Re: [PATCH] sched/fair: Fix detection of per-CPU kthreads waking a task

[Vincent Guittot <vincent.guittot@linaro.org>]

On Mon, 29 Nov 2021 at 17:54, Vincent Donnefort
<vincent.donnefort@arm.com> wrote:
>
> [...]
>
> > > > >
> > > > > still i don't see the need of !is_idle_task(current)
> > > > >
> > > >
> > > > Admittedly, belts and braces. The existing condition checks rq->nr_running <= 1
> > > > which can lead to coscheduling when the wakeup is issued by the idle task
> > > > (or even if rq->nr_running == 0, you can have rq->ttwu_pending without
> > > > having sent an IPI due to polling). Essentially this overrides the first
> > > > check in sis() that uses idle_cpu(target) (prev == smp_processor_id() ==
> > > > target).
> > > >
> > > > I couldn't prove such wakeups can happen right now, but if/when they do
> > > > (AIUI it would just take someone to add a wake_up_process() down some
> > > > smp_call_function() callback) then we'll need the above. If you're still
> > > > not convinced by now, I won't push it further.
> > >
> > > From a quick experiment, even with the asym_fits_capacity(), I can trigger
> > > the following:
> > >
> > > [    0.118855] select_idle_sibling: wakee=kthreadd:2 nr_cpus_allowed=8 current=swapper/0:1 in_task=1
> > > [    0.128214] select_idle_sibling: wakee=rcu_gp:3 nr_cpus_allowed=8 current=swapper/0:1 in_task=1
> > > [    0.137327] select_idle_sibling: wakee=rcu_par_gp:4 nr_cpus_allowed=8 current=swapper/0:1 in_task=1
> > > [    0.147221] select_idle_sibling: wakee=kworker/u16:0:7 nr_cpus_allowed=8 current=swapper/0:1 in_task=1
> > > [    0.156994] select_idle_sibling: wakee=mm_percpu_wq:8 nr_cpus_allowed=8 current=swapper/0:1 in_task=1
> >
> > Timestamp shows its booting phase and thread name above shows per cpu
> > thread. Could it happen just while creating per cpu thread at boot and
> > as a result not relevant ?
>
> I have more of those logs a bit later in the boot:
>
> [    0.484791] select_idle_sibling: wakee=kthreadd:2 nr_cpus_allowed=8 current=swapper/0:1 in_task=1
> [    0.516495] select_idle_sibling: wakee=kthreadd:2 nr_cpus_allowed=8 current=swapper/0:1 in_task=1
> [    0.525758] select_idle_sibling: wakee=kthreadd:2 nr_cpus_allowed=8 current=swapper/0:1 in_task=1
> [    0.535078] select_idle_sibling: wakee=kthreadd:2 nr_cpus_allowed=8 current=swapper/0:1 in_task=1
> [    0.547486] select_idle_sibling: wakee=kthreadd:2 nr_cpus_allowed=8 current=swapper/0:1 in_task=1
> [    0.579192] select_idle_sibling: wakee=kthreadd:2 nr_cpus_allowed=8 current=swapper/0:1 in_task=1
>
> The nr_cpus_allowed=8 suggest that none of the threads from the logs I
> shared are per-CPU. Sorry if the format is confusing, I used:
>
>   wakee=<comm>:<pid> current=<comm>:<pid>.
>
> >
> > Can you see similar things later after booting ?
>
> I tried few scenarios other than the boot time but none of them produced
> "current=swapper/X:1 in_task=1"
>
> >
> > I have tried to trigger the situation but failed to get wrong
> > sequence. All are coming from interrupt while idle.
> > After adding in_task() condition, I haven't been able to trigger the
> > warn() that I added to catch the wrong situations on SMP, Heterogenous
> > or NUMA system. Could you share more details on your setup ?
> >
>
> This is just my Hikey960 with the asym_fits_capacity() fix [1] to make sure I
> don't simply hit the other issue with asym platforms.

I ran my previous tests on dragonboard 845c which is dynamiQ and I
have tried on my hikey960 since  but without any success so far. This
is what i use:

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6397,9 +6397,12 @@ static int select_idle_sibling(struct
task_struct *p, int prev, int target)
         * essentially a sync wakeup. An obvious example of this
         * pattern is IO completions.
         */
-       if (is_per_cpu_kthread(current) &&
+       if (in_task() &&
+           is_per_cpu_kthread(current) &&
            prev == smp_processor_id() &&
            this_rq()->nr_running <= 1) {
+
+               WARN(is_idle_task(current), "idle per cpu kthread: cpu
%d task: %s", prev, p->comm);
                return prev;
        }


Without in_task() condition, i've got warnings from interrupt context
but nothing else.
Note that I don't even have the asym_fits_capacity() condition

>
> Then I just added my log in the per-CPU kthread wakee stacking exit path
>
>     printk("%s: wakee=%s:%d nr_cpus_allowed=%d current=%s:%d in_task=%d\n",
>             __func__, p->comm, p->pid, p->nr_cpus_allowed, current->comm, current->pid, in_task());
>
>
> [1] https://lore.kernel.org/all/20211125101239.3248857-1-vincent.donnefort@arm.com/
>
>
> From the same logs I also see:
>
>   wakee=xfsaild/mmcblk0:4855 nr_cpus_allowed=8 current=kworker/1:1:1070 in_task=0
>
> Doesn't that look like a genuine wakeup that would escape the per-CPU kthread
> stacking exit path because of the in_task test?