The patch titled
Subject: sysvipc: make get_maxid O(1) again
has been added to the -mm tree. Its filename is
sysvipc-make-get_maxid-o1-again.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/sysvipc-make-get_maxid-o1-again.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/sysvipc-make-get_maxid-o1-again.patch
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------------------------------------------------------
From: Davidlohr Bueso <dave@xxxxxxxxxxxx>
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 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.
Link: http://lkml.kernel.org/r/20170831172049.14576-5-dave@xxxxxxxxxxxx
Signed-off-by: Davidlohr Bueso <dbueso@xxxxxxx>
Cc: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/ipc_namespace.h | 1
ipc/util.c | 43 +++++++++-----------------------
ipc/util.h | 21 +++++++++++++--
3 files changed, 32 insertions(+), 33 deletions(-)
diff -puN include/linux/ipc_namespace.h~sysvipc-make-get_maxid-o1-again include/linux/ipc_namespace.h
--- a/include/linux/ipc_namespace.h~sysvipc-make-get_maxid-o1-again
+++ a/include/linux/ipc_namespace.h
@@ -18,6 +18,7 @@ struct ipc_ids {
bool tables_initialized;
struct rw_semaphore rwsem;
struct idr ipcs_idr;
+ int max_id;
#ifdef CONFIG_CHECKPOINT_RESTORE
int next_id;
#endif
diff -puN ipc/util.c~sysvipc-make-get_maxid-o1-again ipc/util.c
--- a/ipc/util.c~sysvipc-make-get_maxid-o1-again
+++ a/ipc/util.c
@@ -121,6 +121,7 @@ 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
@@ -187,36 +188,6 @@ static struct kern_ipc_perm *ipc_findkey
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.
@@ -312,6 +283,9 @@ int ipc_addid(struct ipc_ids *ids, struc
}
ids->in_use++;
+ if (id > ids->max_id)
+ ids->max_id = id;
+
new->id = ipc_buildid(id, ids, new);
return id;
@@ -458,6 +432,15 @@ void ipc_rmid(struct ipc_ids *ids, struc
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;
+ }
}
/**
diff -puN ipc/util.h~sysvipc-make-get_maxid-o1-again ipc/util.h
--- a/ipc/util.h~sysvipc-make-get_maxid-o1-again
+++ a/ipc/util.h
@@ -12,6 +12,7 @@
#include <linux/unistd.h>
#include <linux/err.h>
+#include <linux/ipc_namespace.h>
#define SEQ_MULTIPLIER (IPCMNI)
@@ -98,9 +99,6 @@ void __init ipc_init_proc_interface(cons
/* 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 *);
@@ -110,6 +108,23 @@ void ipc_set_key_private(struct ipc_ids
/* 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.
_
Patches currently in -mm which might be from dave@xxxxxxxxxxxx are
mmcompaction-serialize-waitqueue_active-checks-for-real.patch
sysvipc-unteach-ids-next_id-for-checkpoint_restore.patch
sysvipc-duplicate-lock-comments-wrt-ipc_addid.patch
sysvipc-properly-name-ipc_addid-limit-parameter.patch
sysvipc-make-get_maxid-o1-again.patch
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