syscall exit path optimization

Chen, Kenneth W
2005-01-26T21:02:26Z

Follow up on previous discussion, this patch optimize how we handle
r8/r10 in syscall return path. If there are no pending work to be
done, we will skip storing/loading r8/r10, cutting out 4 memory
references in the fast path. This resulted a net of 4 cycles saving.

Signed-off-by: Ken Chen <kenneth.w.chen@intel.com>
Signed-off-by: Rohit Seth <rohit.seth@intel.com>

--- linux-ia64-release/arch/ia64/kernel/entry.S.orig 2005-01-26 11:41:24.000000000 -0800
+++ linux-ia64-release/arch/ia64/kernel/entry.S 2005-01-26 12:31:52.000000000 -0800
@@ -558,7 +558,8 @@ GLOBAL_ENTRY(ia64_trace_syscall)
.mem.offset 0,0; st8.spill [r2]=r8 // store return value in slot for r8
.mem.offset 8,0; st8.spill [r3]=r10 // clear error indication in slot for r10
br.call.sptk.many rp=syscall_trace_leave // give parent a chance to catch return value
-.ret3: br.cond.sptk ia64_leave_syscall
+ cmp.eq p9,p8=r0,r0
+ br.cond.sptk ia64_leave_syscall

strace_error:
ld8 r3=[r2] // load pt_regs.r8
@@ -619,12 +620,10 @@ END(ia64_ret_from_clone)
// fall through
GLOBAL_ENTRY(ia64_ret_from_syscall)
PT_REGS_UNWIND_INFO(0)
+ cmp.eq p8,p9=r0,r0 // p8: ret val in live reg, p9: ret val in pt_regs
cmp.ge p6,p7=r8,r0 // syscall executed successfully?
adds r2=PT(R8)+16,sp // r2 = &pt_regs.r8
- adds r3=PT(R10)+16,sp // r3 = &pt_regs.r10
- ;;
-(p6) st8 [r2]=r8 // store return value in slot for r8
-(p6) st8 [r3]=r0 // clear error indication in slot for r10
+ mov r10=r0 // clear error indication in r10
(p7) br.cond.spnt handle_syscall_error // handle potential syscall failure
END(ia64_ret_from_syscall)
// fall through
@@ -715,10 +714,10 @@ ENTRY(ia64_leave_syscall)
;;
mov r16=ar.bsp // M2 get existing backing store pointer
(p6) cmp4.ne.unc p6,p0=r15, r0 // any special work pending?
-(p6) br.cond.spnt .work_pending
+(p6) br.cond.spnt .work_pending_syscall
;;
// start restoring the state saved on the kernel stack (struct pt_regs):
- ld8 r8=[r2],16
+(p9) ld8 r8=[r2],16
ld8 r9=[r3],16
mov f6ð // clear f6
;;
@@ -726,9 +725,10 @@ ENTRY(ia64_leave_syscall)
rsm psr.i | psr.ic // M2 initiate turning off of interrupt and interruption collection
mov f9ð // clear f9

- ld8 r10=[r2],16
+ .pred.rel.mutex p8,p9
+(p9) ld8 r10=[r2],16
ld8 r11=[r3],16
- mov f7ð // clear f7
+(p8) add r22,r2
;;
ld8 r29=[r2],16 // load cr.ipsr
ld8 r28=[r3],16 // load cr.iip
@@ -760,7 +760,7 @@ ENTRY(ia64_leave_syscall)
;;
srlz.d // M0 ensure interruption collection is off
ld8.fill r13=[r3],16
- nop.i 0
+ mov f7ð // clear f7
;;
ld8.fill r12=[r2] // restore r12 (sp)
ld8.fill r15=[r3] // restore r15
@@ -770,8 +770,8 @@ ENTRY(ia64_leave_syscall)
(pUStk) st1 [r14]=r17
mov b6=r18 // I0 restore b6
;;
- shr.u r18=r19,16 // I0|1 get byte size of existing "dirty" partition
mov r14=r0 // clear r14
+ shr.u r18=r19,16 // I0|1 get byte size of existing "dirty" partition
(pKStk) br.cond.dpnt.many skip_rbs_switch

mov.m ar.ccv=r0 // clear ar.ccv
@@ -1083,6 +1083,10 @@ skip_rbs_switch:
* On exit:
* p6 = TRUE if work-pending-check needs to be redone
*/
+.work_pending_syscall:
+(p8) st8 [r2]=r8,16
+ ;;
+(p8) st8 [r2]=r10,16
.work_pending:
tbit.nz p6,p0=r31,TIF_SIGDELAYED // signal delayed from MCA/INIT/NMI/PMI context?
(p6) br.cond.sptk.few .sigdelayed
@@ -1104,12 +1108,14 @@ skip_rbs_switch:
;;
(pKStk) st4 [r20]=r0 // preempt_count() <- 0
#endif
+ cmp.eq p9,p8=r0,r0
(pLvSys)br.cond.sptk.many .work_processed_syscall // re-check
br.cond.sptk.many .work_processed_kernel // re-check

.notify:
(pUStk) br.call.spnt.many rp=notify_resume_user
.ret10: cmp.ne p6,p0=r0,r0 // p6 <- 0
+ cmp.eq p9,p8=r0,r0
(pLvSys)br.cond.sptk.many .work_processed_syscall // don't re-check
br.cond.sptk.many .work_processed_kernel // don't re-check

@@ -1121,6 +1127,7 @@ skip_rbs_switch:
.sigdelayed:
br.call.sptk.many rp=do_sigdelayed
cmp.eq p6,p0=r0,r0 // p6 <- 1, always re-check
+ cmp.eq p9,p8=r0,r0
(pLvSys)br.cond.sptk.many .work_processed_syscall // re-check
br.cond.sptk.many .work_processed_kernel // re-check

@@ -1135,17 +1142,11 @@ ENTRY(handle_syscall_error)
*/
PT_REGS_UNWIND_INFO(0)
ld8 r3=[r2] // load pt_regs.r8
- sub r9=0,r8 // negate return value to get errno
;;
- mov r10=-1 // return -1 in pt_regs.r10 to indicate error
cmp.eq p6,p7=r3,r0 // is pt_regs.r8=0?
- adds r3\x16,r2 // r3=&pt_regs.r10
- ;;
-(p6) mov r9=r8
-(p6) mov r10=0
;;
- st8 [r2]=r9 // store errno in pt_regs.r8
- st8 [r3]=r10 // store error indication in pt_regs.r10
+(p7) mov r10=-1
+(p7) sub r8=0,r8 // negate return value to get errno
br.cond.sptk ia64_leave_syscall
END(handle_syscall_error)