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Commit bf17bb71 authored by Nick Piggin's avatar Nick Piggin Committed by Linus Torvalds
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ipc/sem.c: sem optimise undo list search 



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: default avatarNick Piggin <npiggin@suse.de>
Signed-off-by: default avatarManfred Spraul <manfred@colorfullife.com>
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 7d6feeb2

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.