[PATCH 2/3] powerpc: Create context switch helpers save_sprs() and restore_sprs()
Anton Blanchard
anton at samba.org
Sun Jul 26 02:57:44 AEST 2015
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 --type=yield 0 0
shows an improvement of almost 10% on POWER8.
Signed-off-by: Anton Blanchard <anton at samba.org>
---
arch/powerpc/include/asm/switch_to.h | 11 -----
arch/powerpc/kernel/entry_64.S | 60 +---------------------
arch/powerpc/kernel/process.c | 96 +++++++++++++++++++++++++++++++-----
3 files changed, 85 insertions(+), 82 deletions(-)
diff --git a/arch/powerpc/include/asm/switch_to.h b/arch/powerpc/include/asm/switch_to.h
index 58abeda..a65f78c 100644
--- a/arch/powerpc/include/asm/switch_to.h
+++ b/arch/powerpc/include/asm/switch_to.h
@@ -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);
diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index daf00f5..8428280 100644
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -441,29 +441,12 @@ _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
@@ -551,47 +534,6 @@ 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
diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index 8005e18..b9b690f 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -745,26 +745,96 @@ void restore_tm_state(struct pt_regs *regs)
#define __switch_to_tm(prev)
#endif /* CONFIG_PPC_TRANSACTIONAL_MEM */
+struct spr_save_t {
+ u64 fscr;
+};
+
+static inline void save_sprs(struct thread_struct *t, struct spr_save_t *save)
+{
+#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);
+
+ save->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,
+ struct spr_save_t *save)
+{
+#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;
+
+ if (new_thread->dscr_inherit) {
+ dscr = new_thread->dscr;
+
+ if (!(save->fscr & FSCR_DSCR))
+ mtspr(SPRN_FSCR, save->fscr | FSCR_DSCR);
+ }
+
+ if (old_thread->dscr != new_thread->dscr)
+ mtspr(SPRN_DSCR, new_thread->dscr);
+ }
+
+ 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)
{
struct thread_struct *new_thread, *old_thread;
struct task_struct *last;
+ struct spr_save_t spr_save = {0, };
#ifdef CONFIG_PPC_BOOK3S_64
struct ppc64_tlb_batch *batch;
#endif
+ new_thread = &new->thread;
+ old_thread = ¤t->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, &spr_save);
__switch_to_tm(prev);
@@ -847,10 +917,6 @@ struct task_struct *__switch_to(struct task_struct *prev,
#endif /* CONFIG_HAVE_HW_BREAKPOINT */
#endif
-
- new_thread = &new->thread;
- old_thread = ¤t->thread;
-
#ifdef CONFIG_PPC64
/*
* Collect processor utilization data per process
@@ -886,6 +952,10 @@ 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 = ¤t->thread;
+
#ifdef CONFIG_PPC_BOOK3S_64
if (current_thread_info()->local_flags & _TLF_LAZY_MMU) {
current_thread_info()->local_flags &= ~_TLF_LAZY_MMU;
@@ -894,6 +964,8 @@ struct task_struct *__switch_to(struct task_struct *prev,
}
#endif /* CONFIG_PPC_BOOK3S_64 */
+ restore_sprs(old_thread, new_thread, &spr_save);
+
return last;
}
--
2.1.4
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