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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: 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>
This commit is contained in:
Nick Piggin 2009-12-15 16:47:28 -08:00 committed by Linus Torvalds
parent 7d6feeb287
commit bf17bb7177
1 changed files with 20 additions and 6 deletions
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26
ipc/sem.c
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@ -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.