Re: [PATCH v4 2/2] arm64: support batched/deferred tlb shootdown during page reclamation

On Thu, Oct 27, 2022 at 11:42 PM Anshuman Khandual
[off-list ref] wrote:

On 9/28/22 05:53, Barry Song wrote:

quoted

On Tue, Sep 27, 2022 at 10:15 PM Yicong Yang [off-list ref] wrote:

quoted

On 2022/9/27 14:16, Anshuman Khandual wrote:

quoted

[...]

On 9/21/22 14:13, Yicong Yang wrote:

quoted

+static inline bool arch_tlbbatch_should_defer(struct mm_struct *mm)
+{
+    /* for small systems with small number of CPUs, TLB shootdown is cheap */
+    if (num_online_cpus() <= 4)

It would be great to have some more inputs from others, whether 4 (which should
to be codified into a macro e.g ARM64_NR_CPU_DEFERRED_TLB, or something similar)
is optimal for an wide range of arm64 platforms.

I have tested it on a 4-cpus and 8-cpus machine. but i have no machine
with 5,6,7
cores.
I saw improvement on 8-cpus machines and I found 4-cpus machines don't need
this patch.

so it seems safe to have
if (num_online_cpus()  < 8)

quoted

Do you prefer this macro to be static or make it configurable through kconfig then
different platforms can make choice based on their own situations? It maybe hard to
test on all the arm64 platforms.

Maybe we can have this default enabled on machines with 8 and more cpus and
provide a tlbflush_batched = on or off to allow users enable or
disable it according
to their hardware and products. Similar example: rodata=on or off.

No, sounds bit excessive. Kernel command line options should not be added
for every possible run time switch options.

quoted

Hi Anshuman, Will,  Catalin, Andrew,
what do you think about this approach?

BTW, haoxin mentioned another important user scenarios for tlb bach on arm64:
https://lore.kernel.org/lkml/393d6318-aa38-01ed-6ad8-f9eac89bf0fc@linux.alibaba.com/ (local)

I do believe we need it based on the expensive cost of tlb shootdown in arm64
even by hardware broadcast.

Alright, for now could we enable ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH selectively
with CONFIG_EXPERT and for num_online_cpus()  > 8 ?

Sounds good to me. It is a good start to bring up tlb batched flush in
ARM64. Later on, we
might want to see it in both memory reclamation and migration.

Thanks
Barry