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    SubjectRe: [RFC 0/6] x86: prefetch_page() vDSO call
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    > On Feb 25, 2021, at 12:40 AM, Peter Zijlstra <peterz@infradead.org> wrote:
    >
    > On Wed, Feb 24, 2021 at 11:29:04PM -0800, Nadav Amit wrote:
    >> From: Nadav Amit <namit@vmware.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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