Re: [RFC PATCH 05/28] sched: Add cond_resched_rwlock

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Thu, Sep 26, 2019 at 04:18:01PM -0700, Ben Gardon wrote:
> Rescheduling while holding a spin lock is essential for keeping long
> running kernel operations running smoothly. Add the facility to
> cond_resched read/write spin locks.
> 
> RFC_NOTE: The current implementation of this patch set uses a read/write
> lock to replace the existing MMU spin lock. See the next patch in this
> series for more on why a read/write lock was chosen, and possible
> alternatives.

This definitely needs to be run by the sched/locking folks sooner rather
than later.

> Signed-off-by: Ben Gardon <bgardon@google.com>
> ---
>  include/linux/sched.h | 11 +++++++++++
>  kernel/sched/core.c   | 23 +++++++++++++++++++++++
>  2 files changed, 34 insertions(+)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 70db597d6fd4f..4d1fd96693d9b 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1767,12 +1767,23 @@ static inline int _cond_resched(void) { return 0; }
>  })
>  
>  extern int __cond_resched_lock(spinlock_t *lock);
> +extern int __cond_resched_rwlock(rwlock_t *lock, bool write_lock);
>  
>  #define cond_resched_lock(lock) ({				\
>  	___might_sleep(__FILE__, __LINE__, PREEMPT_LOCK_OFFSET);\
>  	__cond_resched_lock(lock);				\
>  })
>  
> +#define cond_resched_rwlock_read(lock) ({			\
> +	__might_sleep(__FILE__, __LINE__, PREEMPT_LOCK_OFFSET);	\
> +	__cond_resched_rwlock(lock, false);			\
> +})
> +
> +#define cond_resched_rwlock_write(lock) ({			\
> +	__might_sleep(__FILE__, __LINE__, PREEMPT_LOCK_OFFSET);	\
> +	__cond_resched_rwlock(lock, true);			\
> +})
> +
>  static inline void cond_resched_rcu(void)
>  {
>  #if defined(CONFIG_DEBUG_ATOMIC_SLEEP) || !defined(CONFIG_PREEMPT_RCU)
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index f9a1346a5fa95..ba7ed4bed5036 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -5663,6 +5663,29 @@ int __cond_resched_lock(spinlock_t *lock)
>  }
>  EXPORT_SYMBOL(__cond_resched_lock);
>  
> +int __cond_resched_rwlock(rwlock_t *lock, bool write_lock)
> +{
> +	int ret = 0;
> +
> +	lockdep_assert_held(lock);
> +	if (should_resched(PREEMPT_LOCK_OFFSET)) {
> +		if (write_lock) {

The existing __cond_resched_lock() checks for resched *or* lock
contention.  Is lock contention not something that needs (or can't?) be
considered?

> +			write_unlock(lock);
> +			preempt_schedule_common();
> +			write_lock(lock);
> +		} else {
> +			read_unlock(lock);
> +			preempt_schedule_common();
> +			read_lock(lock);
> +		}
> +
> +		ret = 1;
> +	}
> +
> +	return ret;
> +}
> +EXPORT_SYMBOL(__cond_resched_rwlock);
> +
>  /**
>   * yield - yield the current processor to other threads.
>   *
> -- 
> 2.23.0.444.g18eeb5a265-goog
>