linux-kernel - Re: [PATCH 5/8] x86/clear_page: add clear_page

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Date:   Wed, 14 Oct 2020 20:21:57 -0700
From:   Ankur Arora <ankur.a.arora@...cle.com>
To:     Andy Lutomirski <luto@...capital.net>,
        Borislav Petkov <bp@...en8.de>
Cc:     Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Michal Hocko <mhocko@...nel.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, X86 ML <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, Arnd Bergmann <arnd@...db.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ira Weiny <ira.weiny@...el.com>,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH 5/8] x86/clear_page: add clear_page_uncached()

On 2020-10-14 2:07 p.m., Andy Lutomirski wrote:
> 
> 
> 
>> On Oct 14, 2020, at 12:58 PM, Borislav Petkov <bp@...en8.de> wrote:
>>
>> On Wed, Oct 14, 2020 at 08:45:37AM -0700, Andy Lutomirski wrote:
>>>> On Wed, Oct 14, 2020 at 1:33 AM Ankur Arora <ankur.a.arora@...cle.com> wrote:
>>>>
>>>> Define clear_page_uncached() as an alternative_call() to clear_page_nt()
>>>> if the CPU sets X86_FEATURE_NT_GOOD and fallback to clear_page() if it
>>>> doesn't.
>>>>
>>>> Similarly define clear_page_uncached_flush() which provides an SFENCE
>>>> if the CPU sets X86_FEATURE_NT_GOOD.
>>>
>>> As long as you keep "NT" or "MOVNTI" in the names and keep functions
>>> in arch/x86, I think it's reasonable to expect that callers understand
>>> that MOVNTI has bizarre memory ordering rules.  But once you give
>>> something a generic name like "clear_page_uncached" and stick it in
>>> generic code, I think the semantics should be more obvious.
>>
>> Why does it have to be a separate call? Why isn't it behind the
>> clear_page() alternative machinery so that the proper function is
>> selected at boot? IOW, why does a user of clear_page functionality need
>> to know at all about an "uncached" variant?
>
> I assume it’s for a little optimization of clearing more than one page
> per SFENCE.
>
> In any event, based on the benchmark data upthread, we only want to do
> NT clears when they’re rather large, so this shouldn’t be just an
> alternative. I assume this is because a page or two will fit in cache
> and, for most uses that allocate zeroed pages, we prefer cache-hot
> pages. When clearing 1G, on the other hand, cache-hot is impossible
> and we prefer the improved bandwidth and less cache trashing of NT
> clears.

Also, if we did extend clear_page() to take the page-size as parameter
we still might not have enough information (ex. a 4K or a 2MB page that
clear_page() sees could be part of a GUP of a much larger extent) to
decide whether to go uncached or not.

> Perhaps SFENCE is so fast that this is a silly optimization, though,
> and we don’t lose anything measurable by SFENCEing once per page.
Alas, no. I tried that and dropped about 15% performance on Rome.

Thanks
Ankur