linux-kernel - [bug report] sched/fair: Prefer prev cpu in asymmetric wakeup path

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Date:   Fri, 13 Nov 2020 11:46:57 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     vincent.guittot@...aro.org
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Valentin Schneider <valentin.schneider@....com>,
        linux-kernel@...r.kernel.org
Subject: [bug report] sched/fair: Prefer prev cpu in asymmetric wakeup path

Hello Vincent Guittot,

The patch b4c9c9f15649: "sched/fair: Prefer prev cpu in asymmetric
wakeup path" from Oct 29, 2020, leads to the following static checker
warning:

	kernel/sched/fair.c:6249 select_idle_sibling()
	error: uninitialized symbol 'task_util'.

kernel/sched/fair.c
  6233  static int select_idle_sibling(struct task_struct *p, int prev, int target)
  6234  {
  6235          struct sched_domain *sd;
  6236          unsigned long task_util;
  6237          int i, recent_used_cpu;
  6238  
  6239          /*
  6240           * On asymmetric system, update task utilization because we will check
  6241           * that the task fits with cpu's capacity.
  6242           */

The original comment was a bit more clear...  Perhaps "On asymmetric
system[s], [record the] task utilization because we will check that the
task [can be done within] the cpu's capacity."

  6243          if (static_branch_unlikely(&sched_asym_cpucapacity)) {
  6244                  sync_entity_load_avg(&p->se);
  6245                  task_util = uclamp_task_util(p);
  6246          }

"task_util" is not initialized on the else path.

  6247  
  6248          if ((available_idle_cpu(target) || sched_idle_cpu(target)) &&
  6249              asym_fits_capacity(task_util, target))
                                       ^^^^^^^^^
Uninitialized variable warning.

  6250                  return target;
  6251  
  6252          /*
  6253           * If the previous CPU is cache affine and idle, don't be stupid:
  6254           */
  6255          if (prev != target && cpus_share_cache(prev, target) &&
  6256              (available_idle_cpu(prev) || sched_idle_cpu(prev)) &&
  6257              asym_fits_capacity(task_util, prev))
  6258                  return prev;
  6259  
  6260          /*
  6261           * Allow a per-cpu kthread to stack with the wakee if the

regards,
dan carpenter