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The spinlock's? >>> >>> The queue data needs to move from one CPU to the other in either case. >>> It's the same data that is being moved with spinlock protection. >>> >>> A spinlock doesn't lead to more cache line moves as long as there's >>> still just a single consumer / producer. >> >> Looking at a couple of examples: >> >> 1. spinlock + double linked list (which is what the scheduler used initially). >> >>    You have to touch 3-4 different cache lines, the lock, the previous, the current and the next element (next and prev are usually the same with the lock). > > list when pushing: > > Lock + head (same cache line) + head->next > head->next->next > > when popping: > > Lock + head + head->previous > head->previous->previous > > I don't see why you need a "current" element when you're always only > touching head or tail. The current element is the one you insert or remove. >> >> 2. kfifo (attempt #2): >> >>    3 cache lines, one for the array, one for the rptr/wptr and one for the element. >>    Plus the problem that you need to come up with some upper bound for it. >> >> 3. spsc (attempt #3) >> >>    2-3 cache lines, one for the queue (head/tail), one for the element and one for the previous element (but it is quite likely that this is pre-fetched). >> >> Saying this I just realized we could potentially trivially replace the spsc with an single linked list+pointer to the end+spinlock and have the same efficiency. We don't need all the lockless stuff for that at all. >> > > Now we're speaking mostly the same language :] > > If you could RB my DRM TODO patches we'd have a section for drm/sched, > and there we could then soonish add an item for getting rid of spsc. > > https://lore.kernel.org/dri-devel/20251107135701.244659-2-phasta@kernel.org/ I can't find that in my inbox anywhere. Can you send it out one more with my AMD mail address on explicit CC? Thanks in advance. >>>> The problem is really to separate the push from the pop side so that as few cache lines as possible are transferred from one CPU to another. >>> >>> With a doubly linked list you can attach at the front and pull from the >>> tail. How is that transferring many cache lines? >> >> See above. >> >> We have some tests for old and trivial use cases (e.g. GLmark2) which on todays standards pretty much only depend on how fast you can push things to the HW. >> >> We could just extend the scheduler test cases to see how many submissions per second we can pump through a dummy implementation where both producer and consumer are nailed to separate CPUs. >> > > I disagree. That would be a microbenchmark for a very narrow use case. That is actually a rather common use case (unfortunately). > It would only tell us that a specific patch slows things done for the > microbenchmark, and we could only detect that if a developer runs the > unit tests with and without his patches. I could trigger adding that to AMDs CI systems. > > The few major reworks that touch such essentials have good realistic > tests anyways, see Tvrtko's CFS series. > > > Lockless magic should always be justified by real world use cases. > > By the way, back when spsc_queue was implemented, how large were the > real world performance gains you meassured by saving that 1 cache line? That was actually quite a bit. If you want a real world test case use glMark2 on any modern HW. And yeah I know how ridicules that is, the problem is that we still have people using this as indicator for the command submission overhead. Regards, Christian. > > > P.