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

[Christophe Leroy]

Michael Ellerman <mpe@xxxxxxxxxxxxxx> a ÃcritÂ:

> Christophe Leroy <christophe.leroy@xxxxxx> writes:
> > This is copied and adapted from commit 5c929885f1bb ("powerpc/vdso64:
> > Add support for CLOCK_{REALTIME/MONOTONIC}_COARSE")
> > from Santosh Sivaraj <santosh@xxxxxxxxxx>
> >
> > 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