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authorDavidlohr Bueso <dave@stgolabs.net>2017-11-17 15:31:18 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2017-11-17 16:10:04 -0800
commit15df03c87983660a4d1eedb4541778592bd97684 (patch)
treefb8e7ae25cee35177eb45e11bb3435f79ccc009f /ipc/util.h
parentebf66799acfb5f52ada4ff96ecc9579867941ea9 (diff)
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 <dbueso@suse.de> Cc: Manfred Spraul <manfred@colorfullife.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'ipc/util.h')
-rw-r--r--ipc/util.h21
1 files changed, 18 insertions, 3 deletions
diff --git a/ipc/util.h b/ipc/util.h
index 0cd6201fe63a..89b8ec176fc4 100644
--- a/ipc/util.h
+++ b/ipc/util.h
@@ -13,6 +13,7 @@
#include <linux/unistd.h>
#include <linux/err.h>
+#include <linux/ipc_namespace.h>
#define SEQ_MULTIPLIER (IPCMNI)
@@ -99,9 +100,6 @@ void __init ipc_init_proc_interface(const char *path, const char *header,
/* must be called with ids->rwsem acquired for writing */
int ipc_addid(struct ipc_ids *, struct kern_ipc_perm *, int);
-/* must be called with ids->rwsem acquired for reading */
-int ipc_get_maxid(struct ipc_ids *);
-
/* must be called with both locks acquired. */
void ipc_rmid(struct ipc_ids *, struct kern_ipc_perm *);
@@ -111,6 +109,23 @@ void ipc_set_key_private(struct ipc_ids *, struct kern_ipc_perm *);
/* must be called with ipcp locked */
int ipcperms(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp, short flg);
+/**
+ * ipc_get_maxid - get the last assigned id
+ * @ids: ipc identifier set
+ *
+ * Called with ipc_ids.rwsem held for reading.
+ */
+static inline int ipc_get_maxid(struct ipc_ids *ids)
+{
+ if (ids->in_use == 0)
+ return -1;
+
+ if (ids->in_use == IPCMNI)
+ return IPCMNI - 1;
+
+ return ids->max_id;
+}
+
/*
* For allocation that need to be freed by RCU.
* Objects are reference counted, they start with reference count 1.

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