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SubjectRe: [PATCH 5/8] lazy tlb: allow lazy tlb mm switching to be configurable
Excerpts from Andy Lutomirski's message of November 29, 2020 10:36 am:
> On Sat, Nov 28, 2020 at 8:02 AM Nicholas Piggin <npiggin@gmail.com> wrote:
>>
>> NOMMU systems could easily go without this and save a bit of code
>> and the refcount atomics, because their mm switch is a no-op. I
>> haven't flipped them over because haven't audited all arch code to
>> convert over to using the _lazy_tlb refcounting.
>>
>> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
>> ---
>> arch/Kconfig | 11 +++++++
>> include/linux/sched/mm.h | 13 ++++++--
>> kernel/sched/core.c | 68 +++++++++++++++++++++++++++++-----------
>> kernel/sched/sched.h | 4 ++-
>> 4 files changed, 75 insertions(+), 21 deletions(-)
>>
>> diff --git a/arch/Kconfig b/arch/Kconfig
>> index 56b6ccc0e32d..596bf589d74b 100644
>> --- a/arch/Kconfig
>> +++ b/arch/Kconfig
>> @@ -430,6 +430,17 @@ config ARCH_WANT_IRQS_OFF_ACTIVATE_MM
>> irqs disabled over activate_mm. Architectures that do IPI based TLB
>> shootdowns should enable this.
>>
>> +# Should make this depend on MMU, because there is little use for lazy mm switching
>> +# with NOMMU. Must audit NOMMU architecture code for lazy mm refcounting first.
>> +config MMU_LAZY_TLB
>> + def_bool y
>> + help
>> + Enable "lazy TLB" mmu context switching for kernel threads.
>> +
>> +config MMU_LAZY_TLB_REFCOUNT
>> + def_bool y
>> + depends on MMU_LAZY_TLB
>> +
>
> This could use some documentation as to what "no" means.

Sure I can add a bit more.

>
>> config ARCH_HAVE_NMI_SAFE_CMPXCHG
>> bool
>>
>> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
>> index 7157c0f6fef8..bd0f27402d4b 100644
>> --- a/include/linux/sched/mm.h
>> +++ b/include/linux/sched/mm.h
>> @@ -51,12 +51,21 @@ static inline void mmdrop(struct mm_struct *mm)
>> /* Helpers for lazy TLB mm refcounting */
>> static inline void mmgrab_lazy_tlb(struct mm_struct *mm)
>> {
>> - mmgrab(mm);
>> + if (IS_ENABLED(CONFIG_MMU_LAZY_TLB_REFCOUNT))
>> + mmgrab(mm);
>> }
>>
>> static inline void mmdrop_lazy_tlb(struct mm_struct *mm)
>> {
>> - mmdrop(mm);
>> + if (IS_ENABLED(CONFIG_MMU_LAZY_TLB_REFCOUNT)) {
>> + mmdrop(mm);
>> + } else {
>> + /*
>> + * mmdrop_lazy_tlb must provide a full memory barrier, see the
>> + * membarrier comment finish_task_switch.
>
> "membarrier comment in finish_task_switch()", perhaps?

Sure.

Thanks,
Nick

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