ipc-replace-costly-bailout-check-in-sysvipc_find_ipc.patch removed from -mm tree

The patch titled
Subject: ipc: replace costly bailout check in sysvipc_find_ipc()
has been removed from the -mm tree. Its filename was
ipc-replace-costly-bailout-check-in-sysvipc_find_ipc.patch

This patch was dropped because it was merged into mainline or a subsystem tree

------------------------------------------------------
From: Rafael Aquini <[email protected]>
Subject: ipc: replace costly bailout check in sysvipc_find_ipc()

sysvipc_find_ipc() was left with a costly way to check if the offset position fed to it is bigger than the total number of IPC IDs in use. So much so that the time it takes to iterate over /proc/sysvipc/* files grows exponentially for a custom benchmark that creates "N" SYSV shm segments and then times the read of /proc/sysvipc/shm (milliseconds):

    12 msecs to read   1024 segs from /proc/sysvipc/shm
    18 msecs to read   2048 segs from /proc/sysvipc/shm
    65 msecs to read   4096 segs from /proc/sysvipc/shm
   325 msecs to read   8192 segs from /proc/sysvipc/shm

1303 msecs to read 16384 segs from /proc/sysvipc/shm 5182 msecs to read 32768 segs from /proc/sysvipc/shm

The root problem lies with the loop that computes the total amount of ids in use to check if the "pos" feeded to sysvipc_find_ipc() grew bigger than "ids->in_use". That is a quite inneficient way to get to the maximum index in the id lookup table, specially when that value is already provided by struct ipc_ids.max_idx.

This patch follows up on the optimization introduced via commit 15df03c879836 ("sysvipc: make get_maxid O(1) again") and gets rid of the aforementioned costly loop replacing it by a simpler checkpoint based on ipc_get_maxidx() returned value, which allows for a smooth linear increase in time complexity for the same custom benchmark:

     2 msecs to read   1024 segs from /proc/sysvipc/shm
     2 msecs to read   2048 segs from /proc/sysvipc/shm
     4 msecs to read   4096 segs from /proc/sysvipc/shm
     9 msecs to read   8192 segs from /proc/sysvipc/shm

19 msecs to read 16384 segs from /proc/sysvipc/shm 39 msecs to read 32768 segs from /proc/sysvipc/shm

---

 ipc/util.c |   16 ++++------------
 1 file changed, 4 insertions(+), 12 deletions(-)

@@ -788,21 +788,13 @@ struct pid_namespace *ipc_seq_pid_ns(str
 static struct kern_ipc_perm *sysvipc_find_ipc(struct ipc_ids *ids, loff_t pos,
 					      loff_t *new_pos)
 {
-	struct kern_ipc_perm *ipc;
-	int total, id;
+	struct kern_ipc_perm *ipc = NULL;
+	int max_idx = ipc_get_maxidx(ids);
 
-	total = 0;
-	for (id = 0; id < pos && total < ids->in_use; id++) {
-		ipc = idr_find(&ids->ipcs_idr, id);
-		if (ipc != NULL)
-			total++;
-	}
-
-	ipc = NULL;
-	if (total >= ids->in_use)
+	if (max_idx == -1 || pos > max_idx)
 		goto out;
 
-	for (; pos < ipc_mni; pos++) {
+	for (; pos <= max_idx; pos++) {
 		ipc = idr_find(&ids->ipcs_idr, pos);
 		if (ipc != NULL) {
 			rcu_read_lock();

_

Patches currently in -mm which might be from [email protected] are