Re: Revert "aio: block exit_aio() until all context requests are completed"

[Jeff Moyer <jmoyer@redhat.com>]

Christian Borntraeger <borntraeger@de.ibm.com> writes:

> I see a significant latency (can be minutes with 2000 disks and HZ=100)
> when exiting a QEMU process that has lots of disk devices via aio. The
> process sits idle doing nothing as zombie in exit_aio waiting for the
> completion.
>
> Turns out that 
> commit 6098b45b32 ("aio: block exit_aio() until all context requests are
> completed") caused the delay.
>
> Patch description was:
>
> It seems that exit_aio() also needs to wait for all iocbs to complete (like
> io_destroy), but we missed the wait step in current implemention, so fix
> it in the same way as we did in io_destroy.
>
> Now: io_destroy requires to block until everything is cleaned up from its
> interface description in the manpage:
> DESCRIPTION
> The  io_destroy()  system call will attempt to cancel all outstanding
> asynchronous I/O operations against ctx_id, will block on the completion
> of all operations that could not be canceled, and will destroy the ctx_id.
>
> Does process exit require the same full blocking? We might be able to
> cleanup the process and let the aio data structures be freed lazily.
> Opinions or better ideas?

This has already been fixed:

commit dc48e56d761610da4ea1088d1bea0a030b8e3e43
Author: Jens Axboe <axboe@fb.com>
Date:   Wed Apr 15 11:17:23 2015 -0600

    aio: fix serial draining in exit_aio()

Cheers,
Jeff