On Thu, 2015-10-29 at 11:43 +1100, Anton Blanchard wrote: > Move all our context switch SPR save and restore code into two > helpers. We do a few optimisations:
To avoid confusion with other places where we might save and restore SPRs for things like power management etc... can you name these save_task_sprs and restore_task_sprs or something similar ? At least for me it makes things a bit clearer :) > - 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 <an...@samba.org> > --- > arch/powerpc/include/asm/processor.h | 1 + > arch/powerpc/include/asm/switch_to.h | 11 ----- > arch/powerpc/kernel/entry_64.S | 60 +---------------------- > arch/powerpc/kernel/process.c | 92 > +++++++++++++++++++++++++++++++----- > 4 files changed, 82 insertions(+), 82 deletions(-) > > diff --git a/arch/powerpc/include/asm/processor.h > b/arch/powerpc/include/asm/processor.h > index 5afea36..c273f3e 100644 > --- a/arch/powerpc/include/asm/processor.h > +++ b/arch/powerpc/include/asm/processor.h > @@ -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 > diff --git a/arch/powerpc/include/asm/switch_to.h > b/arch/powerpc/include/asm/switch_to.h > index 15cca17..33a071d 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 93bb284..e84e5bc 100644 > --- a/arch/powerpc/kernel/entry_64.S > +++ b/arch/powerpc/kernel/entry_64.S > @@ -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 > > diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c > index 75b6676..3aabed4 100644 > --- a/arch/powerpc/kernel/process.c > +++ b/arch/powerpc/kernel/process.c > @@ -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 = ¤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); > > __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 = ¤t->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 = ¤t->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; > } >
