Re: [PATCH v2 04/19] sched: Prepare for Core-wide rq->lock

[Aubrey Li]

On Fri, May 7, 2021 at 8:34 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> Tested-by: Don Hiatt <dhiatt@xxxxxxxxxxxxxxxx>
> Tested-by: Hongyu Ning <hongyu.ning@xxxxxxxxxxxxxxx>
> ---
>  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