Re: [PATCH 2/5] io_uring: mark exit side kworkers as task_work capable

[Jens Axboe <axboe@kernel.dk>]

On 4/11/25 7:55 AM, Christian Brauner wrote:
> On Wed, Apr 09, 2025 at 07:35:20AM -0600, Jens Axboe wrote:
>> There are two types of work here:
>>
>> 1) Fallback work, if the task is exiting
>> 2) The exit side cancelations
>>
>> and both of them may do the final fput() of a file. When this happens,
>> fput() will schedule delayed work. This slows down exits when io_uring
> 
> I was a bit surprised by this because it means that all those __fput()s
> are done with kthread credentials which is a bit surprising (but
> harmless afaict).

Sure hope it is, because that's already what happens off the delayed
fput that it'd otherwise go through!

>> needs to wait for that work to finish. It is possible to flush this via
>> flush_delayed_fput(), but that's a big hammer as other unrelated files
>> could be involved, and from other tasks as well.
>>
>> Add two io_uring helpers to temporarily clear PF_NO_TASKWORK for the
>> worker threads, and run any queued task_work before setting the flag
>> again. Then we can ensure we only flush related items that received
>> their final fput as part of work cancelation and flushing.
> 
> Ok, so the only change is that this isn't offloaded to the global
> delayed fput workqueue but to the task work that you're running off of
> your kthread helpers.

Exactly

-- 
Jens Axboe