powerpc: Create context switch helpers save_sprs() and restore_sprs()

Move all our context switch SPR save and restore code into two
helpers. We do a few optimisations:

- Group all mfsprs and all mtsprs. In many cases an mtspr sets a
scoreboarding bit that an mfspr waits on, so the current practise of
mfspr A; mtspr A; mfpsr B; mtspr B is the worst scheduling we can
do.

- SPR writes are slow, so check that the value is changing before
writing it.

A context switch microbenchmark using yield():

http://ozlabs.org/~anton/junkcode/context_switch2.c

./context_switch2 --test=yield 0 0

shows an improvement of almost 10% on POWER8.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
This commit is contained in:
Anton Blanchard 2015-10-29 11:43:55 +11:00 committed by Michael Ellerman
parent af72ab646a
commit 152d523e63
4 changed files with 82 additions and 82 deletions

View File

@ -294,6 +294,7 @@ struct thread_struct {
#endif
#ifdef CONFIG_PPC64
unsigned long dscr;
unsigned long fscr;
/*
* This member element dscr_inherit indicates that the process
* has explicitly attempted and changed the DSCR register value

View File

@ -15,17 +15,6 @@ extern struct task_struct *__switch_to(struct task_struct *,
struct thread_struct;
extern struct task_struct *_switch(struct thread_struct *prev,
struct thread_struct *next);
#ifdef CONFIG_PPC_BOOK3S_64
static inline void save_early_sprs(struct thread_struct *prev)
{
if (cpu_has_feature(CPU_FTR_ARCH_207S))
prev->tar = mfspr(SPRN_TAR);
if (cpu_has_feature(CPU_FTR_DSCR))
prev->dscr = mfspr(SPRN_DSCR);
}
#else
static inline void save_early_sprs(struct thread_struct *prev) {}
#endif
extern void enable_kernel_fp(void);
extern void enable_kernel_altivec(void);

View File

@ -453,29 +453,12 @@ _GLOBAL(_switch)
SAVE_8GPRS(14, r1)
SAVE_10GPRS(22, r1)
mflr r20 /* Return to switch caller */
#ifdef CONFIG_ALTIVEC
BEGIN_FTR_SECTION
mfspr r24,SPRN_VRSAVE /* save vrsave register value */
std r24,THREAD_VRSAVE(r3)
END_FTR_SECTION_IFSET(CPU_FTR_ALTIVEC)
#endif /* CONFIG_ALTIVEC */
std r20,_NIP(r1)
mfcr r23
std r23,_CCR(r1)
std r1,KSP(r3) /* Set old stack pointer */
#ifdef CONFIG_PPC_BOOK3S_64
BEGIN_FTR_SECTION
/* Event based branch registers */
mfspr r0, SPRN_BESCR
std r0, THREAD_BESCR(r3)
mfspr r0, SPRN_EBBHR
std r0, THREAD_EBBHR(r3)
mfspr r0, SPRN_EBBRR
std r0, THREAD_EBBRR(r3)
END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S)
#endif
#ifdef CONFIG_SMP
/* We need a sync somewhere here to make sure that if the
* previous task gets rescheduled on another CPU, it sees all
@ -563,47 +546,6 @@ END_MMU_FTR_SECTION_IFSET(MMU_FTR_1T_SEGMENT)
mr r1,r8 /* start using new stack pointer */
std r7,PACAKSAVE(r13)
#ifdef CONFIG_PPC_BOOK3S_64
BEGIN_FTR_SECTION
/* Event based branch registers */
ld r0, THREAD_BESCR(r4)
mtspr SPRN_BESCR, r0
ld r0, THREAD_EBBHR(r4)
mtspr SPRN_EBBHR, r0
ld r0, THREAD_EBBRR(r4)
mtspr SPRN_EBBRR, r0
ld r0,THREAD_TAR(r4)
mtspr SPRN_TAR,r0
END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S)
#endif
#ifdef CONFIG_ALTIVEC
BEGIN_FTR_SECTION
ld r0,THREAD_VRSAVE(r4)
mtspr SPRN_VRSAVE,r0 /* if G4, restore VRSAVE reg */
END_FTR_SECTION_IFSET(CPU_FTR_ALTIVEC)
#endif /* CONFIG_ALTIVEC */
#ifdef CONFIG_PPC64
BEGIN_FTR_SECTION
lwz r6,THREAD_DSCR_INHERIT(r4)
ld r0,THREAD_DSCR(r4)
cmpwi r6,0
bne 1f
ld r0,PACA_DSCR_DEFAULT(r13)
1:
BEGIN_FTR_SECTION_NESTED(70)
mfspr r8, SPRN_FSCR
rldimi r8, r6, FSCR_DSCR_LG, (63 - FSCR_DSCR_LG)
mtspr SPRN_FSCR, r8
END_FTR_SECTION_NESTED(CPU_FTR_ARCH_207S, CPU_FTR_ARCH_207S, 70)
cmpd r0,r25
beq 2f
mtspr SPRN_DSCR,r0
2:
END_FTR_SECTION_IFSET(CPU_FTR_DSCR)
#endif
ld r6,_CCR(r1)
mtcrf 0xFF,r6

View File

@ -742,6 +742,73 @@ void restore_tm_state(struct pt_regs *regs)
#define __switch_to_tm(prev)
#endif /* CONFIG_PPC_TRANSACTIONAL_MEM */
static inline void save_sprs(struct thread_struct *t)
{
#ifdef CONFIG_ALTIVEC
if (cpu_has_feature(cpu_has_feature(CPU_FTR_ALTIVEC)))
t->vrsave = mfspr(SPRN_VRSAVE);
#endif
#ifdef CONFIG_PPC_BOOK3S_64
if (cpu_has_feature(CPU_FTR_DSCR))
t->dscr = mfspr(SPRN_DSCR);
if (cpu_has_feature(CPU_FTR_ARCH_207S)) {
t->bescr = mfspr(SPRN_BESCR);
t->ebbhr = mfspr(SPRN_EBBHR);
t->ebbrr = mfspr(SPRN_EBBRR);
t->fscr = mfspr(SPRN_FSCR);
/*
* Note that the TAR is not available for use in the kernel.
* (To provide this, the TAR should be backed up/restored on
* exception entry/exit instead, and be in pt_regs. FIXME,
* this should be in pt_regs anyway (for debug).)
*/
t->tar = mfspr(SPRN_TAR);
}
#endif
}
static inline void restore_sprs(struct thread_struct *old_thread,
struct thread_struct *new_thread)
{
#ifdef CONFIG_ALTIVEC
if (cpu_has_feature(CPU_FTR_ALTIVEC) &&
old_thread->vrsave != new_thread->vrsave)
mtspr(SPRN_VRSAVE, new_thread->vrsave);
#endif
#ifdef CONFIG_PPC_BOOK3S_64
if (cpu_has_feature(CPU_FTR_DSCR)) {
u64 dscr = get_paca()->dscr_default;
u64 fscr = old_thread->fscr & ~FSCR_DSCR;
if (new_thread->dscr_inherit) {
dscr = new_thread->dscr;
fscr |= FSCR_DSCR;
}
if (old_thread->dscr != dscr)
mtspr(SPRN_DSCR, dscr);
if (old_thread->fscr != fscr)
mtspr(SPRN_FSCR, fscr);
}
if (cpu_has_feature(CPU_FTR_ARCH_207S)) {
if (old_thread->bescr != new_thread->bescr)
mtspr(SPRN_BESCR, new_thread->bescr);
if (old_thread->ebbhr != new_thread->ebbhr)
mtspr(SPRN_EBBHR, new_thread->ebbhr);
if (old_thread->ebbrr != new_thread->ebbrr)
mtspr(SPRN_EBBRR, new_thread->ebbrr);
if (old_thread->tar != new_thread->tar)
mtspr(SPRN_TAR, new_thread->tar);
}
#endif
}
struct task_struct *__switch_to(struct task_struct *prev,
struct task_struct *new)
{
@ -751,17 +818,16 @@ struct task_struct *__switch_to(struct task_struct *prev,
struct ppc64_tlb_batch *batch;
#endif
new_thread = &new->thread;
old_thread = &current->thread;
WARN_ON(!irqs_disabled());
/* Back up the TAR and DSCR across context switches.
* Note that the TAR is not available for use in the kernel. (To
* provide this, the TAR should be backed up/restored on exception
* entry/exit instead, and be in pt_regs. FIXME, this should be in
* pt_regs anyway (for debug).)
* Save the TAR and DSCR here before we do treclaim/trecheckpoint as
* these will change them.
/*
* We need to save SPRs before treclaim/trecheckpoint as these will
* change a number of them.
*/
save_early_sprs(&prev->thread);
save_sprs(&prev->thread);
__switch_to_tm(prev);
@ -844,10 +910,6 @@ struct task_struct *__switch_to(struct task_struct *prev,
#endif /* CONFIG_HAVE_HW_BREAKPOINT */
#endif
new_thread = &new->thread;
old_thread = &current->thread;
#ifdef CONFIG_PPC64
/*
* Collect processor utilization data per process
@ -883,6 +945,10 @@ struct task_struct *__switch_to(struct task_struct *prev,
last = _switch(old_thread, new_thread);
/* Need to recalculate these after calling _switch() */
old_thread = &last->thread;
new_thread = &current->thread;
#ifdef CONFIG_PPC_BOOK3S_64
if (current_thread_info()->local_flags & _TLF_LAZY_MMU) {
current_thread_info()->local_flags &= ~_TLF_LAZY_MMU;
@ -891,6 +957,8 @@ struct task_struct *__switch_to(struct task_struct *prev,
}
#endif /* CONFIG_PPC_BOOK3S_64 */
restore_sprs(old_thread, new_thread);
return last;
}