linux-kernel - Re: [RFC PATCH] powerpc/tlb: enable arch want batched unmap tlb flush

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Message-ID: <87ed5bymlo.fsf@gmail.com>
Date: Mon, 23 Sep 2024 10:48:11 +0530
From: Ritesh Harjani (IBM) <ritesh.list@...il.com>
To: Luming Yu <luming.yu@...ngroup.cn>
Cc: linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org, mpe@...erman.id.au, npiggin@...il.com, christophe.leroy@...roup.eu, jialong.yang@...ngroup.cn, luming.yu@...il.com
Subject: Re: [RFC PATCH] powerpc/tlb: enable arch want batched unmap tlb flush

Luming Yu <luming.yu@...ngroup.cn> writes:

> On Sun, Sep 22, 2024 at 04:39:53PM +0530, Ritesh Harjani wrote:
>> Luming Yu <luming.yu@...ngroup.cn> writes:
>> 
>> > From: Yu Luming <luming.yu@...il.com>
>> >
>> > ppc always do its own tracking for batch tlb. By trivially enabling
>> > the ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH in ppc, ppc arch can re-use
>> > common code in rmap and reduce overhead and do optimization it could not
>> > have without a tlb flushing context at low architecture level.
>> 
>> I looked at this patch and other than the compile failure, this patch
>> still won't optimize anything. The idea of this config is that we want
>> to batch all the tlb flush operation at the end. By returning false from
>> should_defer_flush() (in this patch), we are saying we cannot defer
>> the flush and hence we do tlb flush in the same context of unmap.
> not exactly, as false return implies, we currently do nothing but relying on
> book3S_64's tlb batch implementation which contains a bit of defer optimization
> that we need to use a real benchmark to do some performance characterization.
>
> And I need to get my test bed ready for patch testing first. So I have to
> defer the real optimization in this area.
>> 
>> Anyway, I took a quick look at ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
>> and I have a quick PoC for the same. I will soon post it.
> thanks for picking up the barton for the future collaboration on the
> potential common performance benefits among us for powerpc arch.

Sure Thanks, Luming. 
I have posted this work here [1].

[1]: https://lore.kernel.org/linuxppc-dev/cover.1727001426.git.ritesh.list@gmail.com/
-ritesh