On Thu, Nov 17, 2022 at 04:26:48PM +0800, Yicong Yang wrote:

It is tested on 4,8,128 CPU platforms and shows to be beneficial on
large systems but may not have improvement on small systems like on
a 4 CPU platform. So make ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH depends
on CONFIG_EXPERT for this stage and make this disabled on systems
with less than 8 CPUs. User can modify this threshold according to
their own platforms by CONFIG_NR_CPUS_FOR_BATCHED_TLB.

What's the overhead of such batching on systems with 4 or fewer CPUs? If
it isn't noticeable, I'd rather have it always on than some number
chosen on whichever SoC you tested.

Another option would be to make this a sysctl tunable.

 .../features/vm/TLB/arch-support.txt          |  2 +-
 arch/arm64/Kconfig                            |  6 +++
 arch/arm64/include/asm/tlbbatch.h             | 12 +++++
 arch/arm64/include/asm/tlbflush.h             | 52 ++++++++++++++++++-
 arch/x86/include/asm/tlbflush.h               |  5 +-
 include/linux/mm_types_task.h                 |  4 +-
 mm/rmap.c                                     | 10 ++--

Please keep any function prototype changes in a preparatory patch so
that the arm64 one only introduces the arch specific changes. Easier to
review.

+static inline bool arch_tlbbatch_should_defer(struct mm_struct *mm)
+{
+	/*
+	 * TLB batched flush is proved to be beneficial for systems with large
+	 * number of CPUs, especially system with more than 8 CPUs. TLB shutdown
+	 * is cheap on small systems which may not need this feature. So use
+	 * a threshold for enabling this to avoid potential side effects on
+	 * these platforms.
+	 */
+	if (num_online_cpus() < CONFIG_ARM64_NR_CPUS_FOR_BATCHED_TLB)
+		return false;

The x86 implementation tracks the cpumask of where a task has run. We
don't have such tracking on arm64 and I don't think it matters. As
noticed/described in this series, the bottleneck is the actual DSB
synchronisation (which sends a DVM Sync message to all the other CPUs
and waits for a DVM Complete response). So I think it makes sense not to
bother with an mm_cpumask(). What this patch aims to optimise is
actually the number of DSBs issued on an SMP system by
ptep_clear_flush().

The DVM is not an architected concept (well, it's part of AMBA AXI). I'd
be curious to know how such patch behaves on Apple's M1/M2 hardware. My
preference would be to have this always on for num_online_cpus() > 1 if
there's no overhead.

-- 
Catalin