On 2017/09/18 09:23AM, Santosh Sivaraj wrote:
> Current vDSO64 implementation does not have support for coarse clocks
> (CLOCK_MONOTONIC_COARSE, CLOCK_REALTIME_COARSE), for which it falls back
> to system call, increasing the response time, vDSO implementation reduces
> the cycle time. Below is a benchmark of the difference in execution time
> with and without vDSO support.
>
> (Non-coarse clocks are also included just for completion)
>
> Without vDSO support:
> --------------------
> clock-gettime-realtime: syscall: 172 nsec/call
> clock-gettime-realtime: libc: 26 nsec/call
> clock-gettime-realtime: vdso: 21 nsec/call
> clock-gettime-monotonic: syscall: 170 nsec/call
> clock-gettime-monotonic: libc: 30 nsec/call
> clock-gettime-monotonic: vdso: 24 nsec/call
> clock-gettime-realtime-coarse: syscall: 153 nsec/call
> clock-gettime-realtime-coarse: libc: 15 nsec/call
> clock-gettime-realtime-coarse: vdso: 9 nsec/call
> clock-gettime-monotonic-coarse: syscall: 167 nsec/call
> clock-gettime-monotonic-coarse: libc: 15 nsec/call
> clock-gettime-monotonic-coarse: vdso: 11 nsec/call
>
> CC: Benjamin Herrenschmidt <b...@kernel.crashing.org>
> Signed-off-by: Santosh Sivaraj <sant...@fossix.org>
> ---
> arch/powerpc/kernel/asm-offsets.c | 2 ++
> arch/powerpc/kernel/vdso64/gettimeofday.S | 56
> +++++++++++++++++++++++++++++++
> 2 files changed, 58 insertions(+)
>
> diff --git a/arch/powerpc/kernel/asm-offsets.c
> b/arch/powerpc/kernel/asm-offsets.c
> index 8cfb20e38cfe..b55c68c54dc1 100644
> --- a/arch/powerpc/kernel/asm-offsets.c
> +++ b/arch/powerpc/kernel/asm-offsets.c
> @@ -396,6 +396,8 @@ int main(void)
> /* Other bits used by the vdso */
> DEFINE(CLOCK_REALTIME, CLOCK_REALTIME);
> DEFINE(CLOCK_MONOTONIC, CLOCK_MONOTONIC);
> + DEFINE(CLOCK_REALTIME_COARSE, CLOCK_REALTIME_COARSE);
> + DEFINE(CLOCK_MONOTONIC_COARSE, CLOCK_MONOTONIC_COARSE);
> DEFINE(NSEC_PER_SEC, NSEC_PER_SEC);
> DEFINE(CLOCK_REALTIME_RES, MONOTONIC_RES_NSEC);
>
> diff --git a/arch/powerpc/kernel/vdso64/gettimeofday.S
> b/arch/powerpc/kernel/vdso64/gettimeofday.S
> index a0b4943811db..bae197a81add 100644
> --- a/arch/powerpc/kernel/vdso64/gettimeofday.S
> +++ b/arch/powerpc/kernel/vdso64/gettimeofday.S
> @@ -71,6 +71,11 @@ V_FUNCTION_BEGIN(__kernel_clock_gettime)
> cror cr0*4+eq,cr0*4+eq,cr1*4+eq
> beq cr0,49f
>
> + cmpwi cr0,r3,CLOCK_REALTIME_COARSE
> + cmpwi cr1,r3,CLOCK_MONOTONIC_COARSE
> + cror cr0*4+eq,cr0*4+eq,cr1*4+eq
> + beq cr0,65f
> +
> b 99f /* Fallback to syscall */
> .cfi_register lr,r12
> 49: bl V_LOCAL_FUNC(__get_datapage) /* get data page */
> @@ -112,6 +117,57 @@ V_FUNCTION_BEGIN(__kernel_clock_gettime)
> 1: bge cr1,80f
> addi r4,r4,-1
> add r5,r5,r7
> + b 80f
> +
> + /*
> + * For coarse clocks we get data directly from the vdso data page, so
> + * we don't need to call __do_get_tspec, but we still need to do the
> + * counter trick.
> + */
> +65: bl V_LOCAL_FUNC(__get_datapage) /* get data page */
> +70: ld r8,CFG_TB_UPDATE_COUNT(r3)
> + andi. r0,r8,1 /* pending update ? loop */
> + bne- 70b
> + xor r0,r8,r8 /* create dependency */
> + add r3,r3,r0
> +
> + /*
> + * CLOCK_REALTIME_COARSE, below values are needed for MONOTONIC_COARSE
> + * too
> + */
> + ld r4,STAMP_XTIME+TSPC64_TV_SEC(r3)
> + ld r5,STAMP_XTIME+TSPC64_TV_NSEC(r3)
> + bne cr1,78f
> +
> + /* CLOCK_MONOTONIC_COARSE */
> + lwa r6,WTOM_CLOCK_SEC(r3)
> + lwa r9,WTOM_CLOCK_NSEC(r3)
> +
> + /* check if counter has updated */
> +78: or r0,r6,r9
> + xor r0,r0,r0
> + add r3,r3,r0
> + ld r0,CFG_TB_UPDATE_COUNT(r3)
> + cmpld cr0,r0,r8 /* check if updated */
> + bne- 70b
Don't you need a dependency on r4/r5 here for REALTIME_COARSE?
Something like:
/* check if counter has updated */
or r0,r6,r9
78: or r0,r4,r5
xor r0,r0,r0
> +
> + /* Counter has not updated, so continue calculating proper values for
> + * sec and nsec if monotonic coarse, or just return with the proper
> + * values for realtime.
> + */
> + bne cr1,80f
> +
I think the below hunk can surely be shared across the _COARSE and
regular clocks, if not more.
- Naveen
> + /* Add wall->monotonic offset and check for overflow or underflow */
> + add r4,r4,r6
> + add r5,r5,r9
> + cmpd cr0,r5,r7
> + cmpdi cr1,r5,0
> + blt 79f
> + subf r5,r7,r5
> + addi r4,r4,1
> +79: bge cr1,80f
> + addi r4,r4,-1
> + add r5,r5,r7
>
> 80: std r4,TSPC64_TV_SEC(r11)
> std r5,TSPC64_TV_NSEC(r11)
> --
> 2.13.5
>
