linux-kernel - Re: [RFC 0/6] migrate_pages(): batch TLB flushing

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Date:   Wed, 2 Nov 2022 14:13:29 +0000
From:   Hesham Almatary <hesham.almatary@...wei.com>
To:     "Huang, Ying" <ying.huang@...el.com>
CC:     haoxin <xhao@...ux.alibaba.com>, <linux-kernel@...r.kernel.org>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        Zi Yan <ziy@...dia.com>, Yang Shi <shy828301@...il.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        "Oscar Salvador" <osalvador@...e.de>,
        Matthew Wilcox <willy@...radead.org>,
        <yangyicong@...ilicon.com>, <v-songbaohua@...o.com>,
        <21cnbao@...il.com>, <linux-mm@...ck.org>
Subject: Re: [RFC 0/6] migrate_pages(): batch TLB flushing


On 11/2/2022 3:14 AM, Huang, Ying wrote:
> Hesham Almatary <hesham.almatary@...wei.com> writes:
>
>> On 9/27/2022 12:21 PM, haoxin wrote:
>>> Hi, Huang
>>>
>>> ( 2022/9/21 .H2:06, Huang Ying S:
>>>> From: "Huang, Ying" <ying.huang@...el.com>
>>>>
>>>> Now, migrate_pages() migrate pages one by one, like the fake code as
>>>> follows,
>>>>
>>>> ? for each page
>>>> ?? unmap
>>>> ?? flush TLB
>>>> ?? copy
>>>> ?? restore map
>>>>
>>>> If multiple pages are passed to migrate_pages(), there are
>>>> opportunities to batch the TLB flushing and copying. That is, we can
>>>> change the code to something as follows,
>>>>
>>>> ? for each page
>>>> ?? unmap
>>>> ? for each page
>>>> ?? flush TLB
>>>> ? for each page
>>>> ?? copy
>>>> ? for each page
>>>> ?? restore map
>>>>
>>>> The total number of TLB flushing IPI can be reduced considerably. And
>>>> we may use some hardware accelerator such as DSA to accelerate the
>>>> page copying.
>>>>
>>>> So in this patch, we refactor the migrate_pages() implementation and
>>>> implement the TLB flushing batching. Base on this, hardware
>>>> accelerated page copying can be implemented.
>>>>
>>>> If too many pages are passed to migrate_pages(), in the naive batched
>>>> implementation, we may unmap too many pages at the same time. The
>>>> possibility for a task to wait for the migrated pages to be mapped
>>>> again increases. So the latency may be hurt. To deal with this
>>>> issue, the max number of pages be unmapped in batch is restricted to
>>>> no more than HPAGE_PMD_NR. That is, the influence is at the same
>>>> level of THP migration.
>>>>
>>>> We use the following test to measure the performance impact of the
>>>> patchset,
>>>>
>>>> On a 2-socket Intel server,
>>>>
>>>>   - Run pmbench memory accessing benchmark
>>>>
>>>>   - Run `migratepages` to migrate pages of pmbench between node 0 and
>>>> ? node 1 back and forth.
>>>>
>>> As the pmbench can not run on arm64 machine, so i use lmbench instead.
>>> I test case like this: (i am not sure whether it is reasonable,
>>> but it seems worked)
>>> ./bw_mem -N10000 10000m rd &
>>> time migratepages pid node0 node1
>>>
>> FYI, I have ported pmbench to AArch64 [1]. The project seems to be
>> abandoned on bitbucket,
>>
>> I wonder if it makes sense to fork it elsewhere and push the pending PRs there.
>>
>>
>> [1] https://bitbucket.org/jisooy/pmbench/pull-requests/5
> Maybe try to contact the original author with email firstly?

That's  a good idea. I'm not planning to fork/maintain it myself, but if anyone

is interested in doing so, I am happy to help out and submit PRs there.


> Best Regards,
> Huang, Ying
>
>>> o/patch w/patch
>>> real? 0m0.035s?? real? 0m0.024s
>>> user? 0m0.000s?? user? 0m0.000s
>>> sys? 0m0.035s??? sys? 0m0.024s
>>>
>>> the migratepages time is reduced above 32%.
>>>
>>> But there has a problem, i see the batch flush is called by
>>> migrate_pages_batch
>>> ??try_to_unmap_flush
>>> ??? arch_tlbbatch_flush(&tlb_ubc->arch); // there batch flush really work.
>>>
>>> But in arm64, the arch_tlbbatch_flush are not supported, becasue it
>>> not support CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH yet.
>>>
>>> So, the tlb batch flush means no any flush is did, it is a empty func.
>>>
>>> Maybe this patch can help solve this problem.
>>> https://lore.kernel.org/linux-arm-kernel/20220921084302.43631-1-yangyicong@huawei.com/T/
>>>
>>>
>>>
>>>
>>>
>>>