path: root/ipc/sem.c
diff options
authorManfred Spraul <manfred@colorfullife.com>2009-12-15 16:47:34 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2009-12-16 07:20:10 -0800
commitd987f8b213f2cdcc52b2ca9ee67161516e4d256a (patch)
tree12ed1a267c8fd3ea0343d6a77f1c332b775d4dfe /ipc/sem.c
parent636c6be823870d829b37fc96655bb8820a6a9be9 (diff)
ipc/sem.c: optimize single sops when semval is zero
If multiple simple decrements on the same semaphore are pending, then the current code scans all decrement operations, even if the semaphore value is already 0. The patch optimizes that: if the semaphore value is 0, then there is no need to scan the q->alter entries. Note that this is a common case: It happens if 100 decrements by one are pending and now an increment by one increases the semaphore value from 0 to 1. Without this patch, all 100 entries are scanned. With the patch, only one entry is scanned, then woken up. Then the new rule triggers and the scanning is aborted, without looking at the remaining 99 tasks. With this patch, single sop increment/decrement by 1 are now O(1). (same as with Nick's patch) Signed-off-by: Manfred Spraul <manfred@colorfullife.com> Cc: Nick Piggin <npiggin@suse.de> Cc: Pierre Peiffer <peifferp@gmail.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'ipc/sem.c')
1 files changed, 11 insertions, 0 deletions
diff --git a/ipc/sem.c b/ipc/sem.c
index 7eb6f049dd7c..82518d6a96a3 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -474,6 +474,17 @@ again:
q = (struct sem_queue *)((char *)walk - offset);
walk = walk->next;
+ /* If we are scanning the single sop, per-semaphore list of
+ * one semaphore and that semaphore is 0, then it is not
+ * necessary to scan the "alter" entries: simple increments
+ * that affect only one entry succeed immediately and cannot
+ * be in the per semaphore pending queue, and decrements
+ * cannot be successful if the value is already 0.
+ */
+ if (semnum != -1 && sma->sem_base[semnum].semval == 0 &&
+ q->alter)
+ break;
error = try_atomic_semop(sma, q->sops, q->nsops,
q->undo, q->pid);

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