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SubjectRe: [RFC PATCH v2 07/10] lib: vdso: don't use READ_ONCE() in __c_kernel_time()
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Le 11/01/2020 à 12:07, Thomas Gleixner a écrit :
> Christophe Leroy <christophe.leroy@c-s.fr> writes:
>>
>> With READ_ONCE() the 64 bits are being read:
>>
>> Without the READ_ONCE() only 32 bits are read. That's the most optimal.
>>
>> Without READ_ONCE() but with a barrier() after the read, we should get
>> the same result but GCC (GCC 8.1) does less good:
>>
>> Assuming both part of the 64 bits data will fall into a single
>> cacheline, the second read is in the noise.
>
> They definitely are in the same cacheline.
>
>> So agreed to drop this change.
>
> We could be smart about this and force the compiler to issue a 32bit
> read for 32bit builds. See below. Not sure whether it's worth it, but
> OTOH it will take quite a while until the 32bit time interfaces die
> completely.

I don't think it is worth something so big to just save 1 or 2 cycles in
time() function. Lets keep it as it is.

Thanks,
Christophe

>
> Thanks,
>
> tglx
>
> 8<------------
> --- a/include/vdso/datapage.h
> +++ b/include/vdso/datapage.h
> @@ -21,6 +21,18 @@
> #define CS_RAW 1
> #define CS_BASES (CS_RAW + 1)
>
> +#ifdef __LITTLE_ENDIAN
> +struct sec_hl {
> + u32 sec_l;
> + u32 sec_h;
> +};
> +#else
> +struct sec_hl {
> + u32 sec_h;
> + u32 sec_l;
> +};
> +#endif
> +
> /**
> * struct vdso_timestamp - basetime per clock_id
> * @sec: seconds
> @@ -35,7 +47,10 @@
> * vdso_data.cs[x].shift.
> */
> struct vdso_timestamp {
> - u64 sec;
> + union {
> + u64 sec;
> + struct sec_hl sec_hl;
> + };
> u64 nsec;
> };
>
> --- a/lib/vdso/gettimeofday.c
> +++ b/lib/vdso/gettimeofday.c
> @@ -165,8 +165,13 @@ static __maybe_unused int
> static __maybe_unused __kernel_old_time_t __cvdso_time(__kernel_old_time_t *time)
> {
> const struct vdso_data *vd = __arch_get_vdso_data();
> - __kernel_old_time_t t = READ_ONCE(vd[CS_HRES_COARSE].basetime[CLOCK_REALTIME].sec);
> + __kernel_old_time_t t;
>
> +#if BITS_PER_LONG == 32
> + t = READ_ONCE(vd[CS_HRES_COARSE].basetime[CLOCK_REALTIME].sec_hl.sec_l);
> +#else
> + t = READ_ONCE(vd[CS_HRES_COARSE].basetime[CLOCK_REALTIME].sec);
> +#endif
> if (time)
> *time = t;
>
>

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