[tip: sched/core] sched/fair: Fix wrong negative conversion in find_energy_efficient_cpu()

[tip-bot2 for Lukasz Luba]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     da0777d35f47892f359c3f73ea155870bb595700
Gitweb:        https://git.kernel.org/tip/da0777d35f47892f359c3f73ea155870bb595700
Author:        Lukasz Luba <lukasz.luba@xxxxxxx>
AuthorDate:    Mon, 10 Aug 2020 09:30:04 +01:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Wed, 26 Aug 2020 12:41:57 +02:00

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

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@xxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Valentin Schneider <valentin.schneider@xxxxxxx>
Link: https://lkml.kernel.org/r/20200810083004.26420-1-lukasz.luba@xxxxxxx
---
 kernel/sched/fair.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index abdb54e..90ebaa4 100644
--- 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.