linux-kernel - Re: [PATCH 5/8] lazy tlb: allow lazy tlb mm switching to be configurable

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Date:   Wed, 02 Dec 2020 12:49:51 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Anton Blanchard <anton@...abs.org>, Arnd Bergmann <arnd@...db.de>,
        linux-arch <linux-arch@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Peter Zijlstra <peterz@...radead.org>, X86 ML <x86@...nel.org>
Subject: Re: [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@...il.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@...il.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