Commit d987f8b2 authored by Manfred Spraul's avatar Manfred Spraul Committed by Linus Torvalds

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: default avatarManfred Spraul <manfred@colorfullife.com>
Cc: Nick Piggin <npiggin@suse.de>
Cc: Pierre Peiffer <peifferp@gmail.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 636c6be8
...@@ -474,6 +474,17 @@ static void update_queue(struct sem_array *sma, int semnum) ...@@ -474,6 +474,17 @@ static void update_queue(struct sem_array *sma, int semnum)
q = (struct sem_queue *)((char *)walk - offset); q = (struct sem_queue *)((char *)walk - offset);
walk = walk->next; 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, error = try_atomic_semop(sma, q->sops, q->nsops,
q->undo, q->pid); q->undo, q->pid);
......
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