On 2024/1/5 23:42, Alexander H Duyck wrote: > On Wed, 2024-01-03 at 17:56 +0800, Yunsheng Lin wrote: >> The next patch is above to use page_frag_alloc_align() to >> replace vhost_net_page_frag_refill(), the main difference >> between those two frag page implementations is whether we >> use a initial zero offset or not. >> >> It seems more nature to use a initial zero offset, as it >> may enable more correct cache prefetching and skb frag >> coalescing in the networking, so change it to use initial >> zero offset. >> >> Signed-off-by: Yunsheng Lin <linyunsheng@xxxxxxxxxx> >> CC: Alexander Duyck <alexander.duyck@xxxxxxxxx> > > There are several advantages to running the offset as a countdown > rather than count-up value. > > 1. Specifically for the size of the chunks we are allocating doing it > from the bottom up doesn't add any value as we are jumping in large > enough amounts and are being used for DMA so being sequential doesn't > add any value. What is the expected size of the above chunks in your mind? I suppose that is like NAPI_HAS_SMALL_PAGE_FRAG to avoid excessive truesize underestimation? It seems there is no limit for min size of allocation for page_frag_alloc_align() now, and as the page_frag API seems to be only used in networking, should we enforce the min size of allocation at API level? > > 2. By starting at the end and working toward zero we can use built in > functionality of the CPU to only have to check and see if our result > would be signed rather than having to load two registers with the > values and then compare them which saves us a few cycles. In addition > it saves us from having to read both the size and the offset for every > page. I suppose the above is ok if we only use the page_frag_alloc*() API to allocate memory for skb->data, not for the frag in skb_shinfo(), as by starting at the end and working toward zero, it means we can not do skb coalescing. As page_frag_alloc*() is returning va now, I am assuming most of users is using the API for skb->data, I guess it is ok to drop this patch for now. If we allow page_frag_alloc*() to return struct page, we might need this patch to enable coalescing. > > Again this is another code cleanup at the cost of performance. I > realize many of the items you are removing would be considered micro- > optimizations but when we are dealing with millions of packets per > second those optimizations add up. > . >
