From: Ankur Arora <ankur.a.arora@oracle.com>
To: Andy Lutomirski <luto@amacapital.net>, Borislav Petkov <bp@alien8.de>
Cc: Andy Lutomirski <luto@kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
Linux-MM <linux-mm@kvack.org>,
"Kirill A. Shutemov" <kirill@shutemov.name>,
Michal Hocko <mhocko@kernel.org>,
Boris Ostrovsky <boris.ostrovsky@oracle.com>,
Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, X86 ML <x86@kernel.org>,
"H. Peter Anvin" <hpa@zytor.com>, Arnd Bergmann <arnd@arndb.de>,
Andrew Morton <akpm@linux-foundation.org>,
Ira Weiny <ira.weiny@intel.com>,
linux-arch <linux-arch@vger.kernel.org>
Subject: Re: [PATCH 5/8] x86/clear_page: add clear_page_uncached()
Date: Wed, 14 Oct 2020 20:21:57 -0700 [thread overview]
Message-ID: <50286c32-2869-cbd5-b178-0ad0c13584ea@oracle.com> (raw)
In-Reply-To: <22E29783-F1F5-43DA-B35F-D75FB247475D@amacapital.net>
On 2020-10-14 2:07 p.m., Andy Lutomirski wrote:
>
>
>
>> On Oct 14, 2020, at 12:58 PM, Borislav Petkov <bp@alien8.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@oracle.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
next prev parent reply other threads:[~2020-10-15 3:22 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-14 8:32 [PATCH 0/8] Use uncached writes while clearing gigantic pages Ankur Arora
2020-10-14 8:32 ` [PATCH 1/8] x86/cpuid: add X86_FEATURE_NT_GOOD Ankur Arora
2020-10-14 8:32 ` [PATCH 2/8] x86/asm: add memset_movnti() Ankur Arora
2020-10-14 8:32 ` [PATCH 3/8] perf bench: " Ankur Arora
2020-10-14 8:32 ` [PATCH 4/8] x86/asm: add clear_page_nt() Ankur Arora
2020-10-14 19:56 ` Borislav Petkov
2020-10-14 21:11 ` Ankur Arora
2020-10-14 8:32 ` [PATCH 5/8] x86/clear_page: add clear_page_uncached() Ankur Arora
2020-10-14 11:10 ` kernel test robot
2020-10-14 13:04 ` kernel test robot
2020-10-14 15:45 ` Andy Lutomirski
2020-10-14 19:58 ` Borislav Petkov
2020-10-14 21:07 ` Andy Lutomirski
2020-10-14 21:12 ` Borislav Petkov
2020-10-15 3:37 ` Ankur Arora
2020-10-15 10:35 ` Borislav Petkov
2020-10-15 21:20 ` Ankur Arora
2020-10-16 18:21 ` Borislav Petkov
2020-10-15 3:21 ` Ankur Arora [this message]
2020-10-15 10:40 ` Borislav Petkov
2020-10-15 21:40 ` Ankur Arora
2020-10-14 20:54 ` Ankur Arora
2020-10-14 8:32 ` [PATCH 6/8] mm, clear_huge_page: use clear_page_uncached() for gigantic pages Ankur Arora
2020-10-14 15:28 ` Ingo Molnar
2020-10-14 19:15 ` Ankur Arora
2020-10-14 8:32 ` [PATCH 7/8] x86/cpu/intel: enable X86_FEATURE_NT_GOOD on Intel Broadwellx Ankur Arora
2020-10-14 15:31 ` Ingo Molnar
2020-10-14 19:23 ` Ankur Arora
2020-10-14 8:32 ` [PATCH 8/8] x86/cpu/amd: enable X86_FEATURE_NT_GOOD on AMD Zen Ankur Arora
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