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powerpc/64: Avoid restore_math call if possible in syscall exit 

The syscall exit code that branches to restore_math is quite heavy on
Book3S, consisting of 2 mtmsr instructions. Threads that don't use both
FP and vector can get caught here if the kernel ever uses FP or vector.
Lazy-FP/vec context switching also trips this case.

So check for lazy FP and vector before switching RI for restore_math.
Move most of this case out of line.

For threads that do want to restore math registers, the MSR switches are
still suboptimal. Future direction may be to use a soft-RI bit to avoid
MSR switches in kernel (similar to soft-EE), but for now at least the
no-restore

POWER9 context switch rate increases by about 5% due to sched_yield(2)
return performance. I haven't constructed a test to measure the syscall
cost.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
This commit is contained in:
Nicholas Piggin 2017-06-09 01:35:05 +10:00 committed by Michael Ellerman
parent acd7d8cef0
commit bc4f65e4cf
2 changed files with 47 additions and 19 deletions
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60
arch/powerpc/kernel/entry_64.S
View File

@ -210,27 +210,17 @@ system_call: /* label this so stack traces look sane */
andi. r0,r9,(_TIF_SYSCALL_DOTRACE|_TIF_SINGLESTEP|_TIF_USER_WORK_MASK|_TIF_PERSYSCALL_MASK)
bne- syscall_exit_work
andi. r0,r8,MSR_FP
beq 2f
/* If MSR_FP and MSR_VEC are set in user msr, then no need to restore */
li r7,MSR_FP
#ifdef CONFIG_ALTIVEC
andis. r0,r8,MSR_VEC@h
bne 3f
oris r7,r7,MSR_VEC@h
#endif
2: addi r3,r1,STACK_FRAME_OVERHEAD
#ifdef CONFIG_PPC_BOOK3S
li r10,MSR_RI
mtmsrd r10,1 /* Restore RI */
#endif
bl restore_math
#ifdef CONFIG_PPC_BOOK3S
li r11,0
mtmsrd r11,1
#endif
ld r8,_MSR(r1)
ld r3,RESULT(r1)
li r11,-MAX_ERRNO
and r0,r8,r7
cmpd r0,r7
bne syscall_restore_math
.Lsyscall_restore_math_cont:
3: cmpld r3,r11
cmpld r3,r11
ld r5,_CCR(r1)
bge- syscall_error
.Lsyscall_error_cont:
@ -264,6 +254,40 @@ syscall_error:
std r5,_CCR(r1)
b .Lsyscall_error_cont
syscall_restore_math:
/*
* Some initial tests from restore_math to avoid the heavyweight
* C code entry and MSR manipulations.
*/
LOAD_REG_IMMEDIATE(r0, MSR_TS_MASK)
and. r0,r0,r8
bne 1f
ld r7,PACACURRENT(r13)
lbz r0,THREAD+THREAD_LOAD_FP(r7)
#ifdef CONFIG_ALTIVEC
lbz r6,THREAD+THREAD_LOAD_VEC(r7)
add r0,r0,r6
#endif
cmpdi r0,0
beq .Lsyscall_restore_math_cont
1: addi r3,r1,STACK_FRAME_OVERHEAD
#ifdef CONFIG_PPC_BOOK3S
li r10,MSR_RI
mtmsrd r10,1 /* Restore RI */
#endif
bl restore_math
#ifdef CONFIG_PPC_BOOK3S
li r11,0
mtmsrd r11,1
#endif
/* Restore volatiles, reload MSR from updated one */
ld r8,_MSR(r1)
ld r3,RESULT(r1)
li r11,-MAX_ERRNO
b .Lsyscall_restore_math_cont
/* Traced system call support */
syscall_dotrace:
bl save_nvgprs

4
arch/powerpc/kernel/process.c
View File

@ -511,6 +511,10 @@ void restore_math(struct pt_regs *regs)
{
unsigned long msr;
/*
* Syscall exit makes a similar initial check before branching
* to restore_math. Keep them in synch.
*/
if (!msr_tm_active(regs->msr) &&
!current->thread.load_fp && !loadvec(current->thread))
return;