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Date: Tue, 24 Nov 2020 13:30:38 -0500
From: Joel Fernandes <joel@...lfernandes.org>
To: Balbir Singh <bsingharora@...il.com>
Cc: Nishanth Aravamudan <naravamudan@...italocean.com>,
Julien Desfossez <jdesfossez@...italocean.com>,
Peter Zijlstra <peterz@...radead.org>,
Tim Chen <tim.c.chen@...ux.intel.com>,
Vineeth Pillai <viremana@...ux.microsoft.com>,
Aaron Lu <aaron.lwe@...il.com>,
Aubrey Li <aubrey.intel@...il.com>, tglx@...utronix.de,
linux-kernel@...r.kernel.org, mingo@...nel.org,
torvalds@...ux-foundation.org, fweisbec@...il.com,
keescook@...omium.org, kerrnel@...gle.com,
Phil Auld <pauld@...hat.com>,
Valentin Schneider <valentin.schneider@....com>,
Mel Gorman <mgorman@...hsingularity.net>,
Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
Paolo Bonzini <pbonzini@...hat.com>, vineeth@...byteword.org,
Chen Yu <yu.c.chen@...el.com>,
Christian Brauner <christian.brauner@...ntu.com>,
Agata Gruza <agata.gruza@...el.com>,
Antonio Gomez Iglesias <antonio.gomez.iglesias@...el.com>,
graf@...zon.com, konrad.wilk@...cle.com, dfaggioli@...e.com,
pjt@...gle.com, rostedt@...dmis.org, derkling@...gle.com,
benbjiang@...cent.com,
Alexandre Chartre <alexandre.chartre@...cle.com>,
James.Bottomley@...senpartnership.com, OWeisse@...ch.edu,
Dhaval Giani <dhaval.giani@...cle.com>,
Junaid Shahid <junaids@...gle.com>, jsbarnes@...gle.com,
chris.hyser@...cle.com, Ben Segall <bsegall@...gle.com>,
Josh Don <joshdon@...gle.com>, Hao Luo <haoluo@...gle.com>,
Tom Lendacky <thomas.lendacky@....com>,
Aubrey Li <aubrey.li@...ux.intel.com>,
"Paul E. McKenney" <paulmck@...nel.org>,
Tim Chen <tim.c.chen@...el.com>
Subject: Re: [PATCH -tip 10/32] sched: Fix priority inversion of cookied task
with sibling
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@...radead.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.
---8<-----------------------
From: Peter Zijlstra <peterz@...radead.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@...lfernandes.org>
Signed-off-by: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.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
+ * involved. We may have skipped
+ * non-empty higher priority classes on
+ * siblings, which are empty on this
+ * CPU, so start over.
+ */
+ need_sync = true;
+ goto reset;
+ }
+
next = p;
goto done;
}
@@ -5299,7 +5307,6 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
*/
need_sync = true;
}
-
}
}
next_class:;
--
2.29.2.454.gaff20da3a2-goog
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