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sysvipc: make get_maxid O(1) again

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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 7ca7e564e0 ("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>
This commit is contained in:
Davidlohr Bueso 2017-11-17 15:31:18 -08:00 committed by Linus Torvalds
parent ebf66799ac
commit 15df03c879
3 changed files with 32 additions and 33 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
include/linux/ipc_namespace.h
View File

@ -19,6 +19,7 @@ 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

43
ipc/util.c
View File

@ -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;
}
}
/**

21
ipc/util.h
View File

@ -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.