Re: [PATCH] sched/fair: Skip cpus with no sched domain attached during NOHZ idle balance

[Pierre Gondois]

Hello Rui,

On 8/14/23 10:30, Zhang, Rui wrote:
> On Mon, 2023-08-14 at 11:14 +0800, Aaron Lu wrote:
> > Hi Rui,
> >
> > On Fri, Aug 04, 2023 at 05:08:58PM +0800, Zhang Rui wrote:
> > > Problem statement
> > > -----------------
> > > When using cgroup isolated partition to isolate cpus including
> > > cpu0, it
> > > is observed that cpu0 is woken up frequenctly but doing nothing.
> > > This is
> > > not good for power efficiency.
> > >
> > > <idle>-0     [000]   616.491602: hrtimer_cancel:
> > > hrtimer=0xffff8e8fdf623c10
> > > <idle>-0     [000]   616.491608: hrtimer_start:
> > > hrtimer=0xffff8e8fdf623c10 function=tick_sched_timer/0x0
> > > expires=615996000000 softexpires=615996000000
> > > <idle>-0     [000]   616.491616: rcu_utilization:      Start
> > > context switch
> > > <idle>-0     [000]   616.491618: rcu_utilization:      End context
> > > switch
> > > <idle>-0     [000]   616.491637: tick_stop:            success=1
> > > dependency=NONE
> > > <idle>-0     [000]   616.491637: hrtimer_cancel:
> > > hrtimer=0xffff8e8fdf623c10
> > > <idle>-0     [000]   616.491638: hrtimer_start:
> > > hrtimer=0xffff8e8fdf623c10 function=tick_sched_timer/0x0
> > > expires=616420000000 softexpires=616420000000
> > >
> > > The above pattern repeats every one or multiple ticks, results in
> > > total
> > > 2000+ wakeups on cpu0 in 60 seconds, when running workload on the
> > > cpus that are not in the isolated partition.
> > >
> > > Rootcause
> > > ---------
> > > In NOHZ mode, an active cpu either sends an IPI or touches the idle
> > > cpu's polling flag to wake it up, so that the idle cpu can pull
> > > tasks
> > > from the busy cpu. The logic for selecting the target cpu is to use
> > > the
> > > first idle cpu that presents in both nohz.idle_cpus_mask and
> > > housekeeping_cpumask.
> > >
> > > In the above scenario, when cpu0 is in the cgroup isolated
> > > partition,
> > > its sched domain is deteched, but it is still available in both of
> > > the
> > > above cpumasks. As a result, cpu0
> >
> > I saw in nohz_balance_enter_idle(), if a cpu is isolated, it will not
> > set itself in nohz.idle_cpus_mask and thus should not be chosen as
> > ilb_cpu. I wonder what's stopping this from working?
>
> One thing I forgot to mention is that the problem is gone if we offline
> and re-online those cpus. In that case, the isolated cpus are removed
> from the nohz.idle_cpus_mask in sched_cpu_deactivate() and are never
> added back.
>
> At runtime, the cpus can be removed from the nohz.idle_cpus_mask in
> below case only
> 	trigger_load_balance()
> 	        if (unlikely(on_null_domain(rq) || !cpu_active(cpu_of(rq))))
> 	                return;
> 	        nohz_balancer_kick(rq);
> 			nohz_balance_exit_idle()
>
> My understanding is that if a cpu is in nohz.idle_cpus_mask when it is
> isolated, there is no chance to remove it from that mask later, so the
> check in nohz_balance_enter_idle() does not help.


As you mentioned, once a CPU is isolated, its rq->nohz_tick_stopped is
never updated. Instead of avoiding isolated CPUs at each find_new_ilb() call
as this patch does, maybe it would be better to permanently remove these CPUs
from nohz.idle_cpus_mask. This would lower the number of checks done.
This could be done with something like:
@@ -11576,6 +11586,20 @@ void nohz_balance_enter_idle(int cpu)
          */
         rq->has_blocked_load = 1;
  
+       /* If we're a completely isolated CPU, we don't play: */
+       if (on_null_domain(rq)) {
+               if (cpumask_test_cpu(rq->cpu, nohz.idle_cpus_mask)) {
+                       cpumask_clear_cpu(rq->cpu, nohz.idle_cpus_mask);
+                       atomic_dec(&nohz.nr_cpus);
+               }
+
+               /*
+                * Set nohz_tick_stopped on isolated CPUs to avoid keeping the
+                * value that was stored when rebuild_sched_domains() was called.
+                */
+               rq->nohz_tick_stopped = 1;
+       }
+
         /*
          * The tick is still stopped but load could have been added in the
          * meantime. We set the nohz.has_blocked flag to trig a check of the
@@ -11585,10 +11609,6 @@ void nohz_balance_enter_idle(int cpu)
         if (rq->nohz_tick_stopped)
                 goto out;
  
-       /* If we're a completely isolated CPU, we don't play: */
-       if (on_null_domain(rq))
-               return;
-
         rq->nohz_tick_stopped = 1;
  
         cpumask_set_cpu(cpu, nohz.idle_cpus_mask);

Otherwise I could reproduce the issue and the patch was solving it, so:
Tested-by: Pierre Gondois <pierre.gondois@xxxxxxx>





Also, your patch doesn't aim to solve that, but I think there is an issue
when updating cpuset.cpus when an isolated partition was already created:

// Create an isolated partition containing CPU0
# mkdir cgroup
# mount -t cgroup2 none cgroup/
# mkdir cgroup/Testing
# echo "+cpuset" > cgroup/cgroup.subtree_control
# echo "+cpuset" > cgroup/Testing/cgroup.subtree_control
# echo 0 > cgroup/Testing/cpuset.cpus
# echo isolated > cgroup/Testing/cpuset.cpus.partition

// CPU0's sched domain is detached:
# ls /sys/kernel/debug/sched/domains/cpu0/
# ls /sys/kernel/debug/sched/domains/cpu1/
domain0  domain1

// Change the isolated partition to be CPU1
# echo 1 > cgroup/Testing/cpuset.cpus

// CPU[0-1] sched domains are not updated:
# ls /sys/kernel/debug/sched/domains/cpu0/
# ls /sys/kernel/debug/sched/domains/cpu1/
domain0  domain1

Regards,
Pierre