| lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
|
Open Source and information security mailing list archives
| ||
|
Message-ID: <YShLU3VTtifYU8IR@x61s-fbsd.aquini.net>
Date: Thu, 26 Aug 2021 22:17:55 -0400
From: Rafael Aquini <aquini@...hat.com>
To: Manfred Spraul <manfred@...orfullife.com>
Cc: linux-kernel@...r.kernel.org,
Andrew Morton <akpm@...ux-foundation.org>,
Davidlohr Bueso <dbueso@...e.de>,
Waiman Long <llong@...hat.com>, 1vier1@....de
Subject: Re: [PATCH] ipc: replace costly bailout check in sysvipc_find_ipc()
On Wed, Aug 25, 2021 at 06:15:06PM -0400, Rafael Aquini wrote:
> On Fri, Aug 20, 2021 at 09:41:32PM +0200, Manfred Spraul wrote:
> > 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?
> >
>
> Manfred,
>
> Sorry it took me a while to get back to you here. (coming back from a short
> leave). I'll take a look into your approach and report back in a few days.
>
Manfred,
as promised I re-ran the tests, adjusting the shm workload to leave a gap in
the ids between the runs (effectivelly deleting all previously created segments
before recreating the batch for the next turn), and the outcome between the two
patches is virtually the same:
* your patch:
1 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
24 msecs to read 16384 segs from /proc/sysvipc/shm
44 msecs to read 32768 segs from /proc/sysvipc/shm
* my patch:
1 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
22 msecs to read 16384 segs from /proc/sysvipc/shm
45 msecs to read 32768 segs from /proc/sysvipc/shm
and even perf stat numbers are virtually the same for reading
32768 segments from /proc/sysvipc/shm:
* your patch
Performance counter stats for 'cat /proc/sysvipc/shm' (10 runs):
45.03 msec task-clock # 0.990 CPUs utilized ( +- 0.27% )
0 context-switches # 0.004 K/sec ( +-100.00% )
0 cpu-migrations # 0.004 K/sec ( +-100.00% )
70 page-faults # 0.002 M/sec ( +- 0.61% )
149,631,684 cycles # 3.323 GHz ( +- 0.25% ) (82.23%)
2,148,928 stalled-cycles-frontend # 1.44% frontend cycles idle ( +- 2.21% ) (82.24%)
67,488,510 stalled-cycles-backend # 45.10% backend cycles idle ( +- 0.73% ) (83.32%)
249,186,578 instructions # 1.67 insn per cycle
# 0.27 stalled cycles per insn ( +- 0.03% ) (84.31%)
62,268,386 branches # 1382.790 M/sec ( +- 0.02% ) (84.45%)
83,170 branch-misses # 0.13% of all branches ( +- 1.57% ) (83.44%)
0.045485 +- 0.000142 seconds time elapsed ( +- 0.31% )
* my patch:
Performance counter stats for 'cat /proc/sysvipc/shm' (10 runs):
45.22 msec task-clock # 0.990 CPUs utilized ( +- 0.24% )
0 context-switches # 0.002 K/sec ( +-100.00% )
0 cpu-migrations # 0.000 K/sec
70 page-faults # 0.002 M/sec ( +- 0.92% )
150,273,699 cycles # 3.323 GHz ( +- 0.25% ) (82.31%)
12,288,154 stalled-cycles-frontend # 8.18% frontend cycles idle ( +- 0.95% ) (82.31%)
61,198,482 stalled-cycles-backend # 40.72% backend cycles idle ( +- 0.91% ) (83.14%)
245,485,497 instructions # 1.63 insn per cycle
# 0.25 stalled cycles per insn ( +- 0.03% ) (84.41%)
61,736,363 branches # 1365.267 M/sec ( +- 0.02% ) (84.52%)
82,381 branch-misses # 0.13% of all branches ( +- 1.17% ) (83.31%)
0.045671 +- 0.000125 seconds time elapsed ( +- 0.27% )
I'll leave it up to you, as clearly both approaches do pretty good in reducing
the noted overhead imposed by the old approach.
Since my patch is already in linux-next, may I ask you to ack it and follow it
up there if you decide on going with your approach? The diff below applies
cleanly on top of linux-next head (it's the one I used on tests).
Cheers!
Rafael
--
diff --git a/ipc/util.c b/ipc/util.c
index d48d8cfa1f3f..261540154782 100644
--- a/ipc/util.c
+++ b/ipc/util.c
@@ -788,22 +788,26 @@ struct pid_namespace *ipc_seq_pid_ns(struct seq_file *s)
static struct kern_ipc_perm *sysvipc_find_ipc(struct ipc_ids *ids, loff_t pos,
loff_t *new_pos)
{
- struct kern_ipc_perm *ipc = NULL;
- int max_idx = ipc_get_maxidx(ids);
+ struct kern_ipc_perm *ipc;
+ int tmpidx = pos;
- if (max_idx == -1 || pos > max_idx)
- goto out;
-
- for (; pos <= max_idx; 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;
}
Powered by blists - more mailing lists