* Naveen N. Rao <naveen.n....@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 <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 | 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 > --
