linux-kernel - Re: [PATCH v2 0/5] Introduce DMA_HEAP_ALLOC_AND_READ

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Message-ID: <fe460c96-fb5f-41e7-8312-a2aa30140042@amd.com>
Date: Tue, 30 Jul 2024 12:42:30 +0200
From: Christian König <christian.koenig@....com>
To: Huan Yang <link@...o.com>, Sumit Semwal <sumit.semwal@...aro.org>,
 Benjamin Gaignard <benjamin.gaignard@...labora.com>,
 Brian Starkey <Brian.Starkey@....com>, John Stultz <jstultz@...gle.com>,
 "T.J. Mercier" <tjmercier@...gle.com>, linux-media@...r.kernel.org,
 dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org,
 linux-kernel@...r.kernel.org, opensource.kernel@...o.com
Subject: Re: [PATCH v2 0/5] Introduce DMA_HEAP_ALLOC_AND_READ_FILE heap flag

Am 30.07.24 um 11:05 schrieb Huan Yang:
>
> 在 2024/7/30 16:56, Daniel Vetter 写道:
>> [????????? daniel.vetter@...ll.ch ????????? 
>> https://aka.ms/LearnAboutSenderIdentification?????????????]
>>
>> On Tue, Jul 30, 2024 at 03:57:44PM +0800, Huan Yang wrote:
>>> UDMA-BUF step:
>>>    1. memfd_create
>>>    2. open file(buffer/direct)
>>>    3. udmabuf create
>>>    4. mmap memfd
>>>    5. read file into memfd vaddr
>> Yeah this is really slow and the worst way to do it. You absolutely want
>> to start _all_ the io before you start creating the dma-buf, ideally 
>> with
>> everything running in parallel. But just starting the direct I/O with
>> async and then creating the umdabuf should be a lot faster and avoid
> That's greate,  Let me rephrase that, and please correct me if I'm wrong.
>
> UDMA-BUF step:
>   1. memfd_create
>   2. mmap memfd
>   3. open file(buffer/direct)
>   4. start thread to async read
>   3. udmabuf create
>
> With this, can improve
>
>> needlessly serialization operations.
>>
>> The other issue is that the mmap has some overhead, but might not be too
>> bad.
> Yes, the time spent on page fault in mmap should be negligible 
> compared to the time spent on file read.

You should try to avoid mmap as much as possible. Especially the TLB 
invalidation overhead is really huge on platforms with a large number of 
CPUs.

Regards,
Christian.

>> -Sima
>> -- 
>> Daniel Vetter
>> Software Engineer, Intel Corporation
>> http://blog.ffwll.ch