From: Rafael Aquini <aquini@xxxxxxxxxx>
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
Link: https://lkml.kernel.org/r/20210809203554.1562989-1-aquini@xxxxxxxxxx
Signed-off-by: Rafael Aquini <aquini@xxxxxxxxxx>
Acked-by: Davidlohr Bueso <dbueso@xxxxxxx>
Acked-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
Cc: Waiman Long <llong@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
ipc/util.c | 16 ++++------------
1 file changed, 4 insertions(+), 12 deletions(-)
--- a/ipc/util.c~ipc-replace-costly-bailout-check-in-sysvipc_find_ipc
+++ a/ipc/util.c
@@ -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();
_
