Re: [PATCH] sched: Consider capacity for certain load balancing decisions

[Xi Wang]

On Fri, Feb 3, 2023 at 1:51 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Tue, Jan 31, 2023 at 05:20:32PM -0800, Xi Wang wrote:
> > After load balancing was split into different scenarios, CPU capacity
> > is ignored for the "migrate_task" case, which means a thread can stay
> > on a softirq heavy cpu for an extended amount of time.
> >
> > By comparing nr_running/capacity instead of just nr_running we can add
> > CPU capacity back into "migrate_task" decisions. This benefits
> > workloads running on machines with heavy network traffic. The change
> > is unlikely to cause serious problems for other workloads but maybe
> > some corner cases still need to be considered.
> >
> > Signed-off-by: Xi Wang <xii@xxxxxxxxxx>
> > ---
> >  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 0f8736991427..aad14bc04544 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -10368,8 +10368,9 @@ static struct rq *find_busiest_queue(struct lb_env *env,
> >                       break;
> >
> >               case migrate_task:
> > -                     if (busiest_nr < nr_running) {
> > +                     if (busiest_nr * capacity < nr_running * busiest_capacity) {
> >                               busiest_nr = nr_running;
> > +                             busiest_capacity = capacity;
> >                               busiest = rq;
> >                       }
> >                       break;
>
> I don't think this is correct. The migrate_task case is work-conserving,
> and your change can severely break that I think.
>

I think you meant this kind of scenario:
cpu 0: idle
cpu 1: 2 tasks
cpu 2: 1 task but only has 30% of capacity
Pulling from cpu 2 is good for the task but lowers the overall cpu
throughput.

The problem we have is:
cpu 0: idle
cpu 1: 1 task
cpu 2: 1 task but only has 60% of capacity due to net softirq
The task on cpu 2 stays there and runs slower. (This can also be
considered non work-conserving if we account softirq like a task.)

Maybe the logic can be merged like this: Use capacity but pick from
nr_running > 1 cpus first, then nr_running == 1 cpus if not found.