linux-kernel - Re: [PATCH 2/2] powerpc/64s: Trim offlined CPUs from mm

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Date:   Mon, 14 Dec 2020 21:04:08 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Anton Vorontsov <anton.vorontsov@...aro.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 2/2] powerpc/64s: Trim offlined CPUs from mm_cpumasks

Excerpts from Michael Ellerman's message of December 14, 2020 8:43 pm:
> Nicholas Piggin <npiggin@...il.com> writes:
>> Excerpts from Geert Uytterhoeven's message of December 10, 2020 7:06 pm:
>>> Hi Nicholas,
>>> 
>>> On Fri, Nov 20, 2020 at 4:01 AM Nicholas Piggin <npiggin@...il.com> wrote:
>>>>
>>>> When offlining a CPU, powerpc/64s does not flush TLBs, rather it just
>>>> leaves the CPU set in mm_cpumasks, so it continues to receive TLBIEs
>>>> to manage its TLBs.
>>>>
>>>> However the exit_flush_lazy_tlbs() function expects that after
>>>> returning, all CPUs (except self) have flushed TLBs for that mm, in
>>>> which case TLBIEL can be used for this flush. This breaks for offline
>>>> CPUs because they don't get the IPI to flush their TLB. This can lead
>>>> to stale translations.
>>>>
>>>> Fix this by clearing the CPU from mm_cpumasks, then flushing all TLBs
>>>> before going offline.
>>>>
>>>> These offlined CPU bits stuck in the cpumask also prevents the cpumask
>>>> from being trimmed back to local mode, which means continual broadcast
>>>> IPIs or TLBIEs are needed for TLB flushing. This patch prevents that
>>>> situation too.
>>>>
>>>> Signed-off-by: Nicholas Piggin <npiggin@...il.com>
>>> 
>>> Thanks for your patch!
>>> 
>>>> --- a/arch/powerpc/platforms/powermac/smp.c
>>>> +++ b/arch/powerpc/platforms/powermac/smp.c
>>>> @@ -911,6 +911,8 @@ static int smp_core99_cpu_disable(void)
>>>>
>>>>         mpic_cpu_set_priority(0xf);
>>>>
>>>> +       cleanup_cpu_mmu_context();
>>>> +
>>> 
>>> I guess this change broke pmac32_defconfig+SMP in v5.10-rc7?
>>> 
>>> arch/powerpc/platforms/powermac/smp.c: error: implicit
>>> declaration of function 'cleanup_cpu_mmu_context'
>>> [-Werror=implicit-function-declaration]:  => 914:2
>>> 
>>> http://kisskb.ellerman.id.au/kisskb/buildresult/14423174/
>>
>> Hey, yeah it does thanks for catching it. This patch fixes it for me
>>
>> ---
>> From a9b5ec92ffac975e81c6d7db6ff2b1486b2723f7 Mon Sep 17 00:00:00 2001
>> From: Nicholas Piggin <npiggin@...il.com>
>> Date: Mon, 14 Dec 2020 13:52:39 +1000
>> Subject: [PATCH] powerpc/32s: Fix cleanup_cpu_mmu_context() compile bug
>>
>> 32s has no tlbiel_all() defined, so just disable the cleanup with a
>> comment.
> 
> Or what about just:

That works, I kind of wanted it in there explicit that we don't
clean up on 32s. I don't mind if you prefer this though.

Thanks,
Nick

> 
> diff --git a/arch/powerpc/include/asm/book3s/32/mmu-hash.h b/arch/powerpc/include/asm/book3s/32/mmu-hash.h
> index 331187661236..685c589e723f 100644
> --- a/arch/powerpc/include/asm/book3s/32/mmu-hash.h
> +++ b/arch/powerpc/include/asm/book3s/32/mmu-hash.h
> @@ -94,6 +94,7 @@ typedef struct {
>  } mm_context_t;
> 
>  void update_bats(void);
> +static inline void cleanup_cpu_mmu_context(void) { };
> 
>  /* patch sites */
>  extern s32 patch__hash_page_A0, patch__hash_page_A1, patch__hash_page_A2;
> 
> 
> cheers
> 
>