Re: [PATCH 2/2] powerpc/64s: Trim offlined CPUs from mm_cpumasks

Nicholas Piggin
2020-12-14T04:16:12Z

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@gmail.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@gmail.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@gmail.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@linux-m68k.org>
Signed-off-by: Nicholas Piggin <npiggin@gmail.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