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Message-ID: <CAERHkrvBdFHWxXu=iHAPMe=1dB0qtG_HTMtUbvVviv7_7kc97Q@mail.gmail.com>
Date: Sat, 8 May 2021 16:07:35 +0800
From: Aubrey Li <aubrey.intel@...il.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Joel Fernandes <joel@...lfernandes.org>,
"Hyser,Chris" <chris.hyser@...cle.com>,
Josh Don <joshdon@...gle.com>, Ingo Molnar <mingo@...nel.org>,
Vincent Guittot <vincent.guittot@...aro.org>,
Valentin Schneider <valentin.schneider@....com>,
Mel Gorman <mgorman@...e.de>,
Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 04/19] sched: Prepare for Core-wide rq->lock
On Fri, May 7, 2021 at 8:34 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
>
> When switching on core-sched, CPUs need to agree which lock to use for
> their RQ.
>
> The new rule will be that rq->core_enabled will be toggled while
> holding all rq->__locks that belong to a core. This means we need to
> double check the rq->core_enabled value after each lock acquire and
> retry if it changed.
>
> This also has implications for those sites that take multiple RQ
> locks, they need to be careful that the second lock doesn't end up
> being the first lock.
>
> Verify the lock pointer after acquiring the first lock, because if
> they're on the same core, holding any of the rq->__lock instances will
> pin the core state.
>
> While there, change the rq->__lock order to CPU number, instead of rq
> address, this greatly simplifies the next patch.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Tested-by: Don Hiatt <dhiatt@...italocean.com>
> Tested-by: Hongyu Ning <hongyu.ning@...ux.intel.com>
> ---
> kernel/sched/core.c | 48 ++++++++++++++++++++++++++++++++++++++++++++++--
> kernel/sched/sched.h | 48 +++++++++++++++++-------------------------------
> 2 files changed, 63 insertions(+), 33 deletions(-)
>
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -186,12 +186,37 @@ int sysctl_sched_rt_runtime = 950000;
>
> void raw_spin_rq_lock_nested(struct rq *rq, int subclass)
> {
> - raw_spin_lock_nested(rq_lockp(rq), subclass);
> + raw_spinlock_t *lock;
> +
> + if (sched_core_disabled()) {
> + raw_spin_lock_nested(&rq->__lock, subclass);
> + return;
> + }
> +
> + for (;;) {
> + lock = rq_lockp(rq);
> + raw_spin_lock_nested(lock, subclass);
> + if (likely(lock == rq_lockp(rq)))
> + return;
> + raw_spin_unlock(lock);
> + }
> }
>
> bool raw_spin_rq_trylock(struct rq *rq)
> {
> - return raw_spin_trylock(rq_lockp(rq));
> + raw_spinlock_t *lock;
> + bool ret;
> +
> + if (sched_core_disabled())
> + return raw_spin_trylock(&rq->__lock);
> +
> + for (;;) {
> + lock = rq_lockp(rq);
> + ret = raw_spin_trylock(lock);
> + if (!ret || (likely(lock == rq_lockp(rq))))
> + return ret;
> + raw_spin_unlock(lock);
> + }
> }
>
> void raw_spin_rq_unlock(struct rq *rq)
> @@ -199,6 +224,25 @@ void raw_spin_rq_unlock(struct rq *rq)
> raw_spin_unlock(rq_lockp(rq));
> }
>
> +#ifdef CONFIG_SMP
> +/*
> + * double_rq_lock - safely lock two runqueues
> + */
> +void double_rq_lock(struct rq *rq1, struct rq *rq2)
Do we need the static lock checking here?
__acquires(rq1->lock)
__acquires(rq2->lock)
> +{
> + lockdep_assert_irqs_disabled();
> +
> + if (rq_order_less(rq2, rq1))
> + swap(rq1, rq2);
> +
> + raw_spin_rq_lock(rq1);
> + if (rq_lockp(rq1) == rq_lockp(rq2)) {
And here?
__acquire(rq2->lock);
> + return;
}
> +
> + raw_spin_rq_lock_nested(rq2, SINGLE_DEPTH_NESTING);
> +}
> +#endif
> +
> /*
> * __task_rq_lock - lock the rq @p resides on.
> */
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1113,6 +1113,11 @@ static inline bool is_migration_disabled
> #endif
> }
>
> +static inline bool sched_core_disabled(void)
> +{
> + return true;
> +}
> +
> static inline raw_spinlock_t *rq_lockp(struct rq *rq)
> {
> return &rq->__lock;
> @@ -2231,10 +2236,17 @@ unsigned long arch_scale_freq_capacity(i
> }
> #endif
>
> +
> #ifdef CONFIG_SMP
> -#ifdef CONFIG_PREEMPTION
>
> -static inline void double_rq_lock(struct rq *rq1, struct rq *rq2);
> +static inline bool rq_order_less(struct rq *rq1, struct rq *rq2)
> +{
> + return rq1->cpu < rq2->cpu;
> +}
> +
> +extern void double_rq_lock(struct rq *rq1, struct rq *rq2);
> +
> +#ifdef CONFIG_PREEMPTION
>
> /*
> * fair double_lock_balance: Safely acquires both rq->locks in a fair
> @@ -2274,14 +2286,13 @@ static inline int _double_lock_balance(s
> if (likely(raw_spin_rq_trylock(busiest)))
> return 0;
>
> - if (rq_lockp(busiest) >= rq_lockp(this_rq)) {
> + if (rq_order_less(this_rq, busiest)) {
> raw_spin_rq_lock_nested(busiest, SINGLE_DEPTH_NESTING);
> return 0;
> }
>
> raw_spin_rq_unlock(this_rq);
> - raw_spin_rq_lock(busiest);
> - raw_spin_rq_lock_nested(this_rq, SINGLE_DEPTH_NESTING);
> + double_rq_lock(this_rq, busiest);
>
> return 1;
> }
> @@ -2334,31 +2345,6 @@ static inline void double_raw_lock(raw_s
> }
>
> /*
> - * double_rq_lock - safely lock two runqueues
> - *
> - * Note this does not disable interrupts like task_rq_lock,
> - * you need to do so manually before calling.
> - */
> -static inline void double_rq_lock(struct rq *rq1, struct rq *rq2)
> - __acquires(rq1->lock)
> - __acquires(rq2->lock)
> -{
> - BUG_ON(!irqs_disabled());
> - if (rq_lockp(rq1) == rq_lockp(rq2)) {
> - raw_spin_rq_lock(rq1);
> - __acquire(rq2->lock); /* Fake it out ;) */
> - } else {
> - if (rq_lockp(rq1) < rq_lockp(rq2)) {
> - raw_spin_rq_lock(rq1);
> - raw_spin_rq_lock_nested(rq2, SINGLE_DEPTH_NESTING);
> - } else {
> - raw_spin_rq_lock(rq2);
> - raw_spin_rq_lock_nested(rq1, SINGLE_DEPTH_NESTING);
> - }
> - }
> -}
> -
> -/*
> * double_rq_unlock - safely unlock two runqueues
> *
> * Note this does not restore interrupts like task_rq_unlock,
> @@ -2368,11 +2354,11 @@ static inline void double_rq_unlock(stru
> __releases(rq1->lock)
> __releases(rq2->lock)
> {
> - raw_spin_rq_unlock(rq1);
> if (rq_lockp(rq1) != rq_lockp(rq2))
> raw_spin_rq_unlock(rq2);
> else
> __release(rq2->lock);
> + raw_spin_rq_unlock(rq1);
This change seems not necessary, as the softlockup root cause is not
the misorder lock release.
Thanks,
-Aubrey
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