Re: [PATCH] sched/fair: Fix wrong negative conversion in find_energy_efficient_cpu()

[peterz@infradead.org]

On Mon, Aug 10, 2020 at 12:18:25PM +0100, Valentin Schneider wrote:
> 
> 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>

Thanks guys!