ipc/sem.c: sem optimise undo list search (bf17bb71) · Commits · 戴 / test

Around a month ago, there was some discussion about an improvement of the sysv sem algorithm: Most (at least: some important) users only use simple semaphore operations, therefore it's worthwile to optimize this use case. This patch: Move last looked up sem_undo struct to the head of the task's undo list. Attempt to move common entries to the front of the list so search time is reduced. This reduces lookup_undo on oprofile of problematic SAP workload by 30% (see patch 4 for a description of SAP workload). Signed-off-by:

Nick Piggin <npiggin@suse.de> Signed-off-by:

Manfred Spraul <manfred@colorfullife.com> Cc: Pierre Peiffer <peifferp@gmail.com> Signed-off-by:

Andrew Morton <akpm@linux-foundation.org> Signed-off-by:

Linus Torvalds <torvalds@linux-foundation.org>

ipc/sem.c

+20 −6

Original line number	Diff line number	Diff line
		@@ -962,17 +962,31 @@ static inline int get_undo_list(struct sem_undo_list **undo_listp)
		return 0;
		}

		static struct sem_undo lookup_undo(struct sem_undo_list ulp, int semid)
		static struct sem_undo __lookup_undo(struct sem_undo_list ulp, int semid)
		{
		struct sem_undo *walk;
		struct sem_undo *un;

		list_for_each_entry_rcu(walk, &ulp->list_proc, list_proc) {
		if (walk->semid == semid)
		return walk;
		list_for_each_entry_rcu(un, &ulp->list_proc, list_proc) {
		if (un->semid == semid)
		return un;
		}
		return NULL;
		}

		static struct sem_undo lookup_undo(struct sem_undo_list ulp, int semid)
		{
		struct sem_undo *un;

		assert_spin_locked(&ulp->lock);

		un = __lookup_undo(ulp, semid);
		if (un) {
		list_del_rcu(&un->list_proc);
		list_add_rcu(&un->list_proc, &ulp->list_proc);
		}
		return un;
		}

		/**
		* find_alloc_undo - Lookup (and if not present create) undo array
		* @ns: namespace
		@@ -1308,7 +1322,7 @@ void exit_sem(struct task_struct *tsk)
		if (IS_ERR(sma))
		continue;

		un = lookup_undo(ulp, semid);
		un = __lookup_undo(ulp, semid);
		if (un == NULL) {
		/* exit_sem raced with IPC_RMID+semget() that created
		* exactly the same semid. Nothing to do.

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