HAVE_MOVE_PMD enables remapping pages at the PMD level if both the source and destination addresses are PMD-aligned.
HAVE_MOVE_PMD is already enabled on x86. The original patch [1] that introduced this config did not enable it on arm64 at the time because of performance issues with flushing the TLB on every PMD move. These issues have since been addressed in more recent releases with improvements to the arm64 TLB invalidation and core mmu_gather code as Will Deacon mentioned in [2].
From the data below, it can be inferred that there is approximately
8x improvement in performance when HAVE_MOVE_PMD is enabled on arm64.
--------- Test Results ----------
The following results were obtained on an arm64 device running a 5.4 kernel, by remapping a PMD-aligned, 1GB sized region to a PMD-aligned destination. The results from 10 iterations of the test are given below. All times are in nanoseconds.
Control HAVE_MOVE_PMD
9220833 1247761 9002552 1219896 9254115 1094792 8725885 1227760 9308646 1043698 9001667 1101771 8793385 1159896 8774636 1143594 9553125 1025833 9374010 1078125
9100885.4 1134312.6 <-- Mean Time in nanoseconds
Total mremap time for a 1GB sized PMD-aligned region drops from ~9.1 milliseconds to ~1.1 milliseconds. (~8x speedup).
[1] https://lore.kernel.org/r/20181108181201.88826-3-joelaf@google.com [2] https://www.mail-archive.com/linuxppc-dev@lists.ozlabs.org/msg140837.html
Signed-off-by: Kalesh Singh kaleshsingh@google.com --- arch/arm64/Kconfig | 1 + 1 file changed, 1 insertion(+)
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig index 6d232837cbee..844d089668e3 100644 --- a/arch/arm64/Kconfig +++ b/arch/arm64/Kconfig @@ -121,6 +121,7 @@ config ARM64 select GENERIC_VDSO_TIME_NS select HANDLE_DOMAIN_IRQ select HARDIRQS_SW_RESEND + select HAVE_MOVE_PMD select HAVE_PCI select HAVE_ACPI_APEI if (ACPI && EFI) select HAVE_ALIGNED_STRUCT_PAGE if SLUB