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

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Date:   Mon, 30 Oct 2023 18:32:44 +0000
From:   Nadav Amit <namit@...are.com>
To:     Byungchul Park <byungchul@...com>
CC:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        "kernel_team@...ynix.com" <kernel_team@...ynix.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "ying.huang@...el.com" <ying.huang@...el.com>,
        "xhao@...ux.alibaba.com" <xhao@...ux.alibaba.com>,
        "mgorman@...hsingularity.net" <mgorman@...hsingularity.net>,
        Hugh Dickins <hughd@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: [v3 0/3] Reduce TLB flushes under some specific conditions


> On Oct 30, 2023, at 7:55 PM, Dave Hansen <dave.hansen@...el.com> wrote:
> 
> !! External Email
> 
> 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.

I almost forgot that I did (and embarrassingly did not follow) a TLB
flush deferring mechanism mechanism before [*], which was relatively
generic. I did not look at the migration case, but it could have been
relatively easily added - I think.

Feel free to plagiarize if you find it suitable. Note that some of
the patch-set is not relevant (e.g., 20/20 has already been fixed,
3/20 was merged.)

[*] https://lore.kernel.org/linux-mm/20210131001132.3368247-1-namit@vmware.com/