The patch titled ipc/sem.c: optimize single sops when semval is zero has been added to the -mm tree. Its filename is ipc-semc-optimize-single-sops-when-semval-is-zero.patch Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/SubmitChecklist when testing your code *** See http://userweb.kernel.org/~akpm/stuff/added-to-mm.txt to find out what to do about this The current -mm tree may be found at http://userweb.kernel.org/~akpm/mmotm/ ------------------------------------------------------ Subject: ipc/sem.c: optimize single sops when semval is zero From: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxx> Cc: Nick Piggin <npiggin@xxxxxxx> Cc: Pierre Peiffer <peifferp@xxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- ipc/sem.c | 11 +++++++++++ 1 file changed, 11 insertions(+) diff -puN ipc/sem.c~ipc-semc-optimize-single-sops-when-semval-is-zero ipc/sem.c --- a/ipc/sem.c~ipc-semc-optimize-single-sops-when-semval-is-zero +++ a/ipc/sem.c @@ -473,6 +473,17 @@ 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); _ Patches currently in -mm which might be from manfred@xxxxxxxxxxxxxxxx are ipc-semc-sem-optimise-undo-list-search.patch ipc-semc-sem-use-list-operations.patch ipc-semc-sem-preempt-improve.patch ipc-semc-optimize-if-semops-fail.patch ipc-semc-add-a-per-semaphore-pending-list.patch ipc-semc-optimize-single-semop-operations.patch ipc-semc-optimize-single-sops-when-semval-is-zero.patch slab-leaks3-default-y.patch -- To unsubscribe from this list: send the line "unsubscribe mm-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html