linux-kernel - Re: [v3 0/3] Reduce TLB flushes under some specific conditions

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Date:   Tue, 31 Oct 2023 07:55:48 +0900
From:   Byungchul Park <byungchul@...com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        kernel_team@...ynix.com, akpm@...ux-foundation.org,
        ying.huang@...el.com, namit@...are.com, xhao@...ux.alibaba.com,
        mgorman@...hsingularity.net, hughd@...gle.com, willy@...radead.org,
        david@...hat.com, peterz@...radead.org, luto@...nel.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com
Subject: Re: [v3 0/3] Reduce TLB flushes under some specific conditions

On Mon, Oct 30, 2023 at 10:55:07AM -0700, Dave Hansen wrote:
> On 10/30/23 00:25, Byungchul Park wrote:
> > I'm suggesting a mechanism to reduce TLB flushes by keeping source and
> > destination of folios participated in the migrations until all TLB
> > flushes required are done, only if those folios are not mapped with
> > write permission PTE entries at all. I worked Based on v6.6-rc5.
> 
> There's a lot of common overhead here, on top of the complexity in general:
> 
>  * A new page flag
>  * A new cpumask_t in task_struct
>  * A new zone list
>  * Extra (temporary) memory consumption
> 
> and the benefits are ... "performance improved a little bit" on one
> workload.  That doesn't seem like a good overall tradeoff to me.
> 
> There will certainly be workloads that, before this patch, would have
> little or no memory pressure and after this patch would need to do reclaim.

'if (gain - cost) > 0 ?'" is a difficult problem. I think the followings
are already big benefit in general:

	1. big reduction of IPIs #
	2. big reduction of TLB flushes #
	3. big reduction of TLB misses #

Of course, I or we need to keep trying to see a better number in
end-to-end performance.

> Also, looking with my arch/x86 hat on, there's really nothing
> arch-specific here.  Please try to keep stuff out of arch/x86 unless
> it's very much arch-specific.

Okay. I will try to keep it out of arch code. I should give up an
optimization that can be achieved by working on arch code tho.

	Byungchul