Re: [RFC PATCH 00/12] KVM: arm64: Eager huge-page splitting for dirty-logging

[Oliver Upton <oliver.upton@linux.dev>]

Hi Ricardo,

On Sat, Nov 12, 2022 at 08:17:02AM +0000, Ricardo Koller wrote:
> Hi,
> 
> I'm sending this RFC mainly to get some early feedback on the approach used
> for implementing "Eager Page Splitting" on ARM.  "Eager Page Splitting"
> improves the performance of dirty-logging (used in live migrations) when
> guest memory is backed by huge-pages.  It's an optimization used in Google
> Cloud since 2016 on x86, and for the last couple of months on ARM.
> 
> I tried multiple ways of implementing this optimization on ARM: from
> completely reusing the stage2 mapper, to implementing a new walker from
> scratch, and some versions in between. This RFC is one of those in
> between. They all have similar performance benefits, based on some light
> performance testing (mainly dirty_log_perf_test).
> 
> Background and motivation
> =========================
> Dirty logging is typically used for live-migration iterative copying.  KVM
> implements dirty-logging at the PAGE_SIZE granularity (will refer to 4K
> pages from now on).  It does it by faulting on write-protected 4K pages.
> Therefore, enabling dirty-logging on a huge-page requires breaking it into
> 4K pages in the first place.  KVM does this breaking on fault, and because
> it's in the critical path it only maps the 4K page that faulted; every
> other 4K page is left unmapped.  This is not great for performance on ARM
> for a couple of reasons:
> 
> - Splitting on fault can halt vcpus for milliseconds in some
>   implementations. Splitting a block PTE requires using a broadcasted TLB
>   invalidation (TLBI) for every huge-page (due to the break-before-make
>   requirement). Note that x86 doesn't need this. We observed some
>   implementations that take millliseconds to complete broadcasted TLBIs
>   when done in parallel from multiple vcpus.  And that's exactly what
>   happens when doing it on fault: multiple vcpus fault at the same time
>   triggering TLBIs in parallel.
> 
> - Read intensive guest workloads end up paying for dirty-logging.  Only
>   mapping the faulting 4K page means that all the other pages that were
>   part of the huge-page will now be unmapped. The effect is that any
>   access, including reads, now has to fault.
> 
> Eager Page Splitting (on ARM)
> =============================
> Eager Page Splitting fixes the above two issues by eagerly splitting
> huge-pages when enabling dirty logging. The goal is to avoid doing it while
> faulting on write-protected pages. This is what the TDP MMU does for x86
> [0], except that x86 does it for different reasons: to avoid grabbing the
> MMU lock on fault. Note that taking care of write-protection faults still
> requires grabbing the MMU lock on ARM, but not on x86 (with the
> fast_page_fault path).
> 
> An additional benefit of eagerly splitting huge-pages is that it can be
> done in a controlled way (e.g., via an IOCTL). This series provides two
> knobs for doing it, just like its x86 counterpart: when enabling dirty
> logging, and when using the KVM_CLEAR_DIRTY_LOG ioctl. The benefit of doing
> it on KVM_CLEAR_DIRTY_LOG is that this ioctl takes ranges, and not complete
> memslots like when enabling dirty logging. This means that the cost of
> splitting (mainly broadcasted TLBIs) can be throttled: split a range, wait
> for a bit, split another range, etc. The benefits of this approach were
> presented by Oliver Upton at KVM Forum 2022 [1].
> 
> Implementation
> ==============
> Patches 1-4 add a pgtable utility function for splitting huge block PTEs:
> kvm_pgtable_stage2_split(). Patches 5-6 add support for not doing
> break-before-make on huge-page breaking when FEAT_BBM level 2 is supported.

I would suggest you split up FEAT_BBM=2 and eager page splitting into
two separate series, if possible. IMO, the eager page split is easier to
reason about if it follows the existing pattern of break-before-make.

--
Thanks,
Oliver
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