Re: [v2] locking/percpu-rwsem: Optimize readers and reduce global impact

[Guenter Roeck <linux@roeck-us.net>]

Peter,

On Tue, Aug 09, 2016 at 11:51:12AM +0200, Peter Zijlstra wrote:
> Currently the percpu-rwsem switches to (global) atomic ops while a
> writer is waiting; which could be quite a while and slows down
> releasing the readers.
> 
> This patch cures this problem by ordering the reader-state vs
> reader-count (see the comments in __percpu_down_read() and
> percpu_down_write()). This changes a global atomic op into a full
> memory barrier, which doesn't have the global cacheline contention.
> 
> This also enables using the percpu-rwsem with rcu_sync disabled in order
> to bias the implementation differently, reducing the writer latency by
> adding some cost to readers.
> 
> Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
> Reviewed-by: Oleg Nesterov <oleg@redhat.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  include/linux/percpu-rwsem.h  |   84 +++++++++++++--
>  kernel/locking/percpu-rwsem.c |  228 ++++++++++++++++++++++++------------------
>  2 files changed, 206 insertions(+), 106 deletions(-)
> 
> --- a/include/linux/percpu-rwsem.h
> +++ b/include/linux/percpu-rwsem.h
> @@ -10,30 +10,96 @@
>  
>  struct percpu_rw_semaphore {
>  	struct rcu_sync		rss;
> -	unsigned int __percpu	*fast_read_ctr;
> +	unsigned int __percpu	*read_count;
>  	struct rw_semaphore	rw_sem;
> -	atomic_t		slow_read_ctr;
> -	wait_queue_head_t	write_waitq;
> +	wait_queue_head_t	writer;
> +	int			readers_block;
>  };
>  
> -extern void percpu_down_read(struct percpu_rw_semaphore *);
> -extern int  percpu_down_read_trylock(struct percpu_rw_semaphore *);
> -extern void percpu_up_read(struct percpu_rw_semaphore *);
> +extern int __percpu_down_read(struct percpu_rw_semaphore *, int);
> +extern void __percpu_up_read(struct percpu_rw_semaphore *);
> +
> +static inline void percpu_down_read(struct percpu_rw_semaphore *sem)
> +{
> +	might_sleep();
> +
> +	rwsem_acquire_read(&sem->rw_sem.dep_map, 0, 0, _RET_IP_);
> +
> +	preempt_disable();
> +	/*
> +	 * We are in an RCU-sched read-side critical section, so the writer
> +	 * cannot both change sem->state from readers_fast and start checking
> +	 * counters while we are here. So if we see !sem->state, we know that
> +	 * the writer won't be checking until we're past the preempt_enable()
> +	 * and that one the synchronize_sched() is done, the writer will see
> +	 * anything we did within this RCU-sched read-size critical section.
> +	 */
> +	__this_cpu_inc(*sem->read_count);
> +	if (unlikely(!rcu_sync_is_idle(&sem->rss)))

The call to rcu_sync_is_idle() causes the following build error when building
x86_64:allmodconfig.

ERROR: "rcu_sync_lockdep_assert" [kernel/locking/locktorture.ko] undefined!
ERROR: "rcu_sync_lockdep_assert" [fs/ext4/ext4.ko] undefined!

I think this was also reported by the 0-day build bot.

The simple fix would of course be to export rcu_sync_lockdep_assert. Before I
apply that change to the Android code (where the patch has been aplied and
the problem is seen) - do you by any chance have a better solution in mind ?

Thanks,
Guenter