linux-kernel - Re: [PATCH v3 2/8] powerpc/vdso32: Add support for CLOCK_{REALTIME/MONOTONIC}

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Date:   Thu, 21 Nov 2019 10:53:54 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Christophe Leroy <christophe.leroy@....fr>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        Paul Mackerras <paulus@...ba.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [PATCH v3 2/8] powerpc/vdso32: Add support for CLOCK_{REALTIME/MONOTONIC}_COARSE

Christophe Leroy <christophe.leroy@....fr> writes:
> Michael Ellerman <mpe@...erman.id.au> a écrit :
>> Christophe Leroy <christophe.leroy@....fr> writes:
>>> This is copied and adapted from commit 5c929885f1bb ("powerpc/vdso64:
>>> Add support for CLOCK_{REALTIME/MONOTONIC}_COARSE")
>>> from Santosh Sivaraj <santosh@...six.org>
>>>
>>> Benchmark from vdsotest-all:
>>> clock-gettime-realtime: syscall: 3601 nsec/call
>>> clock-gettime-realtime:    libc: 1072 nsec/call
>>> clock-gettime-realtime:    vdso: 931 nsec/call
>>> clock-gettime-monotonic: syscall: 4034 nsec/call
>>> clock-gettime-monotonic:    libc: 1213 nsec/call
>>> clock-gettime-monotonic:    vdso: 1076 nsec/call
>>> clock-gettime-realtime-coarse: syscall: 2722 nsec/call
>>> clock-gettime-realtime-coarse:    libc: 805 nsec/call
>>> clock-gettime-realtime-coarse:    vdso: 668 nsec/call
>>> clock-gettime-monotonic-coarse: syscall: 2949 nsec/call
>>> clock-gettime-monotonic-coarse:    libc: 882 nsec/call
>>> clock-gettime-monotonic-coarse:    vdso: 745 nsec/call
>>>
>>> Additional test passed with:
>>> 	vdsotest -d 30 clock-gettime-monotonic-coarse verify
>>
>> This broke on 64-bit big endian, which uses the 32-bit VDSO, with errors
>> like:
>>
>>   clock-gettime-monotonic-coarse/verify: 10 failures/inconsistencies  
>> encountered
>>   timestamp obtained from libc/vDSO not normalized:
>>   	[-1574202155, 1061008673]
>>   timestamp obtained from libc/vDSO predates timestamp
>>   previously obtained from kernel:
>>   	[74, 261310747] (kernel)
>>   	[-1574202155, 1061008673] (vDSO)
>>   timestamp obtained from libc/vDSO not normalized:
>>   	[-1574202155, 1061008673]
>>   timestamp obtained from libc/vDSO predates timestamp
>>   previously obtained from kernel:
>>   	[74, 261310747] (kernel)
>>   	[-1574202155, 1061008673] (vDSO)
>>   timestamp obtained from libc/vDSO not normalized:
>>   	[-1574202155, 1061008673]
>>   timestamp obtained from libc/vDSO predates timestamp
>>   previously obtained from kernel:
>>   	[74, 261310747] (kernel)
>>   	[-1574202155, 1061008673] (vDSO)
>>   timestamp obtained from libc/vDSO not normalized:
>>   	[-1574202155, 1061008673]
>>   timestamp obtained from libc/vDSO predates timestamp
>>   previously obtained from kernel:
>>   	[74, 261310747] (kernel)
>>   	[-1574202155, 1061008673] (vDSO)
>>   timestamp obtained from libc/vDSO not normalized:
>>   	[-1574202155, 1061008673]
>>   timestamp obtained from libc/vDSO predates timestamp
>>   previously obtained from kernel:
>>   	[74, 261310747] (kernel)
>>   	[-1574202155, 1061008673] (vDSO)
>>   Failure threshold (10) reached; stopping test.
>>
>>
>> The diff below seems to fix it, but I'm not sure it's correct. ie. we
>> just ignore the top part of the values, how does that work?
>
> Your change makes sense, it is consistent with other functions using  
> STAMP_XTIME.
>
> It works because nanoseconds are max 999999999, it fits 32 bits regs.

Yeah, but for seconds? I guess this is the whole Y2038 problem, though
I'm not clear how it's going to work for compat.

Anyway I'll squash that in and get this merged.

cheers