Re: [PATCHSET 0/4] Use io_wq_work_list for task_work

[Pavel Begunkov <asml.silence@gmail.com>]

On 3/26/24 18:42, Jens Axboe wrote:
> Hi,
> 
> This converts the deferred, normal, and fallback task_work to use a
> normal io_wq_work_list, rather than an llist.
> 
> The main motivation behind this is to get rid of the need to reverse
> the list once it's deleted and run. I tested this basic conversion of
> just switching it from an llist to an io_wq_work_list with a spinlock,
> and I don't see any benefits from the lockless list. And for cases where
> we get a bursty addition of task_work, this approach is faster as it
> avoids the need to iterate the list upfront while reversing it.

I'm curious how you benchmarked it including accounting of irq/softirq
where tw add usually happens?

One known problem with the current list approach I mentioned several
times before is that it peeks at the previous queued tw to count them.
It's not nice, but that can be easily done with cmpxchg double. I
wonder how much of an issue is that.

> And this is less code and simpler, so I'd prefer to go that route.

I'm not sure it's less code, if you return optimisations that I
believe were killed, see comments to patch 2, it might turn out to
be even bulkier and not that simpler.


>   include/linux/io_uring_types.h |  13 +--
>   io_uring/io_uring.c            | 175 ++++++++++++++++-----------------
>   io_uring/io_uring.h            |   8 +-
>   io_uring/sqpoll.c              |   8 +-
>   io_uring/tctx.c                |   3 +-
>   5 files changed, 102 insertions(+), 105 deletions(-)
> 

-- 
Pavel Begunkov