6.5-stable review patch. If anyone has any objections, please let me know.
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From: Qais Yousef qyousef@layalina.io
[ Upstream commit 6b00a40147653c8ea748e8f4396510f252763364 ]
When uclamp_max is being used, the util of the task could be higher than the spare capacity of the CPU, but due to uclamp_max value we force-fit it there.
The way the condition for checking for max_spare_cap in find_energy_efficient_cpu() was constructed; it ignored any CPU that has its spare_cap less than or _equal_ to max_spare_cap. Since we initialize max_spare_cap to 0; this lead to never setting max_spare_cap_cpu and hence ending up never performing compute_energy() for this cluster and missing an opportunity for a better energy efficient placement to honour uclamp_max setting.
max_spare_cap = 0; cpu_cap = capacity_of(cpu) - cpu_util(p); // 0 if cpu_util(p) is high
...
util_fits_cpu(...); // will return true if uclamp_max forces it to fit
...
// this logic will fail to update max_spare_cap_cpu if cpu_cap is 0 if (cpu_cap > max_spare_cap) { max_spare_cap = cpu_cap; max_spare_cap_cpu = cpu; }
prev_spare_cap suffers from a similar problem.
Fix the logic by converting the variables into long and treating -1 value as 'not populated' instead of 0 which is a viable and correct spare capacity value. We need to be careful signed comparison is used when comparing with cpu_cap in one of the conditions.
Fixes: 1d42509e475c ("sched/fair: Make EAS wakeup placement consider uclamp restrictions") Signed-off-by: Qais Yousef (Google) qyousef@layalina.io Signed-off-by: Ingo Molnar mingo@kernel.org Reviewed-by: Vincent Guittot vincent.guittot@linaro.org Reviewed-by: Dietmar Eggemann dietmar.eggemann@arm.com Acked-by: Peter Zijlstra (Intel) peterz@infradead.org Link: https://lore.kernel.org/r/20230916232955.2099394-2-qyousef@layalina.io Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/sched/fair.c | 11 +++++------ 1 file changed, 5 insertions(+), 6 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 7e3ed4f447fdf..4ee95125d17a0 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -7562,11 +7562,10 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu) for (; pd; pd = pd->next) { unsigned long util_min = p_util_min, util_max = p_util_max; unsigned long cpu_cap, cpu_thermal_cap, util; - unsigned long cur_delta, max_spare_cap = 0; + long prev_spare_cap = -1, max_spare_cap = -1; unsigned long rq_util_min, rq_util_max; - unsigned long prev_spare_cap = 0; + unsigned long cur_delta, base_energy; int max_spare_cap_cpu = -1; - unsigned long base_energy; int fits, max_fits = -1;
cpumask_and(cpus, perf_domain_span(pd), cpu_online_mask); @@ -7629,7 +7628,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu) prev_spare_cap = cpu_cap; prev_fits = fits; } else if ((fits > max_fits) || - ((fits == max_fits) && (cpu_cap > max_spare_cap))) { + ((fits == max_fits) && ((long)cpu_cap > max_spare_cap))) { /* * Find the CPU with the maximum spare capacity * among the remaining CPUs in the performance @@ -7641,7 +7640,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu) } }
- if (max_spare_cap_cpu < 0 && prev_spare_cap == 0) + if (max_spare_cap_cpu < 0 && prev_spare_cap < 0) continue;
eenv_pd_busy_time(&eenv, cpus, p); @@ -7649,7 +7648,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu) base_energy = compute_energy(&eenv, pd, cpus, p, -1);
/* Evaluate the energy impact of using prev_cpu. */ - if (prev_spare_cap > 0) { + if (prev_spare_cap > -1) { prev_delta = compute_energy(&eenv, pd, cpus, p, prev_cpu); /* CPU utilization has changed */