Re: [PATCH v4] powerpc/vdso64: Add support for CLOCK_{REALTIME/MONOTONIC}_COARSE

[Santosh Sivaraj <santosh@fossix.org>]

* Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com> wrote (on 2017-10-09 10:39:18 +0000):

> 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@kernel.crashing.org>
> > Signed-off-by: Santosh Sivaraj <santosh@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@h	/* want nanoseconds */
> >  	ori	r7,r7,NSEC_PER_SEC@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...

Yeah, I too missed it.
> 
> > +	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.
>
In this case we will be adding more checks and branches in order to reuse
the code. If we want to keep the code common we will have to do a lot of
jumping around, code will contain a bunch of branches, which I feel will make
the code/flow hard to understand. (Q: Does lot of branches have bad effect on
branch prediction?)

Will wait for your thoughts, before respinning.

Thanks,
Santosh
> 
> - Naveen
> 

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