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

Phil Auld
2020-05-07T15:18:05Z

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.

No problem. I had to see it all myself anyway :)

>
> >
> >
> >
> > 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 ?

I'll give it a try this way and let you know. I had to give that machine
away. I'll get another and make sure it hits and then we'll see.

Thanks,
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
> > > >
> > >
> >
> > --
> >
>

--