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SubjectRe: [External] Re: [PATCH] sched/core: Avoid obvious double update_rq_clock warning
On 20/04/2022 10:29, Hao Jia wrote:
> On 4/19/22 6:48 PM, Peter Zijlstra wrote:
>> On Mon, Apr 18, 2022 at 05:09:29PM +0800, Hao Jia wrote:

[...]

>> I'm really not sure about this part though. This is a bit of a mess. The
>> balancer doesn't really need the pinning stuff. I realize you did that
>> because we got the clock annotation mixed up with that, but urgh.
>>
>> Basically we want double_rq_lock() / double_lock_balance() to clear
>> RQCF_UPDATED, right? Perhaps do that directly?
>>
>> (maybe with an inline helper and a wee comment?)
>>
>> The only immediate problem with this would appear to be that
>> _double_rq_lock() behaves differently when it returns 0. Not sure that
>> matters.
>>
>> Hmm?
>
> Thanks for your review comments.
> As you have prompted, the WARN_DOUBLE_CLOCK warning is still triggered
> when _double_rq_lock() returns 0.
> Please review the solution below, and based on your review, I will
> submit the v2 patch as soon as possible.
> Thanks.


[...]

Maybe something like this:

-->8--

From: Dietmar Eggemann <dietmar.eggemann@arm.com>
Date: Wed, 20 Apr 2022 11:12:10 +0200
Subject: [PATCH] sched/core: Clear RQCF_UPDATED in _double_lock_balance() &
double_rq_lock()

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
---
kernel/sched/core.c | 6 +++---
kernel/sched/sched.h | 20 ++++++++++++++++----
2 files changed, 19 insertions(+), 7 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 068c088e9584..f4cfe7eea861 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -610,10 +610,10 @@ void double_rq_lock(struct rq *rq1, struct rq *rq2)
swap(rq1, rq2);

raw_spin_rq_lock(rq1);
- if (__rq_lockp(rq1) == __rq_lockp(rq2))
- return;
+ if (__rq_lockp(rq1) != __rq_lockp(rq2))
+ raw_spin_rq_lock_nested(rq2, SINGLE_DEPTH_NESTING);

- raw_spin_rq_lock_nested(rq2, SINGLE_DEPTH_NESTING);
+ rq_clock_clear_update(rq1, rq2);
}
#endif

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 58263f90c559..3a77b10d7cc4 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2515,6 +2515,16 @@ static inline bool rq_order_less(struct rq *rq1, struct rq *rq2)

extern void double_rq_lock(struct rq *rq1, struct rq *rq2);

+#ifdef CONFIG_SCHED_DEBUG
+static inline void rq_clock_clear_update(struct rq *rq1, struct rq *rq2)
+{
+ rq1->clock_update_flags &= (RQCF_REQ_SKIP|RQCF_ACT_SKIP);
+ rq2->clock_update_flags &= (RQCF_REQ_SKIP|RQCF_ACT_SKIP);
+}
+#else
+static inline void rq_clock_clear_update(struct rq *rq1, struct rq *rq2) {}
+#endif
+
#ifdef CONFIG_PREEMPTION

/*
@@ -2549,14 +2559,15 @@ static inline int _double_lock_balance(struct rq *this_rq, struct rq *busiest)
__acquires(busiest->lock)
__acquires(this_rq->lock)
{
- if (__rq_lockp(this_rq) == __rq_lockp(busiest))
- return 0;
-
- if (likely(raw_spin_rq_trylock(busiest)))
+ if (__rq_lockp(this_rq) == __rq_lockp(busiest) ||
+ likely(raw_spin_rq_trylock(busiest))) {
+ rq_clock_clear_update(this_rq, busiest);
return 0;
+ }

if (rq_order_less(this_rq, busiest)) {
raw_spin_rq_lock_nested(busiest, SINGLE_DEPTH_NESTING);
+ rq_clock_clear_update(this_rq, busiest);
return 0;
}

@@ -2650,6 +2661,7 @@ static inline void double_rq_lock(struct rq *rq1, struct rq *rq2)
BUG_ON(rq1 != rq2);
raw_spin_rq_lock(rq1);
__acquire(rq2->lock); /* Fake it out ;) */
+ rq_clock_clear_update(rq1, rq2);
}

/*
--
2.25.1
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