Re: [PATCH] sched/fair: Fix enqueue_task_fair warning some more

Vincent Guittot
2020-05-07T15:06:45Z

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 ?

--- 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
> > >
> >
>
> --
>