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

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Date:   Mon, 14 Dec 2020 14:15:04 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     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

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.

Fixes: 01b0f0eae081 ("powerpc/64s: Trim offlined CPUs from mm_cpumasks")
Reported-by: Geert Uytterhoeven <geert@...ux-m68k.org>
Signed-off-by: Nicholas Piggin <npiggin@...il.com>
---
 arch/powerpc/platforms/powermac/smp.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/powerpc/platforms/powermac/smp.c b/arch/powerpc/platforms/powermac/smp.c
index adae2a6712e1..66ef5f8f4445 100644
--- a/arch/powerpc/platforms/powermac/smp.c
+++ b/arch/powerpc/platforms/powermac/smp.c
@@ -911,7 +911,16 @@ static int smp_core99_cpu_disable(void)
 
 	mpic_cpu_set_priority(0xf);
 
+	/*
+	 * Would be nice for consistency if all platforms clear mm_cpumask and
+	 * flush TLBs on unplug, but the TLB invalidation bug described in
+	 * commit 01b0f0eae081 ("powerpc/64s: Trim offlined CPUs from
+	 * mm_cpumasks") only applies to 64s and for now we only have the TLB
+	 * flush code for that platform.
+	 */
+#ifdef CONFIG_PPC64
 	cleanup_cpu_mmu_context();
+#endif
 
 	return 0;
 }
-- 
2.23.0