Re: [PATCH RFC for-next 0/8] io_uring: tw contention improvments

[Hao Xu <hao.xu@linux.dev>]

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?