From: Ankur Arora <ankur.a.arora@oracle.com>
To: Borislav Petkov <bp@alien8.de>, Andy Lutomirski <luto@amacapital.net>
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 20:37:44 -0700 [thread overview]
Message-ID: <3de58840-1f4c-566b-3a66-46d57475820c@oracle.com> (raw)
In-Reply-To: <20201014211214.GD18196@zn.tnic>
On 2020-10-14 2:12 p.m., Borislav Petkov wrote:
> On Wed, Oct 14, 2020 at 02:07:30PM -0700, Andy Lutomirski wrote:
>> 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.
>
> Yeah, use case makes sense but people won't know what to use. At the
> time I was experimenting with this crap, I remember Linus saying that
> that selection should be made based on the size of the area cleared, so
> users should not have to know the difference.
I don't disagree but I think the selection of cached/uncached route should
be made where we have enough context available to be able to choose to do
this.
This could be for example, done in mm_populate() or gup where if say the
extent is larger than LLC-size, it takes the uncached path.
>
> Which perhaps is the only sane use case I see for this.
>
>> Perhaps SFENCE is so fast that this is a silly optimization, though,
>> and we don’t lose anything measurable by SFENCEing once per page.
>
> Yes, I'd like to see real use cases showing improvement from this, not
> just microbenchmarks.
Sure will add.
Thanks
Ankur
>
next prev parent reply other threads:[~2020-10-15 3:38 UTC|newest]
Thread overview: 32+ 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 11:10 ` kernel test robot
2020-10-14 13:04 ` kernel test robot
2020-10-14 13:04 ` kernel test robot
2020-10-14 15:45 ` Andy Lutomirski
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 [this message]
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
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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