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
