Re: [PATCH] ipc: replace costly bailout check in sysvipc_find_ipc()

[Manfred Spraul <manfred@colorfullife.com>]

[-- Attachment #1: Type: text/plain, Size: 2371 bytes --]

Hi Rafael,

On 8/9/21 10:35 PM, Rafael Aquini wrote:
> 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

Could you run your test with the attached patch?

The patch switches the code to idr_get_next(), and I see a speedup of 
factor 400 for this test:

- boot with ipcmni_extend

- create ipc object

- echo 16000000 > /proc/sys/kernel/msg_next_id

- create ipc object

- time cat /proc/sysvipc/msg

with current mainline: 8.65 seconds

with the patch: 0.02 seconds


If there are no gaps, then I would assume there is no speed-up compared 
to your patch, but it would be create if you could check

[and check that there is no slow-down]


Thanks,

--

     Manfred


[-- Attachment #2: 0001-PATCH-Improve-sysvipc_find_ipc.patch --]
[-- Type: text/x-patch, Size: 2760 bytes --]

From 4b7975d712db27c3d08731e0ebe4efd684256ca4 Mon Sep 17 00:00:00 2001
From: Manfred Spraul <manfred@colorfullife.com>
Date: Fri, 20 Aug 2021 21:08:12 +0200
Subject: [PATCH] [PATCH] Improve sysvipc_find_ipc()

Initially noticed by Rafael Aquini, see
https://lore.kernel.org/lkml/20210809203554.1562989-1-aquini@redhat.com/

The algorithm used in sysvipc_find_ipc() is highly inefficient.
It actually needs to find the next used index in an idr, and it uses
a for loop to locate that entry.

But: The IDR API contains idr_get_next(), thus switch the code to use
idr_get_next().

In addition: Update a few comments.

Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
---
 ipc/util.c | 43 ++++++++++++++++++++-----------------------
 1 file changed, 20 insertions(+), 23 deletions(-)

diff --git a/ipc/util.c b/ipc/util.c
index 0027e47626b7..083fd6dba1a1 100644
--- a/ipc/util.c
+++ b/ipc/util.c
@@ -783,35 +783,32 @@ struct pid_namespace *ipc_seq_pid_ns(struct seq_file *s)
 }
 
 /*
- * This routine locks the ipc structure found at least at position pos.
+ * This routine locks the ipc structure found at least at index pos.
  */
 static struct kern_ipc_perm *sysvipc_find_ipc(struct ipc_ids *ids, loff_t pos,
 					      loff_t *new_pos)
 {
+	int tmpidx;
 	struct kern_ipc_perm *ipc;
-	int total, id;
-
-	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)
-		goto out;
+	tmpidx = pos;
 
-	for (; pos < ipc_mni; pos++) {
-		ipc = idr_find(&ids->ipcs_idr, pos);
-		if (ipc != NULL) {
-			rcu_read_lock();
-			ipc_lock_object(ipc);
-			break;
-		}
+	ipc = idr_get_next(&ids->ipcs_idr, &tmpidx);
+	if (ipc != NULL) {
+		rcu_read_lock();
+		ipc_lock_object(ipc);
+		/*
+		 * We found the object with the index tmpidx.
+		 * For next search, start with tmpidx+1
+		 */
+		*new_pos = tmpidx + 1;
+	} else {
+		/*
+		 * EOF. seq_file can't notice that, thus
+		 * move the offset by one.
+		 */
+		*new_pos = pos + 1;
 	}
-out:
-	*new_pos = pos + 1;
 	return ipc;
 }
 
@@ -829,7 +826,7 @@ static void *sysvipc_proc_next(struct seq_file *s, void *it, loff_t *pos)
 }
 
 /*
- * File positions: pos 0 -> header, pos n -> ipc id = n - 1.
+ * File positions: pos 0 -> header, pos n -> ipc idx = n - 1.
  * SeqFile iterator: iterator value locked ipc pointer or SEQ_TOKEN_START.
  */
 static void *sysvipc_proc_start(struct seq_file *s, loff_t *pos)
@@ -854,7 +851,7 @@ static void *sysvipc_proc_start(struct seq_file *s, loff_t *pos)
 	if (*pos == 0)
 		return SEQ_START_TOKEN;
 
-	/* Find the (pos-1)th ipc */
+	/* Find the ipc object with the index >= (pos-1) */
 	return sysvipc_find_ipc(ids, *pos - 1, pos);
 }
 
-- 
2.31.1