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

[Punit Agrawal <punit.agrawal@bytedance.com>]

[ Apologies for chiming in late in the conversation ]

Anshuman Khandual <anshuman.khandual@arm.com> writes:

> On 9/28/22 05:53, Barry Song wrote:
>> On Tue, Sep 27, 2022 at 10:15 PM Yicong Yang <yangyicong@huawei.com> wrote:
>>>
>>> On 2022/9/27 14:16, Anshuman Khandual wrote:
>>>> [...]
>>>>
>>>> On 9/21/22 14:13, Yicong Yang wrote:
>>>>> +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)
>> 
>>>
>>> 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.
>
>> 
>> 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/
>> 
>> 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 ?

When running the test program in the commit in a VM, I saw benefits from
the patches at all sizes from 2, 4, 8, 32 vcpus. On the test machine,
ptep_clear_flush() went from ~1% in the unpatched version to not showing
up.

Yicong mentioned that he didn't see any benefit for <= 4 CPUs but is
there any overhead? I am wondering what are the downsides of enabling
the config by default.

Thanks,
Punit