From 15df03c87983660a4d1eedb4541778592bd97684 Mon Sep 17 00:00:00 2001 From: Davidlohr Bueso Date: Fri, 17 Nov 2017 15:31:18 -0800 Subject: sysvipc: make get_maxid O(1) again For a custom microbenchmark on a 3.30GHz Xeon SandyBridge, which calls IPC_STAT over and over, it was calculated that, on avg the cost of ipc_get_maxid() for increasing amounts of keys was: 10 keys: ~900 cycles 100 keys: ~15000 cycles 1000 keys: ~150000 cycles 10000 keys: ~2100000 cycles This is unsurprising as maxid is currently O(n). By having the max_id available in O(1) we save all those cycles for each semctl(_STAT) command, the idr_find can be expensive -- which some real (customer) workloads actually poll on. Note that this used to be the case, until commit 7ca7e564e04 ("ipc: store ipcs into IDRs"). The cost is the extra idr_find when doing RMIDs, but we simply go backwards, and should not take too many iterations to find the new value. [akpm@linux-foundation.org: coding-style fixes] Link: http://lkml.kernel.org/r/20170831172049.14576-5-dave@stgolabs.net Signed-off-by: Davidlohr Bueso Cc: Manfred Spraul Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- ipc/util.c | 43 +++++++++++++------------------------------ 1 file changed, 13 insertions(+), 30 deletions(-) (limited to 'ipc/util.c') diff --git a/ipc/util.c b/ipc/util.c index e09bf76610ef..ff045fec8d83 100644 --- a/ipc/util.c +++ b/ipc/util.c @@ -122,6 +122,7 @@ int ipc_init_ids(struct ipc_ids *ids) return err; idr_init(&ids->ipcs_idr); ids->tables_initialized = true; + ids->max_id = -1; #ifdef CONFIG_CHECKPOINT_RESTORE ids->next_id = -1; #endif @@ -188,36 +189,6 @@ static struct kern_ipc_perm *ipc_findkey(struct ipc_ids *ids, key_t key) return NULL; } -/** - * ipc_get_maxid - get the last assigned id - * @ids: ipc identifier set - * - * Called with ipc_ids.rwsem held. - */ -int ipc_get_maxid(struct ipc_ids *ids) -{ - struct kern_ipc_perm *ipc; - int max_id = -1; - int total, id; - - if (ids->in_use == 0) - return -1; - - if (ids->in_use == IPCMNI) - return IPCMNI - 1; - - /* Look for the last assigned id */ - total = 0; - for (id = 0; id < IPCMNI && total < ids->in_use; id++) { - ipc = idr_find(&ids->ipcs_idr, id); - if (ipc != NULL) { - max_id = id; - total++; - } - } - return max_id; -} - #ifdef CONFIG_CHECKPOINT_RESTORE /* * Specify desired id for next allocated IPC object. @@ -313,6 +284,9 @@ int ipc_addid(struct ipc_ids *ids, struct kern_ipc_perm *new, int limit) } ids->in_use++; + if (id > ids->max_id) + ids->max_id = id; + new->id = ipc_buildid(id, ids, new); return id; @@ -459,6 +433,15 @@ void ipc_rmid(struct ipc_ids *ids, struct kern_ipc_perm *ipcp) ipc_kht_remove(ids, ipcp); ids->in_use--; ipcp->deleted = true; + + if (unlikely(lid == ids->max_id)) { + do { + lid--; + if (lid == -1) + break; + } while (!idr_find(&ids->ipcs_idr, lid)); + ids->max_id = lid; + } } /** -- cgit v1.2.3-59-g8ed1b