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

[Punit Agrawal]

Anshuman Khandual <anshuman.khandual@xxxxxxx> writes:

> On 10/28/22 03:25, Barry Song wrote:
>> On Fri, Oct 28, 2022 at 3:19 AM Punit Agrawal
>> <punit.agrawal@xxxxxxxxxxxxx> wrote:
>>>
>>> [ Apologies for chiming in late in the conversation ]
>>>
>>> Anshuman Khandual <anshuman.khandual@xxxxxxx> writes:
>>>
>>>> On 9/28/22 05:53, Barry Song wrote:
>>>>> On Tue, Sep 27, 2022 at 10:15 PM Yicong Yang <yangyicong@xxxxxxxxxx> 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@xxxxxxxxxxxxxxxxx/
>>>>>
>>>>> 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.
>> As we are deferring tlb flush, but sometimes while we are modifying the vma
>> which are deferred, we need to do a sync by flush_tlb_batched_pending() in
>> mprotect() , madvise() to make sure they can see the flushed result. if nobody
>> is doing mprotect(), madvise() etc in the deferred period, the overhead is zero.
>
> Right, it is difficult to justify this overhead for smaller systems,
> which for sure would not benefit from this batched TLB framework.

Thank you for the pointers to the overhead.

Having looked at this more closely, I also see that
flush_tlb_batched_pending() discards the entire mm vs just flushing the
page being unmapped (as is done with ptep_clear_flush()).