE_WAKE)
>  		load_idx = sd->wake_idx;
>  
>  	do {
>  		unsigned long load, avg_load, runnable_load;
>  		unsigned long spare_cap, max_spare_cap;
> -		int local_group;
> +		bool group_is_local = group == local_group;
>  		int i;
>  
>  		/* Skip over this group if it has no CPUs allowed */
> @@ -5409,9 +5412,6 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,
>  					&p->cpus_allowed))
>  			continue;
>  
> -		local_group = cpumask_test_cpu(this_cpu,
> -					       sched_group_span(group));
> -

This isn't right is it?  cpu isn't necessarily in the very first group of a sd
right?  Thanks,

Josef