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

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Date:   Mon, 14 Dec 2020 21:43:21 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Nicholas Piggin <npiggin@...il.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>
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

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:

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