Re: [PATCH -tip 10/32] sched: Fix priority inversion of cookied task with sibling

[Balbir Singh <bsingharora@gmail.com>]

On Tue, Nov 24, 2020 at 01:30:38PM -0500, Joel Fernandes wrote:
> On Mon, Nov 23, 2020 at 09:41:23AM +1100, Balbir Singh wrote:
> > On Tue, Nov 17, 2020 at 06:19:40PM -0500, Joel Fernandes (Google) wrote:
> > > From: Peter Zijlstra <peterz@infradead.org>
> > > 
> > > The rationale is as follows. In the core-wide pick logic, even if
> > > need_sync == false, we need to go look at other CPUs (non-local CPUs) to
> > > see if they could be running RT.
> > > 
> > > Say the RQs in a particular core look like this:
> > > Let CFS1 and CFS2 be 2 tagged CFS tags. Let RT1 be an untagged RT task.
> > > 
> > > rq0            rq1
> > > CFS1 (tagged)  RT1 (not tag)
> > > CFS2 (tagged)
> > > 
> > > Say schedule() runs on rq0. Now, it will enter the above loop and
> > > pick_task(RT) will return NULL for 'p'. It will enter the above if() block
> > > and see that need_sync == false and will skip RT entirely.
> > > 
> > > The end result of the selection will be (say prio(CFS1) > prio(CFS2)):
> > > rq0             rq1
> > > CFS1            IDLE
> > > 
> > > When it should have selected:
> > > rq0             r1
> > > IDLE            RT
> > > 
> > > Joel saw this issue on real-world usecases in ChromeOS where an RT task
> > > gets constantly force-idled and breaks RT. Lets cure it.
> > > 
> > > NOTE: This problem will be fixed differently in a later patch. It just
> > >       kept here for reference purposes about this issue, and to make
> > >       applying later patches easier.
> > >
> > 
> > The changelog is hard to read, it refers to above if(), whereas there
> > is no code snippet in the changelog.
> 
> Yeah sorry, it comes from this email where I described the issue:
> http://lore.kernel.org/r/20201023175724.GA3563800@google.com
> 
> I corrected the changelog and appended the patch below. Also pushed it to:
> https://git.kernel.org/pub/scm/linux/kernel/git/jfern/linux.git/log/?h=coresched
> 
> > Also, from what I can see following
> > the series, p->core_cookie is not yet set anywhere (unless I missed it),
> > so fixing it in here did not make sense just reading the series.
> 
> The interface patches for core_cookie are added later, that's how it is. The
> infrastructure comes first here. It would also not make sense to add
> interface first as well so I think the current ordering is fine.
>

Some comments below to help make the code easier to understand

> ---8<-----------------------
> 
> From: Peter Zijlstra <peterz@infradead.org>
> Subject: [PATCH] sched: Fix priority inversion of cookied task with sibling
> 
> The rationale is as follows. In the core-wide pick logic, even if
> need_sync == false, we need to go look at other CPUs (non-local CPUs) to
> see if they could be running RT.
> 
> Say the RQs in a particular core look like this:
> Let CFS1 and CFS2 be 2 tagged CFS tags. Let RT1 be an untagged RT task.
> 
> rq0            rq1
> CFS1 (tagged)  RT1 (not tag)
> CFS2 (tagged)
> 
> The end result of the selection will be (say prio(CFS1) > prio(CFS2)):
> rq0             rq1
> CFS1            IDLE
> 
> When it should have selected:
> rq0             r1
> IDLE            RT
> 
> Fix this issue by forcing need_sync and restarting the search if a
> cookied task was discovered. This will avoid this optimization from
> making incorrect picks.
> 
> Joel saw this issue on real-world usecases in ChromeOS where an RT task
> gets constantly force-idled and breaks RT. Lets cure it.
> 
> NOTE: This problem will be fixed differently in a later patch. It just
>       kept here for reference purposes about this issue, and to make
>       applying later patches easier.
> 
> Reported-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> Signed-off-by: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> ---
>  kernel/sched/core.c | 25 ++++++++++++++++---------
>  1 file changed, 16 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 4ee4902c2cf5..53af817740c0 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -5195,6 +5195,7 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>  	need_sync = !!rq->core->core_cookie;
>  
>  	/* reset state */
> +reset:
>  	rq->core->core_cookie = 0UL;
>  	if (rq->core->core_forceidle) {
>  		need_sync = true;
> @@ -5242,14 +5243,8 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>  				/*
>  				 * If there weren't no cookies; we don't need to
>  				 * bother with the other siblings.
> -				 * If the rest of the core is not running a tagged
> -				 * task, i.e.  need_sync == 0, and the current CPU
> -				 * which called into the schedule() loop does not
> -				 * have any tasks for this class, skip selecting for
> -				 * other siblings since there's no point. We don't skip
> -				 * for RT/DL because that could make CFS force-idle RT.
>  				 */
> -				if (i == cpu && !need_sync && class == &fair_sched_class)
> +				if (i == cpu && !need_sync)
>  					goto next_class;
>  
>  				continue;
> @@ -5259,7 +5254,20 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>  			 * Optimize the 'normal' case where there aren't any
>  			 * cookies and we don't need to sync up.
>  			 */
> -			if (i == cpu && !need_sync && !p->core_cookie) {
> +			if (i == cpu && !need_sync) {
> +				if (p->core_cookie) {
> +					/*
> +					 * This optimization is only valid as
> +					 * long as there are no cookies

This is not entirely true, need_sync is a function of core cookies, so I
think this needs more clarification, it sounds like we enter this when
the core has no cookies, but the task has a core_cookie? The term cookie
is quite overloaded when used in the context of core vs task.

Effectively from what I understand this means that p wants to be
coscheduled, but the core itself is not coscheduling anything at the
moment, so we need to see if we should do a sync and that sync might
cause p to get kicked out and a higher priority class to come in?

Balbir Singh.