Re: [PATCH v5 6/6] sched/fair: Consider SMT in ASYM_PACKING load balance

[Guillaume Tucker]

Hi Ricardo,

Please see the bisection report below about a boot failure on
rk3288-rock64.

Reports aren't automatically sent to the public while we're
trialing new bisection features on kernelci.org but this one
looks valid.

Some more details can be found here:

  https://linux.kernelci.org/test/case/id/6155a4b0836c79a98f99a31d/

It looks like some i.MX arm64 platforms also regressed with
next-20210920 although it hasn't been verified yet whether that's
due to the same commit:

  https://linux.kernelci.org/test/job/next/branch/master/kernel/next-20210930/plan/baseline/

The x86 platforms don't seem to be impacted though.

Please let us know if you need help debugging the issue or if you
have a fix to try.

Best wishes,
Guillaume


GitHub: https://github.com/kernelci/kernelci-project/issues/65

-------------------------------------------------------------------------------


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* This automated bisection report was sent to you on the basis  *
* that you may be involved with the breaking commit it has      *
* found.  No manual investigation has been done to verify it,   *
* and the root cause of the problem may be somewhere else.      *
*                                                               *
* If you do send a fix, please include this trailer:            *
*   Reported-by: "kernelci.org bot" <bot@xxxxxxxxxxxx>          *
*                                                               *
* Hope this helps!                                              *
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *

next/master bisection: baseline.login on rk3328-rock64

Summary:
  Start:      2d02a18f75fc Add linux-next specific files for 20210929
  Plain log:  https://storage.kernelci.org/next/master/next-20210929/arm64/defconfig+CONFIG_RANDOMIZE_BASE=y/gcc-8/lab-baylibre/baseline-rk3328-rock64.txt
  HTML log:   https://storage.kernelci.org/next/master/next-20210929/arm64/defconfig+CONFIG_RANDOMIZE_BASE=y/gcc-8/lab-baylibre/baseline-rk3328-rock64.html
  Result:     eac6f3841f1d sched/fair: Consider SMT in ASYM_PACKING load balance

Checks:
  revert:     PASS
  verify:     PASS

Parameters:
  Tree:       next
  URL:        https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git
  Branch:     master
  Target:     rk3328-rock64
  CPU arch:   arm64
  Lab:        lab-baylibre
  Compiler:   gcc-8
  Config:     defconfig+CONFIG_RANDOMIZE_BASE=y
  Test case:  baseline.login

Breaking commit found:

-------------------------------------------------------------------------------
commit eac6f3841f1dac7b6f43002056b63f44cc1f1543
Author: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
Date:   Fri Sep 10 18:18:19 2021 -0700

    sched/fair: Consider SMT in ASYM_PACKING load balance


On 11/09/2021 03:18, Ricardo Neri wrote:
> When deciding to pull tasks in ASYM_PACKING, it is necessary not only to
> check for the idle state of the destination CPU, dst_cpu, but also of
> its SMT siblings.
> 
> If dst_cpu is idle but its SMT siblings are busy, performance suffers
> if it pulls tasks from a medium priority CPU that does not have SMT
> siblings.
> 
> Implement asym_smt_can_pull_tasks() to inspect the state of the SMT
> siblings of both dst_cpu and the CPUs in the candidate busiest group.
> 
> Cc: Aubrey Li <aubrey.li@xxxxxxxxx>
> Cc: Ben Segall <bsegall@xxxxxxxxxx>
> Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
> Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Quentin Perret <qperret@xxxxxxxxxx>
> Cc: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> Cc: Srinivas Pandruvada <srinivas.pandruvada@xxxxxxxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> Reviewed-by: Len Brown <len.brown@xxxxxxxxx>
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
> ---
> Changes since v4:
>   * Use sg_lb_stats::sum_nr_running the idle state of a scheduling group.
>     (Vincent, Peter)
>   * Do not even idle CPUs in asym_smt_can_pull_tasks(). (Vincent)
>   * Updated function documentation and corrected a typo.
> 
> Changes since v3:
>   * Removed the arch_asym_check_smt_siblings() hook. Discussions with the
>     powerpc folks showed that this patch should not impact them. Also, more
>     recent powerpc processor no longer use asym_packing. (PeterZ)
>   * Removed unnecessary local variable in asym_can_pull_tasks(). (Dietmar)
>   * Removed unnecessary check for local CPUs when the local group has zero
>     utilization. (Joel)
>   * Renamed asym_can_pull_tasks() as asym_smt_can_pull_tasks() to reflect
>     the fact that it deals with SMT cases.
>   * Made asym_smt_can_pull_tasks() return false for !CONFIG_SCHED_SMT so
>     that callers can deal with non-SMT cases.
> 
> Changes since v2:
>   * Reworded the commit message to reflect updates in code.
>   * Corrected misrepresentation of dst_cpu as the CPU doing the load
>     balancing. (PeterZ)
>   * Removed call to arch_asym_check_smt_siblings() as it is now called in
>     sched_asym().
> 
> Changes since v1:
>   * Don't bailout in update_sd_pick_busiest() if dst_cpu cannot pull
>     tasks. Instead, reclassify the candidate busiest group, as it
>     may still be selected. (PeterZ)
>   * Avoid an expensive and unnecessary call to cpumask_weight() when
>     determining if a sched_group is comprised of SMT siblings.
>     (PeterZ).
> ---
>  kernel/sched/fair.c | 94 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 94 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 26db017c14a3..8d763dd0174b 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8597,10 +8597,98 @@ group_type group_classify(unsigned int imbalance_pct,
>  	return group_has_spare;
>  }
>  
> +/**
> + * asym_smt_can_pull_tasks - Check whether the load balancing CPU can pull tasks
> + * @dst_cpu:	Destination CPU of the load balancing
> + * @sds:	Load-balancing data with statistics of the local group
> + * @sgs:	Load-balancing statistics of the candidate busiest group
> + * @sg:		The candidate busiest group
> + *
> + * Check the state of the SMT siblings of both @sds::local and @sg and decide
> + * if @dst_cpu can pull tasks.
> + *
> + * If @dst_cpu does not have SMT siblings, it can pull tasks if two or more of
> + * the SMT siblings of @sg are busy. If only one CPU in @sg is busy, pull tasks
> + * only if @dst_cpu has higher priority.
> + *
> + * If both @dst_cpu and @sg have SMT siblings, and @sg has exactly one more
> + * busy CPU than @sds::local, let @dst_cpu pull tasks if it has higher priority.
> + * Bigger imbalances in the number of busy CPUs will be dealt with in
> + * update_sd_pick_busiest().
> + *
> + * If @sg does not have SMT siblings, only pull tasks if all of the SMT siblings
> + * of @dst_cpu are idle and @sg has lower priority.
> + */
> +static bool asym_smt_can_pull_tasks(int dst_cpu, struct sd_lb_stats *sds,
> +				    struct sg_lb_stats *sgs,
> +				    struct sched_group *sg)
> +{
> +#ifdef CONFIG_SCHED_SMT
> +	bool local_is_smt, sg_is_smt;
> +	int sg_busy_cpus;
> +
> +	local_is_smt = sds->local->flags & SD_SHARE_CPUCAPACITY;
> +	sg_is_smt = sg->flags & SD_SHARE_CPUCAPACITY;
> +
> +	sg_busy_cpus = sgs->group_weight - sgs->idle_cpus;
> +
> +	if (!local_is_smt) {
> +		/*
> +		 * If we are here, @dst_cpu is idle and does not have SMT
> +		 * siblings. Pull tasks if candidate group has two or more
> +		 * busy CPUs.
> +		 */
> +		if (sg_is_smt && sg_busy_cpus >= 2)
> +			return true;
> +
> +		/*
> +		 * @dst_cpu does not have SMT siblings. @sg may have SMT
> +		 * siblings and only one is busy. In such case, @dst_cpu
> +		 * can help if it has higher priority and is idle (i.e.,
> +		 * it has no running tasks).
> +		 */
> +		return !sds->local_stat.sum_nr_running &&
> +		       sched_asym_prefer(dst_cpu, sg->asym_prefer_cpu);
> +	}
> +
> +	/* @dst_cpu has SMT siblings. */
> +
> +	if (sg_is_smt) {
> +		int local_busy_cpus = sds->local->group_weight -
> +				      sds->local_stat.idle_cpus;
> +		int busy_cpus_delta = sg_busy_cpus - local_busy_cpus;
> +
> +		if (busy_cpus_delta == 1)
> +			return sched_asym_prefer(dst_cpu,
> +						 sg->asym_prefer_cpu);
> +
> +		return false;
> +	}
> +
> +	/*
> +	 * @sg does not have SMT siblings. Ensure that @sds::local does not end
> +	 * up with more than one busy SMT sibling and only pull tasks if there
> +	 * are not busy CPUs (i.e., no CPU has running tasks).
> +	 */
> +	if (!sds->local_stat.sum_nr_running)
> +		return sched_asym_prefer(dst_cpu, sg->asym_prefer_cpu);
> +
> +	return false;
> +#else
> +	/* Always return false so that callers deal with non-SMT cases. */
> +	return false;
> +#endif
> +}
> +
>  static inline bool
>  sched_asym(struct lb_env *env, struct sd_lb_stats *sds,  struct sg_lb_stats *sgs,
>  	   struct sched_group *group)
>  {
> +	/* Only do SMT checks if either local or candidate have SMT siblings */
> +	if ((sds->local->flags & SD_SHARE_CPUCAPACITY) ||
> +	    (group->flags & SD_SHARE_CPUCAPACITY))
> +		return asym_smt_can_pull_tasks(env->dst_cpu, sds, sgs, group);
> +
>  	return sched_asym_prefer(env->dst_cpu, group->asym_prefer_cpu);
>  }
>  
> @@ -9606,6 +9694,12 @@ static struct rq *find_busiest_queue(struct lb_env *env,
>  		    nr_running == 1)
>  			continue;
>  
> +		/* Make sure we only pull tasks from a CPU of lower priority */
> +		if ((env->sd->flags & SD_ASYM_PACKING) &&
> +		    sched_asym_prefer(i, env->dst_cpu) &&
> +		    nr_running == 1)
> +			continue;
> +
>  		switch (env->migration_type) {
>  		case migrate_load:
>  			/*
>