netdev - Re: [PATCH net-next 06/11] net: page_pool: avoid calling no-op externals when possible

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Date: Thu, 18 May 2023 12:54:37 +0800
From: Yunsheng Lin <linyunsheng@...wei.com>
To: Jakub Kicinski <kuba@...nel.org>, Alexander Lobakin
	<aleksander.lobakin@...el.com>
CC: "David S. Miller" <davem@...emloft.net>, Eric Dumazet
	<edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>, Maciej Fijalkowski
	<maciej.fijalkowski@...el.com>, Magnus Karlsson <magnus.karlsson@...el.com>,
	Michal Kubiak <michal.kubiak@...el.com>, Larysa Zaremba
	<larysa.zaremba@...el.com>, Jesper Dangaard Brouer <hawk@...nel.org>, Ilias
 Apalodimas <ilias.apalodimas@...aro.org>, Christoph Hellwig <hch@....de>,
	<netdev@...r.kernel.org>, <intel-wired-lan@...ts.osuosl.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next 06/11] net: page_pool: avoid calling no-op
 externals when possible

On 2023/5/18 12:08, Jakub Kicinski wrote:
> On Tue, 16 May 2023 18:18:36 +0200 Alexander Lobakin wrote:
>> +		/* Try to avoid calling no-op syncs */
>> +		pool->p.flags |= PP_FLAG_DMA_MAYBE_SYNC;
>> +		pool->p.flags &= ~PP_FLAG_DMA_SYNC_DEV;
>>  	}
>>  
>>  	if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT &&
>> @@ -323,6 +327,12 @@ static bool page_pool_dma_map(struct page_pool *pool, struct page *page)
>>  
>>  	page_pool_set_dma_addr(page, dma);
>>  
>> +	if ((pool->p.flags & PP_FLAG_DMA_MAYBE_SYNC) &&
>> +	    dma_need_sync(pool->p.dev, dma)) {
>> +		pool->p.flags |= PP_FLAG_DMA_SYNC_DEV;
>> +		pool->p.flags &= ~PP_FLAG_DMA_MAYBE_SYNC;
>> +	}
> 
> is it just me or does it feel cleaner to allocate a page at init,
> and throw it into the cache, rather than adding a condition to a
> fast(ish) path?

Is dma_need_sync() not reliable until a dma map is called?
Is there any reason why not just clear PP_FLAG_DMA_SYNC_DEV if
dma_need_sync() is false without introducing the PP_FLAG_DMA_MAYBE_SYNC
flag?

> 
> .
>