Re: [RFC 0/6] x86: prefetch_page() vDSO call

Nadav Amit
2021-02-25T08:53:10Z

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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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