Re: [PATCHv4 1/8] mm: Add support for unaccepted memory

From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Dave Hansen <dave.hansen@intel.com>
Cc: David Hildenbrand <david@redhat.com>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Borislav Petkov <bp@alien8.de>, Andy Lutomirski <luto@kernel.org>,
	Sean Christopherson <seanjc@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Joerg Roedel <jroedel@suse.de>, Ard Biesheuvel <ardb@kernel.org>,
	Andi Kleen <ak@linux.intel.com>,
	Kuppuswamy Sathyanarayanan
	<sathyanarayanan.kuppuswamy@linux.intel.com>,
	David Rientjes <rientjes@google.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Ingo Molnar <mingo@redhat.com>,
	Varad Gautam <varad.gautam@suse.com>,
	Dario Faggioli <dfaggioli@suse.com>,
	Brijesh Singh <brijesh.singh@amd.com>,
	Mike Rapoport <rppt@kernel.org>,
	x86@kernel.org, linux-mm@kvack.org, linux-coco@lists.linux.dev,
	linux-efi@vger.kernel.org, linux-kernel@vger.kernel.org,
	Mike Rapoport <rppt@linux.ibm.com>
Subject: Re: [PATCHv4 1/8] mm: Add support for unaccepted memory
Date: Wed, 13 Apr 2022 19:24:26 +0300	[thread overview]
Message-ID: <20220413162426.3sy6kjkaqvdgdbl6@box.shutemov.name> (raw)
In-Reply-To: <cfcf9f82-7e5f-58b6-7b47-9ac552832596@intel.com>

On Wed, Apr 13, 2022 at 08:36:52AM -0700, Dave Hansen wrote:
> On 4/13/22 04:30, Kirill A. Shutemov wrote:
> >> 2) Fast boot; after boot, all memory will slowly but steadily get
> >>    accepted in the background. After a while, all memory is accepted and
> >>    can be signaled to user space.
> ...
> > Frankly, I think option 2 is the worst one. You still CPU cycles from the
> > workload after boot to do the job that may or may not be needed. It is an
> > half-measure that helps nobody.
> 
> Let's not be too hyperbolic here.  "Worst" is entirely subjective and it
> totally depends on your perspective and what you care about.
> 
> There are basically four options:
> 
>  * Accept everything in early boot
>  * Accept with deferred page free
>  * Accept with kthread after boot
>  * Accept on demand
> 
> and four things that matter:
> 
>  * Code complexity
>  * Time to a shell prompt
>  * CPU/Memory waste
>  * Deterministic overhead
> 
> Did I miss any?

"Time to shell" is not equal to "time to do the job". Real workloads do
stuff beyond memory allocations. But, yes, it is harder quantify.

> News flash: none of the options wins on all the things that matter.
> We're going to have to pick one (or maybe two).  I'm also not horribly
> convinced that there's a problem here worth solving, especially one that
> requires surgery in the core of the buddy allocator.
> 
> This is essentially making a performance argument: it takes too long to
> boot if we go with a simpler solution.  Yet, I haven't seen any data.  I
> think we need to go with the simplest approach(es) until there's some
> actual data to guide us here.
> 
> Here's another way to look at it:
> 
> > https://docs.google.com/spreadsheets/d/1Fpv0Yp0CTF5_JXHR2pywvNtImTwUVGTxDMlJ5t8qiis/edit?usp=sharing

The link is view-only.

AFAICS, complexity of the kthread approach is on par or greater comparing
to on-demand. You need coordination between allocator and the thread.
It can be hard to hit right balance for the kthread between being CPU hog
and not providing enough accepted memory.

-- 
 Kirill A. Shutemov