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SubjectRe: recvfrom/recvmsg performance and CONFIG_HARDENED_USERCOPY
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On 12/6/19 5:39 AM, David Laight wrote:
> Some tests I've done seem to show that recvmsg() is much slower that recvfrom()
> even though most of what they do is the same.

Not really.

> One thought is that the difference is all the extra copy_from_user() needed by
> recvmsg. CONFIG_HARDENED_USERCOPY can add a significant cost.
>
> I've built rebuilt my 5.4-rc7 kernel with all the copy_to/from_user() in net/socket.c
> replaced with the '_' prefixed versions (that don't call check_object()).
> And also changed rw_copy_check_uvector() in fs/read_write.c.
>
> Schedviz then showed the time spent by the application thread that calls
> recvmsg() (about) 225 times being reduced from 0.9ms to 0.75ms.
>
> I've now instrumented the actual recv calls. It show some differences,
> but now enough to explain the 20% difference above.
> (This is all made more difficult because my Ivy Bridge i7-3770 refuses
> to run at a fixed frequency.)
>
> Anyway using PERF_COUNT_HW_CPU_CYCLES I've got the following
> histograms for the number of cycles in each recv call.
> There are about the same number (2.8M) in each column over
> an elapsed time of 20 seconds.
> There are 450 active UDP sockets, each receives 1 message every 20ms.
> Every 10ms a RT thread that is pinned to a cpu reads all the pending messages.
> This is a 4 core hyperthreading (8 cpu) system.
> During these tests 5 other threads are also busy.
> There are no sends (on those sockets).
>
> | recvfrom | recvmsg
> cycles | unhard | hard | unhard | hard
> -----------------------------------------------------
> 1472: 29 1 0 0
> 1600: 8980 4887 3 0
> 1728: 112540 159518 5393 2895
> 1856: 174555 270148 119054 111230
> 1984: 126007 168383 152310 195288
> 2112: 80249 87045 118941 168801
> 2240: 61570 54790 81847 110561
> 2368: 95088 61796 57496 71732
> 2496: 193633 155870 54020 54801
> 2624: 274997 284921 102465 74626
> 2752: 276661 295715 160492 119498
> 2880: 248751 264174 206327 186028
> 3008: 207532 213067 230704 229232
> 3136: 167976 164804 226493 238555
> 3264: 133708 124857 202639 220574
> 3392: 107859 95696 172949 189475
> 3520: 88599 75943 141056 153524
> 3648: 74290 61586 115873 120994
> 3776: 62253 50891 96061 95040
> 3904: 52213 42482 81113 76577
> 4032: 42920 34632 69077 63131
> 4160: 35472 28327 60074 53631
> 4288: 28787 22603 51345 46620
> 4416: 24072 18496 44006 40325
> 4544: 20107 14886 37185 34516
> 4672: 16759 12206 31408 29031
> 4800: 14195 9991 26843 24396
> 4928: 12356 8167 22775 20165
> 5056: 10387 6931 19404 16591
> 5184: 9284 5916 16817 13743
> 5312: 7994 5116 14737 11452
> 5440: 7152 4495 12592 9607
> 5568: 6300 3969 11117 8592
> 5696: 5445 3421 9988 7237
> 5824: 4683 2829 8839 6368
> 5952: 3959 2643 7652 5652
> 6080: 3454 2377 6442 4814
> 6208: 3041 2219 5735 4170
> 6336: 2840 2060 5059 3615
> 6464: 2428 1975 4433 3201
> 6592: 2109 1794 4078 2823
> 6720: 1871 1382 3549 2558
> 6848: 1706 1262 3110 2328
> 6976: 1567 1001 2733 1991
> 7104: 1436 873 2436 1819
> 7232: 1417 860 2102 1652
> 7360: 1414 741 1823 1429
> 7488: 1372 814 1663 1239
> 7616: 1201 896 1430 1152
> 7744: 1275 1008 1364 1049
> 7872: 1382 1120 1367 925
> 8000: 1316 1282 1253 815
> 8128: 1264 1266 1313 792
> 8256+: 19252 19450 34703 30228
> ----------------------------------------------------
> Total: 2847707 2863582 2853688 2877088
>
> This does show a few interesting things:
> 1) The 'hardened' kernel is slower, especially for recvmsg.
> 2) The difference for recvfrom isn't enough for the 20% reduction I saw.
> 3) There are two peaks at the top a 'not insubstantial' number are a lot
> faster than the main peak.
> 4) There is second peak way down at 8000 cycles.
> This is repeatable.
>
> Any idea what is actually going on??
>

Real question is : Do you actually need to use recvmsg() instead of recvfrom() ?

If recvmsg() provides additional cmsg, this is not surprising it is more expensive.

recvmsg() also uses an indirect call, so CONFIG_RETPOLINE=y is probably hurting.

err = (nosec ? sock_recvmsg_nosec : sock_recvmsg)(sock, msg_sys, flags);

Maybe a INDIRECT_CALL annotation could help, or rewriting this to not let gcc
use an indirect call.


diff --git a/net/socket.c b/net/socket.c
index ea28cbb9e2e7a7180ee63de2d09a81aacb001ab7..752714281026dab6db850ec7fa75b7aa6240661f 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -2559,7 +2559,10 @@ static int ____sys_recvmsg(struct socket *sock, struct msghdr *msg_sys,

if (sock->file->f_flags & O_NONBLOCK)
flags |= MSG_DONTWAIT;
- err = (nosec ? sock_recvmsg_nosec : sock_recvmsg)(sock, msg_sys, flags);
+ if (nosec)
+ err = sock_recvmsg_nosec(sock, msg_sys, flags);
+ else
+ err = sock_recvmsg(sock, msg_sys, flags);
if (err < 0)
goto out;
len = err;


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