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

[Xi Wang]

On Mon, Feb 6, 2023 at 1:28 AM Vincent Guittot
<vincent.guittot@xxxxxxxxxx> wrote:
>
> On Fri, 3 Feb 2023 at 19:47, Xi Wang <xii@xxxxxxxxxx> wrote:
> >
> > 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.)
>
> When load_balance runs for this 2 cpus, cpu2 should be tagged as
> misfit_task because of reduce_capacity and should be selected in
> priority by cpu0 to pull the task. Do you have more details on your
> topology ?

The topology is 64 core AMD with 2 hyperthreads.

I am not familiar with the related code but I think there are cases
where a task fits cpu capacity but it can still run faster elsewhere,
e.g.: Bursty workloads. Thread pool threads with variable utilization
because it would process more or less requests based on cpu
availability (pick the next request from a shared queue when the
previous one is done). A thread having enough cpu cycles but runs
slower due to softirqs can also directly affect application
performance.

> >
> > 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.