Re: [RFC PATCH 4/9] vringh: unify the APIs for all accessors
From: Shunsuke Mie
Date: Tue Jan 10 2023 - 23:10:37 EST
On 2022/12/28 16:20, Michael S. Tsirkin wrote:
On Wed, Dec 28, 2022 at 11:24:10AM +0900, Shunsuke Mie wrote:
2022年12月27日(火) 23:37 Michael S. Tsirkin <mst@xxxxxxxxxx>:
On Tue, Dec 27, 2022 at 07:22:36PM +0900, Shunsuke Mie wrote:
2022年12月27日(火) 16:49 Shunsuke Mie <mie@xxxxxxxxxx>:
2022年12月27日(火) 16:04 Michael S. Tsirkin <mst@xxxxxxxxxx>:
On Tue, Dec 27, 2022 at 11:25:26AM +0900, Shunsuke Mie wrote:
Each vringh memory accessors that are for user, kern and iotlb has own
interfaces that calls common code. But some codes are duplicated and that
becomes loss extendability.
Introduce a struct vringh_ops and provide a common APIs for all accessors.
It can bee easily extended vringh code for new memory accessor and
simplified a caller code.
Signed-off-by: Shunsuke Mie <mie@xxxxxxxxxx>
---
drivers/vhost/vringh.c | 667 +++++++++++------------------------------
include/linux/vringh.h | 100 +++---
2 files changed, 225 insertions(+), 542 deletions(-)
diff --git a/drivers/vhost/vringh.c b/drivers/vhost/vringh.c
index aa3cd27d2384..ebfd3644a1a3 100644
--- a/drivers/vhost/vringh.c
+++ b/drivers/vhost/vringh.c
@@ -35,15 +35,12 @@ static __printf(1,2) __cold void vringh_bad(const char *fmt, ...)
}
/* Returns vring->num if empty, -ve on error. */
-static inline int __vringh_get_head(const struct vringh *vrh,
- int (*getu16)(const struct vringh *vrh,
- u16 *val, const __virtio16 *p),
- u16 *last_avail_idx)
+static inline int __vringh_get_head(const struct vringh *vrh, u16 *last_avail_idx)
{
u16 avail_idx, i, head;
int err;
- err = getu16(vrh, &avail_idx, &vrh->vring.avail->idx);
+ err = vrh->ops.getu16(vrh, &avail_idx, &vrh->vring.avail->idx);
if (err) {
vringh_bad("Failed to access avail idx at %p",
&vrh->vring.avail->idx);
I like that this patch removes more lines of code than it adds.
However one of the design points of vringh abstractions is that they were
carefully written to be very low overhead.
This is why we are passing function pointers to inline functions -
compiler can optimize that out.
I think that introducing ops indirect functions calls here is going to break
these assumptions and hurt performance.
Unless compiler can somehow figure it out and optimize?
I don't see how it's possible with ops pointer in memory
but maybe I'm wrong.
I think your concern is correct. I have to understand the compiler
optimization and redesign this approach If it is needed.
Was any effort taken to test effect of these patches on performance?
I just tested vringh_test and already faced little performance reduction.
I have to investigate that, as you said.
I attempted to test with perf. I found that the performance of patched code
is almost the same as the upstream one. However, I have to investigate way
this patch leads to this result, also the profiling should be run on
more powerful
machines too.
environment:
$ grep 'model name' /proc/cpuinfo
model name : Intel(R) Core(TM) i3-7020U CPU @ 2.30GHz
model name : Intel(R) Core(TM) i3-7020U CPU @ 2.30GHz
model name : Intel(R) Core(TM) i3-7020U CPU @ 2.30GHz
model name : Intel(R) Core(TM) i3-7020U CPU @ 2.30GHz
results:
* for patched code
Performance counter stats for 'nice -n -20 ./vringh_test_patched
--parallel --eventidx --fast-vringh --indirect --virtio-1' (20 runs):
3,028.05 msec task-clock # 0.995 CPUs
utilized ( +- 0.12% )
78,150 context-switches # 25.691 K/sec
( +- 0.00% )
5 cpu-migrations # 1.644 /sec
( +- 3.33% )
190 page-faults # 62.461 /sec
( +- 0.41% )
6,919,025,222 cycles # 2.275 GHz
( +- 0.13% )
8,990,220,160 instructions # 1.29 insn per
cycle ( +- 0.04% )
1,788,326,786 branches # 587.899 M/sec
( +- 0.05% )
4,557,398 branch-misses # 0.25% of all
branches ( +- 0.43% )
3.04359 +- 0.00378 seconds time elapsed ( +- 0.12% )
* for upstream code
Performance counter stats for 'nice -n -20 ./vringh_test_base
--parallel --eventidx --fast-vringh --indirect --virtio-1' (10 runs):
3,058.41 msec task-clock # 0.999 CPUs
utilized ( +- 0.14% )
78,149 context-switches # 25.545 K/sec
( +- 0.00% )
5 cpu-migrations # 1.634 /sec
( +- 2.67% )
194 page-faults # 63.414 /sec
( +- 0.43% )
6,988,713,963 cycles # 2.284 GHz
( +- 0.14% )
8,512,533,269 instructions # 1.22 insn per
cycle ( +- 0.04% )
1,638,375,371 branches # 535.549 M/sec
( +- 0.05% )
4,428,866 branch-misses # 0.27% of all
branches ( +- 22.57% )
3.06085 +- 0.00420 seconds time elapsed ( +- 0.14% )
How you compiled it also matters. ATM we don't enable retpolines
and it did not matter since we didn't have indirect calls,
but we should. Didn't yet investigate how to do that for virtio tools.
I think the retpolines certainly affect performance. Thank you for pointing
it out. I'd like to start the investigation that how to apply the
retpolines to the
virtio tools.
Thank you for your comments.
Thanks!
Best,
Shunsuke.
This isn't all that trivial if we want this at runtime.
But compile time is kind of easy.
See Documentation/admin-guide/hw-vuln/spectre.rst
Thank you for showing it.
I followed the document and added options to CFLAGS to the tools Makefile.
That is
---
diff --git a/tools/virtio/Makefile b/tools/virtio/Makefile
index 1b25cc7c64bb..7b7139d97d74 100644
--- a/tools/virtio/Makefile
+++ b/tools/virtio/Makefile
@@ -4,7 +4,7 @@ test: virtio_test vringh_test
virtio_test: virtio_ring.o virtio_test.o
vringh_test: vringh_test.o vringh.o virtio_ring.o
-CFLAGS += -g -O2 -Werror -Wno-maybe-uninitialized -Wall -I.
-I../include/ -I ../../usr/include/ -Wno-pointer-sign
-fno-strict-overflow -fno-strict-aliasing -fno-common -MMD
-U_FORTIFY_SOURCE -include ../../include/linux/kconfig.h
+CFLAGS += -g -O2 -Werror -Wno-maybe-uninitialized -Wall -I.
-I../include/ -I ../../usr/include/ -Wno-pointer-sign
-fno-strict-overflow -fno-strict-aliasing -fno-common -MMD
-U_FORTIFY_SOURCE -include ../../include/linux/kconfig.h
-mfunction-return=thunk -fcf-protection=none -mindirect-branch-register
CFLAGS += -pthread
LDFLAGS += -pthread
vpath %.c ../../drivers/virtio ../../drivers/vhost
---
And results of evaluation are following:
- base with retpoline
$ sudo perf stat --repeat 20 -- nice -n -20 ./vringh_test_retp_origin
--parallel --eventidx --fast-vringh
Using CPUS 0 and 3
Guest: notified 0, pinged 98040
Host: notified 98040, pinged 0
...
Performance counter stats for 'nice -n -20 ./vringh_test_retp_origin
--parallel --eventidx --fast-vringh' (20 runs):
6,228.33 msec task-clock # 1.004 CPUs
utilized ( +- 0.05% )
196,110 context-switches # 31.616
K/sec ( +- 0.00% )
6 cpu-migrations # 0.967
/sec ( +- 2.39% )
205 page-faults # 33.049
/sec ( +- 0.46% )
14,218,527,987 cycles # 2.292
GHz ( +- 0.05% )
10,342,897,254 instructions # 0.73 insn per
cycle ( +- 0.02% )
2,310,572,989 branches # 372.500
M/sec ( +- 0.03% )
178,273,068 branch-misses # 7.72% of all
branches ( +- 0.04% )
6.20406 +- 0.00308 seconds time elapsed ( +- 0.05% )
- patched (unified APIs) with retpoline
$ sudo perf stat --repeat 20 -- nice -n -20 ./vringh_test_retp_patched
--parallel --eventidx --fast-vringh
Using CPUS 0 and 3
Guest: notified 0, pinged 98040
Host: notified 98040, pinged 0
...
Performance counter stats for 'nice -n -20 ./vringh_test_retp_patched
--parallel --eventidx --fast-vringh' (20 runs):
6,103.94 msec task-clock # 1.001 CPUs
utilized ( +- 0.03% )
196,125 context-switches # 32.165
K/sec ( +- 0.00% )
7 cpu-migrations # 1.148
/sec ( +- 1.56% )
196 page-faults # 32.144
/sec ( +- 0.41% )
13,933,055,778 cycles # 2.285
GHz ( +- 0.03% )
10,309,004,718 instructions # 0.74 insn per
cycle ( +- 0.03% )
2,368,447,519 branches # 388.425
M/sec ( +- 0.04% )
211,364,886 branch-misses # 8.94% of all
branches ( +- 0.05% )
6.09888 +- 0.00155 seconds time elapsed ( +- 0.03% )
As a result, at the patched code, the branch-misses was increased but
elapsed time became faster than the based code. The number of
page-faults was
a little different. I'm suspicious of that the page-fault penalty leads the
performance result.
I think that a pattern of memory access for data is same with those, but
for instruction is different. Actually a code size (.text segment) was a
little smaller. 0x6a65 and 0x63f5.
$ readelf -a ./vringh_test_retp_origin |grep .text -1
0000000000000008 0000000000000008 AX 0 0 8
[14] .text PROGBITS 0000000000001230 00001230
0000000000006a65 0000000000000000 AX 0 0 16
--
02 .interp .note.gnu.build-id .note.ABI-tag .gnu.hash .dynsym
.dynstr .gnu.version .gnu.version_r .rela.dyn .rela.plt
03 .init .plt .plt.got .text .fini
04 .rodata .eh_frame_hdr .eh_frame
$ readelf -a ./vringh_test_retp_patched |grep .text -1
0000000000000008 0000000000000008 AX 0 0 8
[14] .text PROGBITS 0000000000001230 00001230
00000000000063f5 0000000000000000 AX 0 0 16
--
02 .interp .note.gnu.build-id .note.ABI-tag .gnu.hash .dynsym
.dynstr .gnu.version .gnu.version_r .rela.dyn .rela.plt
03 .init .plt .plt.got .text .fini
04 .rodata .eh_frame_hdr .eh_frame
I'll keep this investigation. I was wondering if you could comment me.
Best