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From: James Bottomley <jejb@linux.ibm.com>
To: Michal Hocko <mhocko@suse.com>, Mike Rapoport <rppt@kernel.org>
Cc: David Hildenbrand <david@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Andy Lutomirski <luto@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
	Borislav Petkov <bp@alien8.de>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Christopher Lameter <cl@linux.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Elena Reshetova <elena.reshetova@intel.com>,
	"H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
	"Kirill A. Shutemov" <kirill@shutemov.name>,
	Matthew Wilcox <willy@infradead.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Mike Rapoport <rppt@linux.ibm.com>,
	Michael Kerrisk <mtk.manpages@gmail.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Rick Edgecombe <rick.p.edgecombe@intel.com>,
	Roman Gushchin <guro@fb.com>, Shakeel Butt <shakeelb@google.com>,
	Shuah Khan <shuah@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Tycho Andersen <tycho@tycho.ws>, Will Deacon <will@kernel.org>,
	linux-api@vger.kernel.org, linux-arch@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
	linux-nvdimm@lists.01.org, linux-riscv@lists.infradead.org,
	x86@kernel.org, Hagen Paul Pfeifer <hagen@jauu.net>,
	Palmer Dabbelt <palmerdabbelt@google.com>
Subject: Re: [PATCH v16 07/11] secretmem: use PMD-size pages to amortize direct map fragmentation
Date: Thu, 28 Jan 2021 07:28:57 -0800
Message-ID: <2b6a5f22f0b062432186b89eeef58e2ba45e09c1.camel@linux.ibm.com> (raw)
In-Reply-To: <YBK1kqL7JA7NePBQ@dhcp22.suse.cz>

On Thu, 2021-01-28 at 14:01 +0100, Michal Hocko wrote:
> On Thu 28-01-21 11:22:59, Mike Rapoport wrote:
[...]
> > One of the major pushbacks on the first RFC [1] of the concept was
> > about the direct map fragmentation. I tried really hard to find
> > data that shows what is the performance difference with different
> > page sizes in the direct map and I didn't find anything.
> > 
> > So presuming that large pages do provide advantage the first
> > implementation of secretmem used PMD_ORDER allocations to amortise
> > the effect of the direct map fragmentation and then handed out 4k
> > pages at each fault. In addition there was an option to reserve a
> > finite pool at boot time and limit secretmem allocations only to
> > that pool.
> > 
> > At some point David suggested to use CMA to improve overall
> > flexibility [3], so I switched secretmem to use CMA.
> > 
> > Now, with the data we have at hand (my benchmarks and Intel's
> > report David mentioned) I'm even not sure this whole pooling even
> > required.
> 
> I would still like to understand whether that data is actually
> representative. With some underlying reasoning rather than I have run
> these XYZ benchmarks and numbers do not look terrible.

My theory, and the reason I made Mike run the benchmarks, is that our
fear of TLB miss has been alleviated by CPU speculation advances over
the years.  You can appreciate this if you think that both Intel and
AMD have increased the number of levels in the page table to
accommodate larger virtual memory size 5 instead of 3.  That increases
the length of the page walk nearly 2x in a physical system and even
more in a virtual system.  Unless this were massively optimized,
systems would have slowed down significantly.  Using 2M pages only
eliminates one level and 2G pages eliminates 2, so I theorized that
actually fragmentation wouldn't be the significant problem we once
thought it was and asked Mike to benchmark it.

The benchmarks show that indeed, it isn't a huge change in the data TLB
miss time, I suspect because data is nicely continuous nowadays and the
prediction that goes into the CPU optimizations quite easy.  ITLB
fragmentation actually seems to be quite a bit worse, likely because we
still don't have branch prediction down to an exact science.

James



  parent reply index

Thread overview: 76+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-21 12:27 [PATCH v16 00/11] mm: introduce memfd_secret system call to create "secret" memory areas Mike Rapoport
2021-01-21 12:27 ` [PATCH v16 01/11] mm: add definition of PMD_PAGE_ORDER Mike Rapoport
2021-01-21 12:27 ` [PATCH v16 02/11] mmap: make mlock_future_check() global Mike Rapoport
2021-01-21 12:27 ` [PATCH v16 03/11] riscv/Kconfig: make direct map manipulation options depend on MMU Mike Rapoport
2021-01-21 12:27 ` [PATCH v16 04/11] set_memory: allow set_direct_map_*_noflush() for multiple pages Mike Rapoport
2021-01-21 12:27 ` [PATCH v16 05/11] set_memory: allow querying whether set_direct_map_*() is actually enabled Mike Rapoport
2021-01-21 12:27 ` [PATCH v16 06/11] mm: introduce memfd_secret system call to create "secret" memory areas Mike Rapoport
2021-01-25 17:01   ` Michal Hocko
2021-01-25 21:36     ` Mike Rapoport
2021-01-26  7:16       ` Michal Hocko
2021-01-26  8:33         ` Mike Rapoport
2021-01-26  9:00           ` Michal Hocko
2021-01-26  9:20             ` Mike Rapoport
2021-01-26  9:49               ` Michal Hocko
2021-01-26  9:53                 ` David Hildenbrand
2021-01-26 10:19                   ` Michal Hocko
2021-01-26  9:20             ` Michal Hocko
2021-02-03 12:15   ` Michal Hocko
2021-02-04 11:34     ` Mike Rapoport
2021-01-21 12:27 ` [PATCH v16 07/11] secretmem: use PMD-size pages to amortize direct map fragmentation Mike Rapoport
2021-01-26 11:46   ` Michal Hocko
2021-01-26 11:56     ` David Hildenbrand
2021-01-26 12:08       ` Michal Hocko
2021-01-28  9:22         ` Mike Rapoport
2021-01-28 13:01           ` Michal Hocko
2021-01-28 13:28             ` Christoph Lameter
2021-01-28 13:49               ` Michal Hocko
2021-01-28 15:56                 ` Christoph Lameter
2021-01-28 16:23                   ` Michal Hocko
2021-01-28 15:28             ` James Bottomley [this message]
2021-01-29  7:03               ` Mike Rapoport
2021-01-28 21:05             ` James Bottomley
     [not found]               ` <YBPF8ETGBHUzxaZR@dhcp22.suse.cz>
2021-02-01 16:56                 ` James Bottomley
2021-02-02  9:35                   ` Michal Hocko
2021-02-02 12:48                     ` Mike Rapoport
2021-02-02 13:14                       ` David Hildenbrand
2021-02-02 13:32                         ` Michal Hocko
2021-02-02 14:12                           ` David Hildenbrand
2021-02-02 14:22                             ` Michal Hocko
2021-02-02 14:26                               ` David Hildenbrand
2021-02-02 14:32                                 ` Michal Hocko
2021-02-02 14:34                                   ` David Hildenbrand
2021-02-02 18:15                                     ` Mike Rapoport
2021-02-02 18:55                                       ` James Bottomley
2021-02-03 12:09                                         ` Michal Hocko
2021-02-04 11:31                                           ` Mike Rapoport
2021-02-02 13:27                       ` Michal Hocko
2021-02-02 19:10                         ` Mike Rapoport
2021-02-03  9:12                           ` Michal Hocko
2021-02-04  9:58                             ` Mike Rapoport
2021-02-04 13:02                               ` Michal Hocko
2021-01-29  7:21             ` Mike Rapoport
2021-01-29  8:51               ` Michal Hocko
2021-02-02 14:42                 ` David Hildenbrand
2021-01-21 12:27 ` [PATCH v16 08/11] secretmem: add memcg accounting Mike Rapoport
2021-01-25 16:17   ` Matthew Wilcox
2021-01-25 17:18     ` Shakeel Butt
2021-01-25 21:35       ` Mike Rapoport
2021-01-28 15:07         ` Shakeel Butt
2021-01-25 16:54   ` Michal Hocko
2021-01-25 21:38     ` Mike Rapoport
2021-01-26  7:31       ` Michal Hocko
2021-01-26  8:56         ` Mike Rapoport
2021-01-26  9:15           ` Michal Hocko
2021-01-26 14:48       ` Matthew Wilcox
2021-01-26 15:05         ` Michal Hocko
2021-01-27 18:42           ` Roman Gushchin
2021-01-28  7:58             ` Michal Hocko
2021-01-28 14:05               ` Shakeel Butt
2021-01-28 14:22                 ` Michal Hocko
2021-01-28 14:57                   ` Shakeel Butt
2021-01-21 12:27 ` [PATCH v16 09/11] PM: hibernate: disable when there are active secretmem users Mike Rapoport
2021-01-21 12:27 ` [PATCH v16 10/11] arch, mm: wire up memfd_secret system call where relevant Mike Rapoport
2021-01-25 18:18   ` Catalin Marinas
2021-01-21 12:27 ` [PATCH v16 11/11] secretmem: test: add basic selftest for memfd_secret(2) Mike Rapoport
2021-01-21 22:18 ` [PATCH v16 00/11] mm: introduce memfd_secret system call to create "secret" memory areas Andrew Morton

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