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Commit bc4f65e4 authored by Nicholas Piggin's avatar Nicholas Piggin Committed by Michael Ellerman
Browse files

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: default avatarNicholas Piggin <npiggin@gmail.com>
Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
parent acd7d8ce

arch/powerpc/kernel/entry_64.S

+43 −19
Original line number Diff line number Diff line
@@ -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

#endif

2:	addi    r3,r1,STACK_FRAME_OVERHEAD

#ifdef CONFIG_PPC_BOOK3S

	li	r10,MSR_RI

	mtmsrd	r10,1		/* Restore RI */

	oris	r7,r7,MSR_VEC@h

#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

arch/powerpc/kernel/process.c

+4 −0
Original line number Diff line number Diff line
@@ -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;