Messages in this thread | | | From | Valentin Schneider <> | Subject | Re: [PATCH] sched/fair: Fix wrong negative conversion in find_energy_efficient_cpu() | Date | Mon, 10 Aug 2020 12:18:25 +0100 |
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On 10/08/20 09:30, Lukasz Luba wrote: > 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")
I was going to say that might even go as far back as:
732cd75b8c92 ("sched/fair: Select an energy-efficient CPU on task wake-up")
but we had a capacity fitness check in the right place back then, which I screwed over with that uclamp_rq_util_with() :/
LGTM, thanks for figuring that one out.
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
> Signed-off-by: Lukasz Luba <lukasz.luba@arm.com> > --- > 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 1a68a0536add..51408ebd76c2 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.
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