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Subject[PATCH 4/4] sysvipc: make get_maxid O(1) again
Date
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.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
arch/x86/Kconfig | 2 +-
include/linux/ipc_namespace.h | 1 +
ipc/util.c | 43 +++++++++++++------------------------------
ipc/util.h | 21 ++++++++++++++++++---
4 files changed, 33 insertions(+), 34 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 44d139b88c32..301fe235367b 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -55,7 +55,7 @@ config X86
select ARCH_HAS_KCOV if X86_64
select ARCH_HAS_MMIO_FLUSH
select ARCH_HAS_PMEM_API if X86_64
- select ARCH_HAS_REFCOUNT
+ select ARCH_HAS_REFCOUNT if BROKEN
select ARCH_HAS_UACCESS_FLUSHCACHE if X86_64
select ARCH_HAS_SET_MEMORY
select ARCH_HAS_SG_CHAIN
diff --git a/include/linux/ipc_namespace.h b/include/linux/ipc_namespace.h
index 058051566ced..c59b8ddee191 100644
--- a/include/linux/ipc_namespace.h
+++ b/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 --git a/ipc/util.c b/ipc/util.c
index d4091665f439..45e84203c3fe 100644
--- a/ipc/util.c
+++ b/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(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.
@@ -312,6 +283,9 @@ 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;
@@ -458,6 +432,15 @@ 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;
+ }
}

/**
diff --git a/ipc/util.h b/ipc/util.h
index 17c8d2f14990..eabca0e37ce1 100644
--- a/ipc/util.h
+++ b/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(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 *);

@@ -110,6 +108,23 @@ 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.
--
2.12.0
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