[PATCH v4] powerpc/vdso64: Add support for CLOCK_{REALTIME/MONOTONIC}_COARSE
Naveen N. Rao
naveen.n.rao at linux.vnet.ibm.com
Mon Oct 9 21:39:18 AEDT 2017
On 2017/10/09 08:09AM, 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 times.
>
> (Non-coarse clocks are also included just for completion)
>
> clock-gettime-realtime: syscall: 172 nsec/call
> clock-gettime-realtime: libc: 28 nsec/call
> clock-gettime-realtime: vdso: 22 nsec/call
> clock-gettime-monotonic: syscall: 171 nsec/call
> clock-gettime-monotonic: libc: 30 nsec/call
> clock-gettime-monotonic: vdso: 25 nsec/call
> clock-gettime-realtime-coarse: syscall: 153 nsec/call
> clock-gettime-realtime-coarse: libc: 16 nsec/call
> clock-gettime-realtime-coarse: vdso: 10 nsec/call
> clock-gettime-monotonic-coarse: syscall: 167 nsec/call
> clock-gettime-monotonic-coarse: libc: 17 nsec/call
> clock-gettime-monotonic-coarse: vdso: 11 nsec/call
>
> CC: Benjamin Herrenschmidt <benh at kernel.crashing.org>
> Signed-off-by: Santosh Sivaraj <santosh at fossix.org>
> ---
> arch/powerpc/kernel/asm-offsets.c | 2 +
> arch/powerpc/kernel/vdso64/gettimeofday.S | 67 ++++++++++++++++++++++++++-----
> 2 files changed, 58 insertions(+), 11 deletions(-)
>
> 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 382021324883..729dded195ce 100644
> --- a/arch/powerpc/kernel/vdso64/gettimeofday.S
> +++ b/arch/powerpc/kernel/vdso64/gettimeofday.S
> @@ -64,6 +64,12 @@ V_FUNCTION_BEGIN(__kernel_clock_gettime)
> cmpwi cr0,r3,CLOCK_REALTIME
> cmpwi cr1,r3,CLOCK_MONOTONIC
> cror cr0*4+eq,cr0*4+eq,cr1*4+eq
> +
> + cmpwi cr5,r3,CLOCK_REALTIME_COARSE
> + cmpwi cr6,r3,CLOCK_MONOTONIC_COARSE
> + cror cr5*4+eq,cr5*4+eq,cr6*4+eq
> +
> + cror cr0*4+eq,cr0*4+eq,cr5*4+eq
> bne cr0,99f
>
> mflr r12 /* r12 saves lr */
> @@ -72,6 +78,7 @@ V_FUNCTION_BEGIN(__kernel_clock_gettime)
> bl V_LOCAL_FUNC(__get_datapage) /* get data page */
> lis r7,NSEC_PER_SEC at h /* want nanoseconds */
> ori r7,r7,NSEC_PER_SEC at l
> + beq cr5,70f
> 50: bl V_LOCAL_FUNC(__do_get_tspec) /* get time from tb & kernel */
> bne cr1,80f /* if not monotonic, all done */
>
> @@ -97,19 +104,57 @@ V_FUNCTION_BEGIN(__kernel_clock_gettime)
> ld r0,CFG_TB_UPDATE_COUNT(r3)
> cmpld cr0,r0,r8 /* check if updated */
> bne- 50b
> + b 78f
>
> - /* Add wall->monotonic offset and check for overflow or underflow.
> + /*
> + * 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.
> */
> - add r4,r4,r6
> - add r5,r5,r9
> - cmpd cr0,r5,r7
> - cmpdi cr1,r5,0
> - blt 1f
> - subf r5,r7,r5
> - addi r4,r4,1
> -1: bge cr1,80f
> - addi r4,r4,-1
> - add r5,r5,r7
> +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 cr6,75f
> +
> + /* CLOCK_MONOTONIC_COARSE */
> + lwa r6,WTOM_CLOCK_SEC(r3)
> + lwa r9,WTOM_CLOCK_NSEC(r3)
> +
> + /* check if counter has updated */
> +75: or r0,r6,r9
> + or r0,r4,r5
> + xor r0,r0,r0
The label '75:' should be on the second instruction since we don't need
to worry about r6/r9 for REALTIME_COARSE.
Also, the above hunk should actually be:
or r0,r6,r9
or r0,r0,r4
or r0,r0,r5
xor r0,r0,r0
Otherwise, the first 'or' will be skipped. I realized this after I
replied to your previous version, but missed letting you know...
> + add r3,r3,r0
> + ld r0,CFG_TB_UPDATE_COUNT(r3)
> + cmpld cr0,r0,r8 /* check if updated */
> + bne- 70b
I also notice that the code for dealing with CLOCK_MONOTONIC is similar
for _COARSE and regular clocks. If possible, we should reuse that as
well.
- Naveen
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