On Tue, 2022-06-21 at 13:10 +0800, Hao Xu wrote: > On 6/21/22 00:18, Dylan Yudaken wrote: > > Task work currently uses a spin lock to guard task_list and > > task_running. Some use cases such as networking can trigger > > task_work_add > > from multiple threads all at once, which suffers from contention > > here. > > > > This can be changed to use a lockless list which seems to have > > better > > performance. Running the micro benchmark in [1] I see 20% > > improvment in > > multithreaded task work add. It required removing the priority tw > > list > > optimisation, however it isn't clear how important that > > optimisation is. > > Additionally it has fairly easy to break semantics. > > > > Patch 1-2 remove the priority tw list optimisation > > Patch 3-5 add lockless lists for task work > > Patch 6 fixes a bug I noticed in io_uring event tracing > > Patch 7-8 adds tracing for task_work_run > > > > Compared to the spinlock overhead, the prio task list optimization is > definitely unimportant, so I agree with removing it here. > Replace the task list with llisy was something I considered but I > gave > it up since it changes the list to a stack which means we have to > handle > the tasks in a reverse order. This may affect the latency, do you > have > some numbers for it, like avg and 99% 95% lat? > Do you have an idea for how to test that? I used a microbenchmark as well as a network benchmark [1] to verify that overall throughput is higher. TW latency sounds a lot more complicated to measure as it's difficult to trigger accurately. My feeling is that with reasonable batching (say 8-16 items) the latency will be low as TW is generally very quick, but if you have an idea for benchmarking I can take a look [1]: https://github.com/DylanZA/netbench
