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 1a68a0536adda5257f44468451501ea1448d8005..51408ebd76c276d902df38227671859788602ea5 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6594,7 +6594,8 @@ 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.