linux-kernel - Re: [RFC 0/6] x86: prefetch

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Message-Id: <64538CCF-AD11-43C2-9632-E054301F9E6D@gmail.com>
Date:   Thu, 25 Feb 2021 00:52:19 -0800
From:   Nadav Amit <nadav.amit@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Linux-MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        Hugh Dickins <hughd@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Sean Christopherson <seanjc@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org
Subject: Re: [RFC 0/6] x86: prefetch_page() vDSO call



> On Feb 25, 2021, at 12:40 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> On Wed, Feb 24, 2021 at 11:29:04PM -0800, Nadav Amit wrote:
>> From: Nadav Amit <namit@...are.com>
>> 
>> Just as applications can use prefetch instructions to overlap
>> computations and memory accesses, applications may want to overlap the
>> page-faults and compute or overlap the I/O accesses that are required
>> for page-faults of different pages.
>> 
>> Applications can use multiple threads and cores for this matter, by
>> running one thread that prefetches the data (i.e., faults in the data)
>> and another that does the compute, but this scheme is inefficient. Using
>> mincore() can tell whether a page is mapped, but might not tell whether
>> the page is in the page-cache and does not fault in the data.
>> 
>> Introduce prefetch_page() vDSO-call to prefetch, i.e. fault-in memory
>> asynchronously. The semantic of this call is: try to prefetch a page of
>> in a given address and return zero if the page is accessible following
>> the call. Start I/O operations to retrieve the page if such operations
>> are required and there is no high memory pressure that might introduce
>> slowdowns.
>> 
>> Note that as usual the page might be paged-out at any point and
>> therefore, similarly to mincore(), there is no guarantee that the page
>> will be present at the time that the user application uses the data that
>> resides on the page. Nevertheless, it is expected that in the vast
>> majority of the cases this would not happen, since prefetch_page()
>> accesses the page and therefore sets the PTE access-bit (if it is
>> clear).
>> 
>> The implementation is as follows. The vDSO code accesses the data,
>> triggering a page-fault it is not present. The handler detects based on
>> the instruction pointer that this is an asynchronous-#PF, using the
>> recently introduce vDSO exception tables. If the page can be brought
>> without waiting (e.g., the page is already in the page-cache), the
>> kernel handles the fault and returns success (zero). If there is memory
>> pressure that prevents the proper handling of the fault (i.e., requires
>> heavy-weight reclamation) it returns a failure. Otherwise, it starts an
>> I/O to bring the page and returns failure.
>> 
>> Compilers can be extended to issue the prefetch_page() calls when
>> needed.
> 
> Interesting, but given we've been removing explicit prefetch from some
> parts of the kernel how useful is this in actual use? I'm thinking there
> should at least be a real user and performance numbers with this before
> merging.

Can you give me a reference to the “removing explicit prefetch from some
parts of the kernel”?

I will work on an llvm/gcc plugin to provide some performance numbers.
I wanted to make sure that the idea is not a complete obscenity first.


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