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Message-ID: <20241125195204.2374458-7-jstultz@google.com>
Date: Mon, 25 Nov 2024 11:52:00 -0800
From: John Stultz <jstultz@...gle.com>
To: LKML <linux-kernel@...r.kernel.org>
Cc: Valentin Schneider <valentin.schneider@....com>, Joel Fernandes <joelaf@...gle.com>,
Qais Yousef <qyousef@...alina.io>, Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>, Dietmar Eggemann <dietmar.eggemann@....com>,
Valentin Schneider <vschneid@...hat.com>, Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>, Zimuzo Ezeozue <zezeozue@...gle.com>, Mel Gorman <mgorman@...e.de>,
Will Deacon <will@...nel.org>, Waiman Long <longman@...hat.com>, Boqun Feng <boqun.feng@...il.com>,
"Paul E. McKenney" <paulmck@...nel.org>, Metin Kaya <Metin.Kaya@....com>,
Xuewen Yan <xuewen.yan94@...il.com>, K Prateek Nayak <kprateek.nayak@....com>,
Thomas Gleixner <tglx@...utronix.de>, Daniel Lezcano <daniel.lezcano@...aro.org>, kernel-team@...roid.com,
"Connor O'Brien" <connoro@...gle.com>, John Stultz <jstultz@...gle.com>
Subject: [RFC][PATCH v14 6/7] sched: Fix proxy/current (push,pull)ability
From: Valentin Schneider <valentin.schneider@....com>
Proxy execution forms atomic pairs of tasks: The waiting donor
task (scheduling context) and a proxy (execution context). The
donor task, along with the rest of the blocked chain, follows
the proxy wrt CPU placement.
They can be the same task, in which case push/pull doesn't need any
modification. When they are different, however,
FIFO1 & FIFO42:
,-> RT42
| | blocked-on
| v
blocked_donor | mutex
| | owner
| v
`-- RT1
RT1
RT42
CPU0 CPU1
^ ^
| |
overloaded !overloaded
rq prio = 42 rq prio = 0
RT1 is eligible to be pushed to CPU1, but should that happen it will
"carry" RT42 along. Clearly here neither RT1 nor RT42 must be seen as
push/pullable.
Unfortunately, only the donor task is usually dequeued from the rq,
and the proxy'ed execution context (rq->curr) remains on the rq.
This can cause RT1 to be selected for migration from logic like the
rt pushable_list.
Thus, adda a dequeue/enqueue cycle on the proxy task before __schedule
returns, which allows the sched class logic to avoid adding the now
current task to the pushable_list.
Furthermore, tasks becoming blocked on a mutex don't need an explicit
dequeue/enqueue cycle to be made (push/pull)able: they have to be running
to block on a mutex, thus they will eventually hit put_prev_task().
Cc: Joel Fernandes <joelaf@...gle.com>
Cc: Qais Yousef <qyousef@...alina.io>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Juri Lelli <juri.lelli@...hat.com>
Cc: Vincent Guittot <vincent.guittot@...aro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>
Cc: Valentin Schneider <vschneid@...hat.com>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Ben Segall <bsegall@...gle.com>
Cc: Zimuzo Ezeozue <zezeozue@...gle.com>
Cc: Mel Gorman <mgorman@...e.de>
Cc: Will Deacon <will@...nel.org>
Cc: Waiman Long <longman@...hat.com>
Cc: Boqun Feng <boqun.feng@...il.com>
Cc: "Paul E. McKenney" <paulmck@...nel.org>
Cc: Metin Kaya <Metin.Kaya@....com>
Cc: Xuewen Yan <xuewen.yan94@...il.com>
Cc: K Prateek Nayak <kprateek.nayak@....com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Daniel Lezcano <daniel.lezcano@...aro.org>
Cc: kernel-team@...roid.com
Signed-off-by: Valentin Schneider <valentin.schneider@....com>
Signed-off-by: Connor O'Brien <connoro@...gle.com>
Signed-off-by: John Stultz <jstultz@...gle.com>
---
v3:
* Tweaked comments & commit message
v5:
* Minor simplifications to utilize the fix earlier
in the patch series.
* Rework the wording of the commit message to match selected/
proxy terminology and expand a bit to make it more clear how
it works.
v6:
* Dropped now-unused proxied value, to be re-added later in the
series when it is used, as caught by Dietmar
v7:
* Unused function argument fixup
* Commit message nit pointed out by Metin Kaya
* Dropped unproven unlikely() and use sched_proxy_exec()
in proxy_tag_curr, suggested by Metin Kaya
v8:
* More cleanups and typo fixes suggested by Metin Kaya
v11:
* Cleanup of comimt message suggested by Metin
v12:
* Rework for rq_selected -> rq->donor renaming
---
kernel/sched/core.c | 25 +++++++++++++++++++++++++
1 file changed, 25 insertions(+)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index b492506d33415..a18523355fb18 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6737,6 +6737,23 @@ find_proxy_task(struct rq *rq, struct task_struct *donor, struct rq_flags *rf)
}
#endif /* SCHED_PROXY_EXEC */
+static inline void proxy_tag_curr(struct rq *rq, struct task_struct *owner)
+{
+ if (!sched_proxy_exec())
+ return;
+ /*
+ * pick_next_task() calls set_next_task() on the chosen task
+ * at some point, which ensures it is not push/pullable.
+ * However, the chosen/donor task *and* the mutex owner form an
+ * atomic pair wrt push/pull.
+ *
+ * Make sure owner we run is not pushable. Unfortunately we can
+ * only deal with that by means of a dequeue/enqueue cycle. :-/
+ */
+ dequeue_task(rq, owner, DEQUEUE_NOCLOCK | DEQUEUE_SAVE);
+ enqueue_task(rq, owner, ENQUEUE_NOCLOCK | ENQUEUE_RESTORE);
+}
+
/*
* __schedule() is the main scheduler function.
*
@@ -6875,6 +6892,10 @@ static void __sched notrace __schedule(int sched_mode)
* changes to task_struct made by pick_next_task().
*/
RCU_INIT_POINTER(rq->curr, next);
+
+ if (!task_current_donor(rq, next))
+ proxy_tag_curr(rq, next);
+
/*
* The membarrier system call requires each architecture
* to have a full memory barrier after updating
@@ -6908,6 +6929,10 @@ static void __sched notrace __schedule(int sched_mode)
/* Also unlocks the rq: */
rq = context_switch(rq, prev, next, &rf);
} else {
+ /* In case next was already curr but just got blocked_donor */
+ if (!task_current_donor(rq, next))
+ proxy_tag_curr(rq, next);
+
rq_unpin_lock(rq, &rf);
__balance_callbacks(rq);
raw_spin_rq_unlock_irq(rq);
--
2.47.0.371.ga323438b13-goog
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