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SubjectRe: [PATCHv7 00/33] kernel: Introduce Time Namespace
On Thu, Oct 17, 2019 at 11:24:45AM +0200, Thomas Gleixner wrote:
> On Fri, 11 Oct 2019, Dmitry Safonov wrote:
> > We wrote two small benchmarks. The first one gettime_perf.c calls
> > clock_gettime() in a loop for 3 seconds. It shows us performance with
> > a hot CPU cache (more clock_gettime() cycles - the better):
> >
> > | before | CONFIG_TIME_NS=n | host | inside timens
> > --------------------------------------------------------------
> > | 153242367 | 153567617 | 150933203 | 139310914
> > | 153324800 | 153115132 | 150919828 | 139299761
> > | 153125401 | 153686868 | 150930471 | 139273917
> > | 153399355 | 153694866 | 151083410 | 139286081
> > | 153489417 | 153739716 | 150997262 | 139146403
> > | 153494270 | 153724332 | 150035651 | 138835612
> > -----------------------------------------------------------
> > avg | 153345935 | 153588088 | 150816637 | 139192114
> > diff % | 100 | 100.1 | 98.3 | 90.7
>
>
> That host 98.3% number is weird and does not match the tests I did with the
> fallback code I provided you. On my limited testing that fallback hidden in
> the slowpath did not show any difference to the TIME_NS=n case when not
> inside a time namespace.

You did your experiments without a small optimization that we introduced
in the 18-th patch:

[PATCHv7 18/33] lib/vdso: Add unlikely() hint into vdso_read_begin()

When I did my measurements in the first time, I found that with this
timens change clock_gettime() shows a better performance when
CONFIG_TIME_NS isn't set. This looked weird for me, because I don't
expect to see this improvement. After analyzing a disassembled code of
vdso.so, I found that we can add the unlikely() hint into
vdso_read_begin() and this gives us 2% improvement of clock_gettime
performance on the upsteam kernel.

In my table, the "before" column is actually for the upstream kernel
with the 18-th patch. Here is the table with the real "before" column:

| before | with 18/33 | CONFIG_TIME_NS=n | host | inside timens
------------------------------------------------------------------------------
avg | 150331408 | 153345935 | 153588088 | 150816637 | 139192114
------------------------------------------------------------------------------
diff % | 98 | 100 | 100.1 | 98.3 | 90.7
------------------------------------------------------------------------------
stdev % | 0.3 | 0.09 | 0.15 | 0.25 | 0.13

If we compare numbers in "before", "host" and "inside timens" columns, we
see the same results that you had. clock_gettime() works with the
same performance in the host namespace and 7% slower in a time
namespace.

Now let's look why we have these 2% degradation in the host time
namespace. For that, we cat look at disassembled code of do_hres:

Before:
0: 55 push %rbp
1: 48 63 f6 movslq %esi,%rsi
4: 49 89 d1 mov %rdx,%r9
7: 49 89 c8 mov %rcx,%r8
a: 48 c1 e6 04 shl $0x4,%rsi
e: 48 01 fe add %rdi,%rsi
11: 48 89 e5 mov %rsp,%rbp
14: 41 54 push %r12
16: 53 push %rbx
17: 44 8b 17 mov (%rdi),%r10d
1a: 41 f6 c2 01 test $0x1,%r10b
1e: 0f 85 fb 00 00 00 jne 11f <do_hres.isra.0+0x11f>
24: 8b 47 04 mov 0x4(%rdi),%eax
27: 83 f8 01 cmp $0x1,%eax
2a: 74 0f je 3b <do_hres.isra.0+0x3b>
2c: 83 f8 02 cmp $0x2,%eax
2f: 74 72 je a3 <do_hres.isra.0+0xa3>
31: 5b pop %rbx
32: b8 ff ff ff ff mov $0xffffffff,%eax
37: 41 5c pop %r12
39: 5d pop %rbp
3a: c3 retq
...

After:
0: 55 push %rbp
1: 4c 63 ce movslq %esi,%r9
4: 49 89 d0 mov %rdx,%r8
7: 49 c1 e1 04 shl $0x4,%r9
b: 49 01 f9 add %rdi,%r9
e: 48 89 e5 mov %rsp,%rbp
11: 41 56 push %r14
13: 41 55 push %r13
15: 41 54 push %r12
17: 53 push %rbx
18: 44 8b 17 mov (%rdi),%r10d
1b: 44 89 d0 mov %r10d,%eax
1e: f7 d0 not %eax
20: 83 e0 01 and $0x1,%eax
23: 89 c3 mov %eax,%ebx
25: 0f 84 03 01 00 00 je 12e <do_hres+0x12e>
2b: 8b 47 04 mov 0x4(%rdi),%eax
2e: 83 f8 01 cmp $0x1,%eax
31: 74 13 je 46 <do_hres+0x46>
33: 83 f8 02 cmp $0x2,%eax
36: 74 7b je b3 <do_hres+0xb3>
38: b8 ff ff ff ff mov $0xffffffff,%eax
3d: 5b pop %rbx
3e: 41 5c pop %r12
40: 41 5d pop %r13
42: 41 5e pop %r14
44: 5d pop %rbp
45: c3 retq
...

So I think we see these 2% degradation in the host time namespace,
because we need to save to extra registers on stack. If we want to avoid
this degradation, we can mark do_hres_timens as noinline. In this case,
the disassembled code will be the same as before these changes:

0000000000000160 <do_hres>:
do_hres():
160: 55 push %rbp
161: 4c 63 ce movslq %esi,%r9
164: 49 89 d0 mov %rdx,%r8
167: 49 c1 e1 04 shl $0x4,%r9
16b: 49 01 f9 add %rdi,%r9
16e: 48 89 e5 mov %rsp,%rbp
171: 41 54 push %r12
173: 53 push %rbx
174: 44 8b 17 mov (%rdi),%r10d
177: 41 f6 c2 01 test $0x1,%r10b
17b: 0f 85 fc 00 00 00 jne 27d <do_hres+0x11d>
181: 8b 47 04 mov 0x4(%rdi),%eax
184: 83 f8 01 cmp $0x1,%eax
187: 74 0f je 198 <do_hres+0x38>
189: 83 f8 02 cmp $0x2,%eax
18c: 74 73 je 201 <do_hres+0xa1>
18e: 5b pop %rbx
18f: b8 ff ff ff ff mov $0xffffffff,%eax
194: 41 5c pop %r12
196: 5d pop %rbp
197: c3 retq
...

But this change will affect the performance of clock_gettime in a time
namespace.

My experiments shows that with the noinline annotation for
do_hres_timens, clock_gettime will work with the same performance in the
host time namespace, but it will be slower on 11% in a time namespace.

Thomas, what do you think about this? Do we need to mark do_hres_timens
as noinline?

Thanks,
Andrei

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