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ipc/sem.c: optimize update_queue() for bulk wakeup calls

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The following series of patches tries to fix the spinlock contention
reported by Chris Mason - his benchmark exposes problems of the current
code:

- In the worst case, the algorithm used by update_queue() is O(N^2).
  Bulk wake-up calls can enter this worst case.  The patch series fix
  that.

  Note that the benchmark app doesn't expose the problem, it just should
  be fixed: Real world apps might do the wake-ups in another order than
  perfect FIFO.

- The part of the code that runs within the semaphore array spinlock is
  significantly larger than necessary.

  The patch series fixes that.  This change is responsible for the main
  improvement.

- The cacheline with the spinlock is also used for a variable that is
  read in the hot path (sem_base) and for a variable that is unnecessarily
  written to multiple times (sem_otime).  The last step of the series
  cacheline-aligns the spinlock.

This patch:

The SysV semaphore code allows to perform multiple operations on all
semaphores in the array as atomic operations.  After a modification,
update_queue() checks which of the waiting tasks can complete.

The algorithm that is used to identify the tasks is O(N^2) in the worst
case.  For some cases, it is simple to avoid the O(N^2).

The patch adds a detection logic for some cases, especially for the case
of an array where all sleeping tasks are single sembuf operations and a
multi-sembuf operation is used to wake up multiple tasks.

A big database application uses that approach.

The patch fixes wakeup due to semctl(,,SETALL,) - the initial version of
the patch breaks that.

[akpm@linux-foundation.org: make do_smart_update() static]
Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Cc: Chris Mason <chris.mason@oracle.com>
Cc: Zach Brown <zach.brown@oracle.com>
Cc: Jens Axboe <jens.axboe@oracle.com>
Cc: Nick Piggin <npiggin@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Manfred Spraul 2010-05-26 14:43:40 -07:00 committed by Linus Torvalds
parent 2dcb22b346
commit fd5db42254
1 changed files with 97 additions and 13 deletions
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110
ipc/sem.c
View file

@ -434,6 +434,69 @@ static void unlink_queue(struct sem_array *sma, struct sem_queue *q)
sma->complex_count--;
}
/** check_restart(sma, q)
* @sma: semaphore array
* @q: the operation that just completed
*
* update_queue is O(N^2) when it restarts scanning the whole queue of
* waiting operations. Therefore this function checks if the restart is
* really necessary. It is called after a previously waiting operation
* was completed.
*/
static int check_restart(struct sem_array *sma, struct sem_queue *q)
{
struct sem *curr;
struct sem_queue *h;
/* if the operation didn't modify the array, then no restart */
if (q->alter == 0)
return 0;
/* pending complex operations are too difficult to analyse */
if (sma->complex_count)
return 1;
/* we were a sleeping complex operation. Too difficult */
if (q->nsops > 1)
return 1;
curr = sma->sem_base + q->sops[0].sem_num;
/* No-one waits on this queue */
if (list_empty(&curr->sem_pending))
return 0;
/* the new semaphore value */
if (curr->semval) {
/* It is impossible that someone waits for the new value:
* - q is a previously sleeping simple operation that
* altered the array. It must be a decrement, because
* simple increments never sleep.
* - The value is not 0, thus wait-for-zero won't proceed.
* - If there are older (higher priority) decrements
* in the queue, then they have observed the original
* semval value and couldn't proceed. The operation
* decremented to value - thus they won't proceed either.
*/
BUG_ON(q->sops[0].sem_op >= 0);
return 0;
}
/*
* semval is 0. Check if there are wait-for-zero semops.
* They must be the first entries in the per-semaphore simple queue
*/
h = list_first_entry(&curr->sem_pending, struct sem_queue, simple_list);
BUG_ON(h->nsops != 1);
BUG_ON(h->sops[0].sem_num != q->sops[0].sem_num);
/* Yes, there is a wait-for-zero semop. Restart */
if (h->sops[0].sem_op == 0)
return 1;
/* Again - no-one is waiting for the new value. */
return 0;
}
/**
* update_queue(sma, semnum): Look for tasks that can be completed.
@ -469,7 +532,7 @@ static void update_queue(struct sem_array *sma, int semnum)
again:
walk = pending_list->next;
while (walk != pending_list) {
int error, alter;
int error, restart;
q = (struct sem_queue *)((char *)walk - offset);
walk = walk->next;
@ -494,22 +557,43 @@ static void update_queue(struct sem_array *sma, int semnum)
unlink_queue(sma, q);
/*
* The next operation that must be checked depends on the type
* of the completed operation:
* - if the operation modified the array, then restart from the
* head of the queue and check for threads that might be
* waiting for the new semaphore values.
* - if the operation didn't modify the array, then just
* continue.
*/
alter = q->alter;
if (error)
restart = 0;
else
restart = check_restart(sma, q);
wake_up_sem_queue(q, error);
if (alter && !error)
if (restart)
goto again;
}
}
/** do_smart_update(sma, sops, nsops): Optimized update_queue
* @sma: semaphore array
* @sops: operations that were performed
* @nsops: number of operations
*
* do_smart_update() does the required called to update_queue, based on the
* actual changes that were performed on the semaphore array.
*/
static void do_smart_update(struct sem_array *sma, struct sembuf *sops, int nsops)
{
int i;
if (sma->complex_count || sops == NULL) {
update_queue(sma, -1);
return;
}
for (i = 0; i < nsops; i++) {
if (sops[i].sem_op > 0 ||
(sops[i].sem_op < 0 &&
sma->sem_base[sops[i].sem_num].semval == 0))
update_queue(sma, sops[i].sem_num);
}
}
/* The following counts are associated to each semaphore:
* semncnt number of tasks waiting on semval being nonzero
* semzcnt number of tasks waiting on semval being zero
@ -1225,7 +1309,7 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops,
error = try_atomic_semop (sma, sops, nsops, un, task_tgid_vnr(current));
if (error <= 0) {
if (alter && error == 0)
update_queue(sma, (nsops == 1) ? sops[0].sem_num : -1);
do_smart_update(sma, sops, nsops);
goto out_unlock_free;
}