xsk: introduce pre-allocated memory per xsk CQ #6504
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This is a prep that will be used to store the addr(s) of descriptors so that each skb going to the end of life can publish corresponding addr(s) in its completion queue that can be read by userspace. Signed-off-by: Jason Xing <kernelxing@tencent.com>
Before the commit 30f241f ("xsk: Fix immature cq descriptor production"), there is one issue[1] which causes the wrong publish of descriptors in race condidtion. The above commit fixes the issue but adds more memory operations in the xmit hot path and interrupt context, which can cause side effect in performance. Based on the existing infrastructure, this patch tries to propose a new solution to fix the problem by using a pre-allocated memory that is local completion queue to avoid frequently performing memory functions. The benefit comes from replacing xsk_tx_generic_cache with local cq. The core logics are as show below: 1. allocate a new local completion queue when setting the real queue. 2. write the descriptors into the local cq in the xmit path. And record the prod as @start_pos that reflects the start position of skb in this queue so that later the skb can easily write the desc addr(s) from local cq to cq addrs in the destruction phase. 3. initialize the upper 24 bits of destructor_arg to store @start_pos in xsk_skb_init_misc(). 4. Initialize the lower 8 bits of destructor_arg to store how many descriptors the skb owns in xsk_inc_num_desc(). 5. write the desc addr(s) from the @start_addr from the local cq one by one into the real cq in xsk_destruct_skb(). In turn sync the global state of the cq as before. The format of destructor_arg is designed as: ------------------------ -------- | start_pos | num | ------------------------ -------- Using upper 24 bits is enough to keep the temporary descriptors. And it's also enough to use lower 8 bits to show the number of descriptors that one skb owns. [1]: https://lore.kernel.org/all/20250530095957.43248-1-e.kubanski@partner.samsung.com/ Signed-off-by: Jason Xing <kernelxing@tencent.com>
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Pull request for series with
subject: xsk: introduce pre-allocated memory per xsk CQ
version: 2
url: https://patchwork.kernel.org/project/netdevbpf/list/?series=1033607