From: David S. Miller Date: Wed, 15 Jun 2016 20:58:34 +0000 (-0700) Subject: Merge branch 'skb_array' X-Git-Url: https://git.stricted.de/?a=commitdiff_plain;h=829e64d16026d381528fe08a65e45942297833c6;p=GitHub%2FLineageOS%2FG12%2Fandroid_kernel_amlogic_linux-4.9.git Merge branch 'skb_array' Michael S. Tsirkin says: ==================== skb_array: array based FIFO for skbs This is in response to the proposal by Jason to make tun rx packet queue lockless using a circular buffer. My testing seems to show that at least for the common usecase in networking, which isn't lockless, circular buffer with indices does not perform that well, because each index access causes a cache line to bounce between CPUs, and index access causes stalls due to the dependency. By comparison, an array of pointers where NULL means invalid and !NULL means valid, can be updated without messing up barriers at all and does not have this issue. On the flip side, cache pressure may be caused by using large queues. tun has a queue of 1000 entries by default and that's 8K. At this point I'm not sure this can be solved efficiently. The correct solution might be sizing the queues appropriately. Here's an implementation of this idea: it can be used more or less whenever sk_buff_head can be used, except you need to know the queue size in advance. As this might be useful outside of networking, I implemented a generic array of void pointers, with a type-safe wrapper for skbs. It remains to be seen whether resizing is required, in case it is I included patches implementing resizing by holding both the consumer and the producer locks. I think this code works fine without any extra memory barriers since we always read and write the same location, so the accesses can not be reordered. Multiple writes of the same value into memory would mess things up for us, I don't think compilers would do it though. But if people feel it's better to be safe wrt compiler optimizations, specifying queue as volatile would probably do it in a cleaner way than converting all accesses to READ_ONCE/WRITE_ONCE. Thoughts? The only issue is with calls within a loop using the __ptr_ring_XXX accessors - in theory compiler could hoist accesses out of the loop. Following volatile-considered-harmful.txt I merely documented that callers that busy-poll should invoke cpu_relax(). Most people will use the external skb_array_XXX APIs with a spinlock, so this should not be an issue for them. Eric Dumazet suggested adding an extra pointer to skb for when we have a single outstanding packet. I could not figure out a way to implement this without a shared consumer/producer lock though, which would cause cache line bounces by itself. Jesper, Jason, I know that both of you tested this, please post Tested-by tags for whatever was tested. changes since v7 fix typos noticed by Jesper Brouer changes since v6 resize implemented. peek/full calls are no longer lockless replaced _FIELD macros with _CALL which invoke a function on the pointer rather than just returning a value destroy now scans the array and frees all queued skbs changes since v5 implemented a generic ptr_ring api, and made skb_array a type-safe wrapper apis for taking the spinlock in different contexts following expected usecase in tun changes since v4 (v3 was never posted) documentation dropped SKB_ARRAY_MIN_SIZE heuristic unit test (in userspace, included as patch 2) changes since v2: fixed integer overflow pointed out by Eric. added some comments. changes since v1: fixed bug pointed out by Eric. ==================== Tested-by: Jesper Dangaard Brouer Signed-off-by: David S. Miller --- 829e64d16026d381528fe08a65e45942297833c6