* glibc tunables to disable axv not updating memcpy
@ 2022-12-08 3:47 Vineet Gupta
2022-12-08 8:31 ` Florian Weimer
0 siblings, 1 reply; 3+ messages in thread
From: Vineet Gupta @ 2022-12-08 3:47 UTC (permalink / raw)
To: libc-help; +Cc: siddhesh, goldstein.w.n, hjl.tools
Hi,
I've been playing with x86_64 tunables to see how they are plumbed into
ifunc machinery, but running into a something which I don't understand
(this is glibc 2.35 so pretty recent).
I'm disabling AVX and friends as:
export
GLIBC_TUNABLES=glibc.cpu.hwcaps=--SSSE3,-SSE4_2,AVX,-AVX2,-AVX512F,-AVX512VL
A simple program which calls memcpy, seems to be calling
__memmove_evex_unaligned_erms() despite my tunable disabling AVX512VL.
|
| main (argc=2, argv=0x7fffffffdce8) at test-mcpu.c:10
| 10 memcpy(dest, src, atoi(argv[1]));
| (gdb) s
| __memmove_evex_unaligned_erms () at
../sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S:307
| 307 ../sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S
glibc dependency on AVX512VL for the said routine/
| IFUNC_IMPL_ADD (array, i, memmove,
| CPU_FEATURE_USABLE (AVX512VL),
| __memmove_evex_unaligned_erms)
|
There used to be AVX*_Usable for tunables but it seems that was changed
in 107e6a3c2212ba7a3a (" x86: Support usable check for all CPU features")
I also don't understand how the memcpy is being redirected to memmove,
since assembler in test has a PLT call to memcpy.
LD_DEBUG=all doesn't print any ifunc related info.
I've tried hacking x86 elf_machine_lazy_rel() add dl_debug_printf()
around R_X86_64_IRELATIVE - which seems to work but there's too less
info to help answer the exact ifunc plumbing.
Any ideas where to look ?
Thx,
-Vineet
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: glibc tunables to disable axv not updating memcpy
2022-12-08 3:47 glibc tunables to disable axv not updating memcpy Vineet Gupta
@ 2022-12-08 8:31 ` Florian Weimer
2023-01-05 0:04 ` glibc ifunc debug (was Re: glibc tunables to disable axv not updating memcpy) Vineet Gupta
0 siblings, 1 reply; 3+ messages in thread
From: Florian Weimer @ 2022-12-08 8:31 UTC (permalink / raw)
To: Vineet Gupta via Libc-help
Cc: Vineet Gupta, siddhesh, goldstein.w.n, hjl.tools
* Vineet Gupta via Libc-help:
> Hi,
>
> I've been playing with x86_64 tunables to see how they are plumbed
> into ifunc machinery, but running into a something which I don't
> understand (this is glibc 2.35 so pretty recent).
>
> I'm disabling AVX and friends as:
> export
> GLIBC_TUNABLES=glibc.cpu.hwcaps=--SSSE3,-SSE4_2,AVX,-AVX2,-AVX512F,-AVX512VL
>
>
> A simple program which calls memcpy, seems to be calling
> __memmove_evex_unaligned_erms() despite my tunable disabling AVX512VL.
>
> |
> | main (argc=2, argv=0x7fffffffdce8) at test-mcpu.c:10
> | 10 memcpy(dest, src, atoi(argv[1]));
> | (gdb) s
> | __memmove_evex_unaligned_erms () at
> ../sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S:307
> | 307 ../sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S
>
>
> glibc dependency on AVX512VL for the said routine/
>
> | IFUNC_IMPL_ADD (array, i, memmove,
> | CPU_FEATURE_USABLE (AVX512VL),
> | __memmove_evex_unaligned_erms)
> |
This is for the test suite and the benchmarks. The production selector
is in sysdeps/x86_64/multiarch/ifunc-memmove.h. You should be able to
set a line-based breakpoint and single-step through it.
> LD_DEBUG=all doesn't print any ifunc related info.
I want to add logging, which is why I asked on the binutils list:
Subject: Named local symbols in the ELF dynamic symbol table
To: binutils@sourceware.org
Date: Tue, 06 Dec 2022 18:11:08 +0100 (1 day, 15 hours, 17 minutes ago)
Message-ID: <87y1rke9ub.fsf@oldenburg.str.redhat.com>
If we have the symbol names in the dynamic symbol tables, we can use
dladdr or a custom implementation of it to get symbol names.
If I don't get a reply, I'll settle for printing relative addresses.
People can then use an external tool to resolve them to symbols.
Thanks,
Florian
^ permalink raw reply [flat|nested] 3+ messages in thread
* glibc ifunc debug (was Re: glibc tunables to disable axv not updating memcpy)
2022-12-08 8:31 ` Florian Weimer
@ 2023-01-05 0:04 ` Vineet Gupta
0 siblings, 0 replies; 3+ messages in thread
From: Vineet Gupta @ 2023-01-05 0:04 UTC (permalink / raw)
To: Florian Weimer, Vineet Gupta via Libc-help
On 12/8/22 00:31, Florian Weimer wrote:
> * Vineet Gupta via Libc-help:
>
>> Hi,
>>
>> I've been playing with x86_64 tunables to see how they are plumbed
>> into ifunc machinery, but running into a something which I don't
>> understand (this is glibc 2.35 so pretty recent).
>>
>> I'm disabling AVX and friends as:
>> export
>> GLIBC_TUNABLES=glibc.cpu.hwcaps=--SSSE3,-SSE4_2,AVX,-AVX2,-AVX512F,-AVX512VL
>>
>>
>> A simple program which calls memcpy, seems to be calling
>> __memmove_evex_unaligned_erms() despite my tunable disabling AVX512VL.
>>
>> |
>> | main (argc=2, argv=0x7fffffffdce8) at test-mcpu.c:10
>> | 10 memcpy(dest, src, atoi(argv[1]));
>> | (gdb) s
>> | __memmove_evex_unaligned_erms () at
>> ../sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S:307
>> | 307 ../sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S
>>
>>
>> glibc dependency on AVX512VL for the said routine/
>>
>> | IFUNC_IMPL_ADD (array, i, memmove,
>> | CPU_FEATURE_USABLE (AVX512VL),
>> | __memmove_evex_unaligned_erms)
>> |
> This is for the test suite and the benchmarks. The production selector
> is in sysdeps/x86_64/multiarch/ifunc-memmove.h. You should be able to
> set a line-based breakpoint and single-step through it.
Unfortunately gdb doesn't want to put a breakpoint in the ifunc header.
I tried a -g3 -O2 build and excellent tips at [1] - Ubuntu 22.04 - gdb 12.1
| gdb elf/ld-linux-x86-64.so.2
| set exec-wrapper env LD_LIBRARY_PATH=....
| b _start
| ..
| b sysdeps/x86_64/multiarch/ifunc-memmove.h:48
| No source file named sysdeps/x86_64/multiarch/ifunc-memmove.h
| b ../sysdeps/x86_64/multiarch/ifunc-memmove.h:48
It does seem to recognize other headers in x86_64.
Do I need a newer gdb ? Also can't build glibc with -O0 anyways.
Thx,
-Vineet
[1] https://sourceware.org/glibc/wiki/Debugging/Development_Debugging
^ permalink raw reply [flat|nested] 3+ messages in thread
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2022-12-08 3:47 glibc tunables to disable axv not updating memcpy Vineet Gupta
2022-12-08 8:31 ` Florian Weimer
2023-01-05 0:04 ` glibc ifunc debug (was Re: glibc tunables to disable axv not updating memcpy) Vineet Gupta
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