Messages in this thread | | | Date | Mon, 19 Dec 2011 17:36:13 -0800 | From | Frank Rowand <> | Subject | Re: [RFC PATCH v2 6/6] ARM: Remove __ARCH_WANT_INTERRUPTS_ON_CTXSW on pre-ARMv6 CPUs |
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On 12/19/11 06:57, Catalin Marinas wrote: > This patch removes the __ARCH_WANT_INTERRUPTS_ON_CTXSW definition for > ARMv5 and earlier processors. On such processors, the context switch > requires a full cache flush. To avoid high interrupt latencies, this > patch defers the mm switching to the post-lock switch hook if the > interrupts are disabled. > > Signed-off-by: Catalin Marinas <catalin.marinas@arm.com> > Cc: Russell King <linux@arm.linux.org.uk> > Cc: Frank Rowand <frank.rowand@am.sony.com> > --- > arch/arm/include/asm/mmu_context.h | 30 +++++++++++++++++++++++++----- > arch/arm/include/asm/system.h | 9 --------- > 2 files changed, 25 insertions(+), 14 deletions(-) > > diff --git a/arch/arm/include/asm/mmu_context.h b/arch/arm/include/asm/mmu_context.h > index fd6eeba..4ac7809 100644 > --- a/arch/arm/include/asm/mmu_context.h > +++ b/arch/arm/include/asm/mmu_context.h > @@ -104,19 +104,39 @@ static inline void finish_arch_post_lock_switch(void) > > #else /* !CONFIG_CPU_HAS_ASID */ > > +#ifdef CONFIG_MMU > + > static inline void check_and_switch_context(struct mm_struct *mm, > struct task_struct *tsk) > { > -#ifdef CONFIG_MMU > if (unlikely(mm->context.kvm_seq != init_mm.context.kvm_seq)) > __check_kvm_seq(mm); > - cpu_switch_mm(mm->pgd, mm); > -#endif > + > + if (irqs_disabled()) > + /* > + * Defer the cpu_switch_mm() call and continue running with > + * the old mm. Since we only support UP systems on non-ASID > + * CPUs, the old mm will remain valid until the > + * finish_arch_post_lock_switch() call.
It would be good to include in this comment the info from the patch header that deferring the cpu_switch_mm() is to avoid high interrupt latencies.
I had applied all six patches so I could see what the end result looked like, and reading the end result was asking myself why cpu_switch_mm() was deferred for !CONFIG_CPU_HAS_ASID (since I was instead focusing on the problem of calling __new_context() with IRQs disabled). Then when I looked at this patch in isolation, the patch header clearly answered the question for me.
> + */ > + set_ti_thread_flag(task_thread_info(tsk), TIF_SWITCH_MM); > + else > + cpu_switch_mm(mm->pgd, mm); > } > > -#define init_new_context(tsk,mm) 0 > +#define finish_arch_post_lock_switch \ > + finish_arch_post_lock_switch > +static inline void finish_arch_post_lock_switch(void) > +{ > + if (test_and_clear_thread_flag(TIF_SWITCH_MM)) { > + struct mm_struct *mm = current->mm; > + cpu_switch_mm(mm->pgd, mm); > + } > +} > > -#define finish_arch_post_lock_switch() do { } while (0) > +#endif /* CONFIG_MMU */ > + > +#define init_new_context(tsk,mm) 0 > > #endif /* CONFIG_CPU_HAS_ASID */ > > diff --git a/arch/arm/include/asm/system.h b/arch/arm/include/asm/system.h > index 3daebde..ac7fade 100644 > --- a/arch/arm/include/asm/system.h > +++ b/arch/arm/include/asm/system.h > @@ -218,15 +218,6 @@ static inline void set_copro_access(unsigned int val) > } > > /* > - * switch_mm() may do a full cache flush over the context switch, > - * so enable interrupts over the context switch to avoid high > - * latency. > - */ > -#ifndef CONFIG_CPU_HAS_ASID > -#define __ARCH_WANT_INTERRUPTS_ON_CTXSW > -#endif > - > -/* > * switch_to(prev, next) should switch from task `prev' to `next' > * `prev' will never be the same as `next'. schedule() itself > * contains the memory barrier to tell GCC not to cache `current'. > > > . >
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