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Message-ID: <8c6d09be-78d0-436e-a5a6-b94fb094b0b3@intel.com>
Date: Wed, 29 Nov 2023 14:17:26 +0100
From: Alexander Lobakin <aleksander.lobakin@...el.com>
To: Yunsheng Lin <linyunsheng@...wei.com>, Christoph Hellwig <hch@....de>
CC: Paul Menzel <pmenzel@...gen.mpg.de>, Maciej Fijalkowski
<maciej.fijalkowski@...el.com>, Jesper Dangaard Brouer <hawk@...nel.org>,
Larysa Zaremba <larysa.zaremba@...el.com>, <netdev@...r.kernel.org>,
Alexander Duyck <alexanderduyck@...com>, Ilias Apalodimas
<ilias.apalodimas@...aro.org>, <linux-kernel@...r.kernel.org>, Eric Dumazet
<edumazet@...gle.com>, Michal Kubiak <michal.kubiak@...el.com>,
<intel-wired-lan@...ts.osuosl.org>, David Christensen
<drc@...ux.vnet.ibm.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni
<pabeni@...hat.com>, "David S. Miller" <davem@...emloft.net>
Subject: Re: [Intel-wired-lan] [PATCH net-next v5 03/14] page_pool: avoid
calling no-op externals when possible
From: Yunsheng Lin <linyunsheng@...wei.com>
Date: Wed, 29 Nov 2023 11:17:50 +0800
> On 2023/11/27 22:32, Alexander Lobakin wrote:
>>
>> Chris, any thoughts on a global flag for skipping DMA syncs ladder?
>
> It seems there was one already in the past:
>
> https://lore.kernel.org/netdev/7c55a4d7-b4aa-25d4-1917-f6f355bd722e@arm.com/T/
It addresses a different problem, meaningless indirect calls, while this
one addresses meaningless direct calls :>
When the above gets merged, we could combine these two into one global,
but Eric wasn't active with his patch and I remember there were some
problems, so I wouldn't count on that it will arrive soon.
>
>>
>>>
>>>
>
>>>> +static inline bool page_pool_set_dma_addr(const struct page_pool *pool,
>>>> + struct page *page,
>>>> + dma_addr_t addr)
>>>> {
>>>> + unsigned long val = addr;
>>>> +
>>>> + if (unlikely(!addr)) {
>>>> + page->dma_addr = 0;
>>>> + return true;
>>>> + }
>>>
>>> The above seems unrelated change?
>>
>> Related. We use page_put_set_dma_addr() to clear ::dma_addr as well
>> (grep for it in page_pool.c). In this case, we don't want
>> dma_need_sync() to be called as we explicitly pass zero. This check
>> zeroes the field and exits as quickly as possible.
>
> The question seems to be about if we need to ensure the LSB of
> page->dma_addr is not set when page_pool releases a page back to page
> allocator?
But why do we need to call dma_need_sync(0) when freeing a page wasting
CPU cycles on relatively hot path?
>
>> In case with the call mentioned above, zero is a compile-time constant
>> there, so that this little branch will be inlined with the rest dropped.
Thanks,
Olek
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