Re: Fuse over io_uring mode cannot handle iodepth > 1 case properly like the default mode

[Bernd Schubert <bernd@bsbernd.com>]

[Added back fsdevel into CC]

Hi Gang,

On 9/18/25 05:05, Gang He wrote:
> Hi Bernd,
> 
> Sorry for interruption again.
> About enable fuse over io_uring feature, I can back-port the related
> kernel patch set.
> But, for user-space libfuse, when will you or the team release a
> tag(e.g. 3.18)? then I can upgrade the libfuse directly, rather than
> back-port patches.
> Second, Fuse over io_uring mode cannot handle iodepth > 1 case
> quickly, I feel this is a by-design issue, the iouring uses the own
> thread queue to handle the requests from the user space. I write a
> kernel patch, which can fix this case, but for the most cases, we
> still use the current design. the patch is attached, could you take a
> look at it, to see if this patch make sense, or not.


I will try to find time to release libfuse-3.18 today, one io-uring
related patch is missing (re-init of some fields in struct fuse_req) and
then just the release process.

Regarding queue balancing, please have a look here:

https://github.com/bsbernd/linux/commits/reduced-nr-ring-queues_3.1/


The tricky part and which is why I didn't publish v2 of this series is
to keep performance for blocking/sync requests (like DIO, metadata,
etc). At DDN we run with an additional workaround patch that disables
migration in fuse_request_end(), which results in kind of
wake_up_on_current_cpu(). Adding in wake_up_on_current_cpu() to fuse is
simple, I have the patch, problem is more that wake_up_on_current_cpu()
is not perfect. I'm just in the middle to walk through scheduler code.
For example, wake_up_on_current_cpu() gets perfect after increasing
/sys/kernel/debug/sched/migration_cost_ns
Now I don't want to change system wide migration costs, but it needs to
be time limited change when waking up from fuse. Additionally it needs
to be a hint, depending on if the queue has more pending work - that is
the part that gets even more complex with reduced queues and queue
balancing.

Also see https://lkml.org/lkml/2023/5/3/646 and related threads for
reference.

Thanks,
Bernd