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Date:   Thu, 7 May 2020 14:04:17 -0400
From:   Phil Auld <pauld@redhat.com>
To:     Vincent Guittot <vincent.guittot@linaro.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Juri Lelli <juri.lelli@redhat.com>
Subject: Re: [PATCH] sched/fair: Fix enqueue_task_fair warning some more
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Hi Vincent,

On Thu, May 07, 2020 at 05:06:29PM +0200 Vincent Guittot wrote:
> Hi Phil,
> 
> On Wed, 6 May 2020 at 20:05, Phil Auld <pauld@redhat.com> wrote:
> >
> > Hi Vincent,
> >
> > Thanks for taking a look. More below...
> >
> > On Wed, May 06, 2020 at 06:36:45PM +0200 Vincent Guittot wrote:
> > > Hi Phil,
> > >
> > > - reply to all this time
> > >
> > > On Wed, 6 May 2020 at 16:18, Phil Auld <pauld@redhat.com> wrote:
> > > >
> > > > sched/fair: Fix enqueue_task_fair warning some more
> > > >
> > > > The recent patch, fe61468b2cb (sched/fair: Fix enqueue_task_fair warning)
> > > > did not fully resolve the issues with the (rq->tmp_alone_branch !=
> > > > &rq->leaf_cfs_rq_list) warning in enqueue_task_fair. There is a case where
> > > > the first for_each_sched_entity loop exits due to on_rq, having incompletely
> > > > updated the list.  In this case the second for_each_sched_entity loop can
> > > > further modify se. The later code to fix up the list management fails to do
> > >
> > > But for the 2nd  for_each_sched_entity, the cfs_rq should already be
> > > in the list, isn't it ?
> >
> > No. In this case we hit the parent not on list case in list_add_leaf_cfs_rq
> > which sets rq-tmp_alone_branch to cfs_rq->leaf_cfs_rq_list which is not
> > the same. It returns false expecting the parent to be added later.
> >
> > But then the parent doens't get there because it's on_rq.
> >
> > >
> > > The third for_each_entity loop is there for the throttled case but is
> > > useless for other case
> > >
> >
> > There actually is a throttling involved usually. The second loop breaks out
> > early because one of the parents is throttled. But not before it advances
> > se at least once.
> 
> Ok, that's even because of the throttling that the problem occurs
> 
> >
> > Then the 3rd loop doesn't fix the tmp_alone_branch because it doesn't start
> > with the right se.
> >
> > > Could you provide us some details about the use case that creates such
> > > a situation ?
> > >
> >
> > I admit I had to add trace_printks to get here. Here's what it showed (sorry
> > for the long lines...)
> >
> > 1)  sh-6271  [044]  1271.322317: bprint: enqueue_task_fair: se 0xffffa085e7e30080 on_rq 0 cfs_rq = 0xffffa085e93da200
> > 2)  sh-6271  [044]  1271.322320: bprint: enqueue_entity: Add_leaf_rq: cpu 17: nr_r 2; cfs 0xffffa085e93da200 onlist 0 tmp_a_b = 0xffffa085ef92c868 &rq->l_c_r_l = 0xffffa085ef92c868
> > 3)  sh-6271  [044]  1271.322322: bprint: enqueue_entity: Add_leaf_rq: cpu 17: nr_r 2: parent not onlist  Set t_a_branch to 0xffffa085e93da340 rq->l_c_r_l = 0xffffa085ef92c868
> > 4)  sh-6271  [044]  1271.322323: bprint: enqueue_task_fair: se 0xffffa085e93d8800 on_rq 1 cfs_rq = 0xffffa085dbfaea00
> > 5)  sh-6271  [044]  1271.322324: bprint: enqueue_task_fair: Done enqueues, se=0xffffa085e93d8800, pid=3642
> > 6)  sh-6271  [044]  1271.322326: bprint: enqueue_task_fair: update: cfs 0xffffa085e48ce000 throttled, se = 0xffffa085dbfafc00
> > 7)  sh-6271  [044]  1271.322326: bprint: enqueue_task_fair: current se = 0xffffa085dbfafc00, orig_se = 0xffffa085e7e30080
> > 8)  sh-6271  [044]  1271.322327: bprint: enqueue_task_fair: Add_leaf_rq: cpu 17: nr_r 2; cfs 0xffffa085e48ce000 onlist 1 tmp_a_b = 0xffffa085e93da340 &rq->l_c_r_l = 0xffffa085ef92c868
> > 9)  sh-6271  [044]  1271.322328: bprint: enqueue_task_fair: Add_leaf_rq: cpu 17: nr_r 0; cfs 0xffffa085ef92bf80 onlist 1 tmp_a_b = 0xffffa085e93da340 &rq->l_c_r_l = 0xffffa085ef92c868
> > 10) sh-6271  [044]  1271.672599: bprint: enqueue_task_fair: cpu 17: rq->tmp_alone_branch = 0xffffa085e93da340 != &rq->leaf_cfs_rq_list = 0xffffa085ef92c868
> >
> >
> > lines 1 and 4 are from the first loop in enqueue_task_fair. Line 2 and 3 are from the
> > first call to list_add_leaf_rq with line 2 being at the start and line 3 showing which
> > of the 3 cases we hit.
> >
> > Line 5 is right after the first loop.
> >
> > Line 6 is the second trip through the 2nd loop and is in the if(throttled) condition.
> > Line 7 is right below the enqueue_throttle label.
> >
> > Lines 8 and 9 are from the fixup loop and since onlist is set for both of these it doesn't
> > do anything. But we've left rq->tmp_alone_branch pointing to the cfs_rq->leaf_cfs_rq_list
> > from the one call to list_add_leaf_rq that did something and so the cleanup doesn't work.
> >
> > Based on the comment at the clean up, it looked like it expected the se to be what it was
> > when the first loop broke not whatever it was left at after the second loop.  Could have
> > been NULL there too I guess but I didn't hit that case.
> >
> > This is 100% reproducible. And completely gone with the fix. I have a trace showing that.
> >
> > Does that make more sense?
> 
> Yes, Good catch
> And thanks for the detailed explanation.
> 
> >
> >
> >
> > Cheers,
> > Phil
> >
> >
> > > > what is needed because se does not point to the sched_entity which broke out
> > > > of the first loop.
> > > >
> > > > Address this issue by saving the se pointer when the first loop exits and
> > > > resetting it before doing the fix up, if needed.
> > > >
> > > > Signed-off-by: Phil Auld <pauld@redhat.com>
> > > > Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
> > > > Cc: Vincent Guittot <vincent.guittot@linaro.org>
> > > > Cc: Ingo Molnar <mingo@kernel.org>
> > > > Cc: Juri Lelli <juri.lelli@redhat.com>
> > > > ---
> > > >  kernel/sched/fair.c | 4 ++++
> > > >  1 file changed, 4 insertions(+)
> > > >
> > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > > index 02f323b85b6d..719c996317e3 100644
> > > > --- a/kernel/sched/fair.c
> > > > +++ b/kernel/sched/fair.c
> > > > @@ -5432,6 +5432,7 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
> > > >  {
> > > >         struct cfs_rq *cfs_rq;
> > > >         struct sched_entity *se = &p->se;
> > > > +       struct sched_entity *saved_se = NULL;
> > > >         int idle_h_nr_running = task_has_idle_policy(p);
> > > >
> > > >         /*
> > > > @@ -5466,6 +5467,7 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
> > > >                 flags = ENQUEUE_WAKEUP;
> > > >         }
> > > >
> > > > +       saved_se = se;
> 
> TBH, I don't like saving and going back to the saved se and loop one
> more time on them
> 
> > > >         for_each_sched_entity(se) {
> > > >                 cfs_rq = cfs_rq_of(se);
> 
> Could you add something like below in the 2nd loop instead ?

This one solves the problem as well with no other visible issues.

Do you want to spin it into a real patch?

You can have my {reported/tested}-by or whatever you need.


Cheers,
Phil

> 
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5486,6 +5486,13 @@ enqueue_task_fair(struct rq *rq, struct
> task_struct *p, int flags)
>                 /* end evaluation on encountering a throttled cfs_rq */
>                 if (cfs_rq_throttled(cfs_rq))
>                         goto enqueue_throttle;
> +
> +               /*
> +                * One parent has been throttled and cfs_rq removed from the
> +                * list. Add it back to not break the leaf list.
> +                */
> +               if (throttled_hierarchy(cfs_rq))
> +                       list_add_leaf_cfs_rq(cfs_rq);
>         }
> 
>  enqueue_throttle:
> 
> > > >
> > > > @@ -5510,6 +5512,8 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
> > > >                  * leaf list maintenance, resulting in triggering the assertion
> > > >                  * below.
> > > >                  */
> > > > +               if (saved_se)
> > > > +                       se = saved_se;
> > > >                 for_each_sched_entity(se) {
> > > >                         cfs_rq = cfs_rq_of(se);
> > > >
> > > > --
> > > > 2.18.0
> > > >
> > > >
> > > > Cheers,
> > > > Phil
> > > >
> > >
> >
> > --
> >
> 

--