lkml.org 
[lkml]   [2018]   [Jul]   [18]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v2] locking/rwsem: Take read lock immediate if queue empty with no writer
On Fri, Jul 13, 2018 at 02:30:53PM -0400, Waiman Long wrote:
> It was discovered that a constant stream of readers might cause the
> count to go negative most of the time after an initial trigger by a
> writer even if no writer was present afterward. As a result, most of the
> readers would have to go through the slowpath reducing their performance.

I'm slightly confused, what happens to trigger this?

(also, I'm forever confused by the whole BIAS scheme)

> To avoid that from happening, an additional check is added to detect
> the special case that the reader in the critical section is the only
> one in the wait queue and no writer is present. When that happens, it
> can just have the lock and return immediately without further action.
> Other incoming readers won't see a waiter is present and be forced
> into the slowpath.
>
> The additional code is in the slowpath and so should not have an impact
> on rwsem performance. However, in the special case listed above, it may
> greatly improve performance.


> diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
> index 3064c50..bf0570e 100644
> --- a/kernel/locking/rwsem-xadd.c
> +++ b/kernel/locking/rwsem-xadd.c
> @@ -233,8 +233,19 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,

your diff function thingy is busted, this is in fact
__rwsem_down_read_failed_common you're patching.

> waiter.type = RWSEM_WAITING_FOR_READ;
>
> raw_spin_lock_irq(&sem->wait_lock);
> - if (list_empty(&sem->wait_list))
> + if (list_empty(&sem->wait_list)) {
> + /*
> + * In the unlikely event that the task is the only one in
> + * the wait queue and a writer isn't present, it can have
> + * the lock and return immediately without going through
> + * the remaining slowpath code.
> + */
> + if (unlikely(atomic_long_read(&sem->count) >= 0)) {
> + raw_spin_unlock_irq(&sem->wait_lock);
> + return sem;
> + }
> adjustment += RWSEM_WAITING_BIAS;
> + }
> list_add_tail(&waiter.list, &sem->wait_list);

So without this, we would add ourselves to the list and then immediately
call __rwsem_mark_wake() on ourselves and fall through the wait-loop, ow
what?

\
 
 \ /
  Last update: 2018-07-18 18:17    [W:0.578 / U:0.024 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site