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Commit 15df03c8 authored by Davidlohr Bueso's avatar Davidlohr Bueso Committed by Linus Torvalds
Browse files

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 7ca7e564 ("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: default avatarDavidlohr Bueso <dbueso@suse.de>
Cc: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent ebf66799

include/linux/ipc_namespace.h

+1 −0
Original line number Diff line number Diff line
@@ -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

ipc/util.c

+13 −30
Original line number Diff line number Diff line
@@ -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;

	}

}



/**

ipc/util.h

+18 −3
Original line number Diff line number Diff line
@@ -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.