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Date: Tue, 5 Feb 2013 21:11:55 +0800
From: Alex Shi <alex.shi@...el.com>
To: mingo@...nel.org, torvalds@...ux-foundation.org
Cc: paul.gortmaker@...driver.com, alex.shi@...el.com,
linux-kernel@...r.kernel.org
Subject: [PATCH] rwsem: steal writing sem for better performance
Commit 5a50508 change to rwsem from mutex, that cause aim7 fork_test
dropped 50%. Yuanhan liu does a good analysis, find it caused by
strict sequential writing. Ingo suggest stealing sem writing from
front task in waiting queue. https://lkml.org/lkml/2013/1/29/84
So has this patch.
In this patch, I just allow writing steal happen when the first waiter
is also writer. Then the performance fully recovered.
Reported-by: lkp@...ux.intel.com
Cc: Ingo Molnar <mingo@...nel.org>
Signed-off-by: Alex Shi <alex.shi@...el.com>
---
lib/rwsem.c | 75 +++++++++++++++++++++++++++++++++++++------------------------
1 file changed, 46 insertions(+), 29 deletions(-)
diff --git a/lib/rwsem.c b/lib/rwsem.c
index 8337e1b..88ba8e5 100644
--- a/lib/rwsem.c
+++ b/lib/rwsem.c
@@ -2,6 +2,8 @@
*
* Written by David Howells (dhowells@...hat.com).
* Derived from arch/i386/kernel/semaphore.c
+ *
+ * Steal writing sem. Alex Shi <alex.shi@...el.com>
*/
#include <linux/rwsem.h>
#include <linux/sched.h>
@@ -60,7 +62,7 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wake_type)
struct rwsem_waiter *waiter;
struct task_struct *tsk;
struct list_head *next;
- signed long oldcount, woken, loop, adjustment;
+ signed long woken, loop, adjustment;
waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
if (!(waiter->flags & RWSEM_WAITING_FOR_WRITE))
@@ -72,30 +74,8 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wake_type)
*/
goto out;
- /* There's a writer at the front of the queue - try to grant it the
- * write lock. However, we only wake this writer if we can transition
- * the active part of the count from 0 -> 1
- */
- adjustment = RWSEM_ACTIVE_WRITE_BIAS;
- if (waiter->list.next == &sem->wait_list)
- adjustment -= RWSEM_WAITING_BIAS;
-
- try_again_write:
- oldcount = rwsem_atomic_update(adjustment, sem) - adjustment;
- if (oldcount & RWSEM_ACTIVE_MASK)
- /* Someone grabbed the sem already */
- goto undo_write;
-
- /* We must be careful not to touch 'waiter' after we set ->task = NULL.
- * It is an allocated on the waiter's stack and may become invalid at
- * any time after that point (due to a wakeup from another source).
- */
- list_del(&waiter->list);
- tsk = waiter->task;
- smp_mb();
- waiter->task = NULL;
- wake_up_process(tsk);
- put_task_struct(tsk);
+ /* wake up the writing waiter and let the task grap sem */
+ wake_up_process(waiter->task);
goto out;
readers_only:
@@ -157,12 +137,40 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wake_type)
out:
return sem;
+}
+
+/* try to get write sem, caller hold sem->wait_lock */
+static int try_get_writer_sem(struct rw_semaphore *sem,
+ struct rwsem_waiter *waiter)
+{
+ struct rwsem_waiter *fwaiter;
+ long oldcount, adjustment;
- /* undo the change to the active count, but check for a transition
- * 1->0 */
- undo_write:
+ /* only steal when first waiter is writing */
+ fwaiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
+ if (!(fwaiter->flags & RWSEM_WAITING_FOR_WRITE))
+ return 0;
+
+ adjustment = RWSEM_ACTIVE_WRITE_BIAS;
+ /* only one waiter in queue */
+ if (fwaiter == waiter && waiter->list.next == &sem->wait_list)
+ adjustment -= RWSEM_WAITING_BIAS;
+
+try_again_write:
+ oldcount = rwsem_atomic_update(adjustment, sem) - adjustment;
+ if (!(oldcount & RWSEM_ACTIVE_MASK)) {
+ /* no active lock */
+ struct task_struct *tsk = waiter->task;
+
+ list_del(&waiter->list);
+ smp_mb();
+ put_task_struct(tsk);
+ tsk->state = TASK_RUNNING;
+ return 1;
+ }
+ /* some one grabbed the sem already */
if (rwsem_atomic_update(-adjustment, sem) & RWSEM_ACTIVE_MASK)
- goto out;
+ return 0;
goto try_again_write;
}
@@ -210,6 +218,15 @@ rwsem_down_failed_common(struct rw_semaphore *sem,
for (;;) {
if (!waiter.task)
break;
+
+ raw_spin_lock_irq(&sem->wait_lock);
+ /* try to get the writer sem, may steal from the head writer */
+ if (flags == RWSEM_WAITING_FOR_WRITE)
+ if (try_get_writer_sem(sem, &waiter)) {
+ raw_spin_unlock_irq(&sem->wait_lock);
+ return sem;
+ }
+ raw_spin_unlock_irq(&sem->wait_lock);
schedule();
set_task_state(tsk, TASK_UNINTERRUPTIBLE);
}
--
1.7.12
--
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