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

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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
(cherry picked from commit da0777d35f47892f359c3f73ea155870bb595700)
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: Ib3e22c155eb031854cfb4428494c1c4da57692b1
This commit is contained in:
Lukasz Luba 2020-08-10 09:30:04 +01:00 committed by Greg Kroah-Hartman
parent 5eee799bec
commit 91c21bea59
1 changed files with 2 additions and 1 deletions
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kernel/sched/fair.c
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@ -6477,7 +6477,8 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu, int sy
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.