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

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Date:   Wed, 20 Nov 2019 16:17:41 +0100
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Michael Ellerman <mpe@...erman.id.au>
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

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.

Christophe