sched/fair: Fix wrong negative conversion in find_energy_efficient_cpu()

[ Upstream commit da0777d35f47892f359c3f73ea155870bb595700 ]

In find_energy_efficient_cpu() 'cpu_cap' could be less that 'util'.
It might be because of RT, DL (so higher sched class than CFS), irq or
thermal pressure signal, which reduce the capacity value.
In such situation the result of 'cpu_cap - util' might be negative but
stored in the unsigned long. Then it might be compared with other unsigned
long when uclamp_rq_util_with() reduced the 'util' such that is passes the
fits_capacity() check.

Prevent this situation and make the arithmetic more safe.

Fixes: 1d42509e475cd ("sched/fair: Make EAS wakeup placement consider uclamp restrictions")
Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Link: https://lkml.kernel.org/r/20200810083004.26420-1-lukasz.luba@arm.com
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6b3b59cc51d6ca4f3a4a9ed8568f0fa5b8634e70..f71e8b0e0346a2c530d0b89f62b2ff886865f19a 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6586,7 +6586,8 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
 
                        util = cpu_util_next(cpu, p, cpu);
                        cpu_cap = capacity_of(cpu);
-                       spare_cap = cpu_cap - util;
+                       spare_cap = cpu_cap;
+                       lsub_positive(&spare_cap, util);
 
                        /*
                         * Skip CPUs that cannot satisfy the capacity request.