linux-kernel - Re: [PATCH v4 01/12] powerpc/64s: Do not re-activate batched TLB flush

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Message-ID: <87cy5t4b0a.ritesh.list@gmail.com>
Date: Sat, 08 Nov 2025 06:05:17 +0530
From: Ritesh Harjani (IBM) <ritesh.list@...il.com>
To: Kevin Brodsky <kevin.brodsky@....com>, linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org, Alexander Gordeev <agordeev@...ux.ibm.com>, 
	Andreas Larsson <andreas@...sler.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Boris Ostrovsky <boris.ostrovsky@...cle.com>, Borislav Petkov <bp@...en8.de>, 
	Catalin Marinas <catalin.marinas@....com>, Christophe Leroy <christophe.leroy@...roup.eu>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, David Hildenbrand <david@...hat.com>, 
	"David S. Miller" <davem@...emloft.net>, David Woodhouse <dwmw2@...radead.org>, 
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>, Jann Horn <jannh@...gle.com>, 
	Juergen Gross <jgross@...e.com>, "Liam R. Howlett" <Liam.Howlett@...cle.com>, 
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Madhavan Srinivasan <maddy@...ux.ibm.com>, 
	Michael Ellerman <mpe@...erman.id.au>, Michal Hocko <mhocko@...e.com>, Mike Rapoport <rppt@...nel.org>, 
	Nicholas Piggin <npiggin@...il.com>, Peter Zijlstra <peterz@...radead.org>, 
	Ryan Roberts <ryan.roberts@....com>, Suren Baghdasaryan <surenb@...gle.com>, 
	Thomas Gleixner <tglx@...utronix.de>, Vlastimil Babka <vbabka@...e.cz>, Will Deacon <will@...nel.org>, 
	Yeoreum Yun <yeoreum.yun@....com>, linux-arm-kernel@...ts.infradead.org, 
	linuxppc-dev@...ts.ozlabs.org, sparclinux@...r.kernel.org, 
	xen-devel@...ts.xenproject.org, x86@...nel.org, 
	Venkat Rao Bagalkote <venkat88@...ux.ibm.com>
Subject: Re: [PATCH v4 01/12] powerpc/64s: Do not re-activate batched TLB flush

Kevin Brodsky <kevin.brodsky@....com> writes:

> On 05/11/2025 02:46, Ritesh Harjani (IBM) wrote:
>> Kevin Brodsky <kevin.brodsky@....com> writes:
>>
>>> From: Alexander Gordeev <agordeev@...ux.ibm.com>
>>>
>>> Since commit b9ef323ea168 ("powerpc/64s: Disable preemption in hash
>>> lazy mmu mode") a task can not be preempted while in lazy MMU mode.
>>> Therefore, the batch re-activation code is never called, so remove it.
>>>
>>> Signed-off-by: Alexander Gordeev <agordeev@...ux.ibm.com>
>>> Signed-off-by: Kevin Brodsky <kevin.brodsky@....com>
>>> ---
>>>  arch/powerpc/include/asm/thread_info.h |  2 --
>>>  arch/powerpc/kernel/process.c          | 25 -------------------------
>>>  2 files changed, 27 deletions(-)
>>>
>> Since the commit referenced in above disables the preemption in
>> arch_enter_lazy_mmu(), so the expectation is that we will never be
>> context switched while in lazy_mmu, hence the code changes in
>> switch_to() around __flush_tlb_pending() should ideally never be called.
>
> Correct, that's the idea.
>
>> With this analysis - the patch looks good to me. I will give this entire
>> patch series a try on Power HW with Hash mmu too (which uses lazy mmu and
>> let you know the results of that)!
>
> That'd be very appreciated, thanks a lot!
>

I did give this patch series a run on Power10 with Hash MMU. I ran the
following stress-ng tests and didn't observe any issues (kernel warnings) so far.

stress-ng --all 0 -t 60s --perf -v --verify \
--tlb-shootdown 0 \
--fault 0 \
--userfaultfd 0 \
--fork 0 \
--exec 0 \
--memfd 0 \
--numa 0 \
--pkey 0 \
--remap 0 \
--vm 0 \
--rmap 0 \
-x swap,pagemove
(Note not all options shown here will work with --verify)

Let me know what else I can run for validation?
Do you know of any specific tests for validation of lazy mmu feature?

>> For this patch please feel free to add:
>> Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@...il.com>
>>
>>
>> CC: Venkat who also runs CI on linux Power HW for upstream testing :)
>
> Ack, will Cc you both in the next version.

Sure. Thanks!

-ritesh