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

Peter Zijlstra
2021-02-25T08:41:24Z

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.