[Qemu-devel] [RFC/ PATCH 0/4] qemu: Extend AIO threading framework to a generic one.

[Gautham R Shenoy]

Hi,

This patch series is strictly RFC only.

It decouples the asynchrnous threading framework implementation
from posix-aio-compat.c to implement a generic asynchrnous task
offloading threading framework which can be used by other subsystems
within QEMU.

Currently within QEMU, the AIO subsystem (paio) creates a bunch of
asynchronous threads to offload any blocking operations so that
the vcpu threads and the IO thread can go back to servicing any
other guest requests.

We would want to make use of this offloading framework in case of
the virtio-9p subsystem, where so that the vcpu thread can offload 
POSIX operations on to the asynchronous threads and resume servicing any other
guest requests. The asynchrnous threads, after finishing the POSIX operations
can then transfer the control over to the IO thread so that the latter
can handle the post_posix_operation().

The post_posix_operation() could in turn offload more work to be handled by
the asynnchronous thread pool or sends the results of the operation to the
guest.

The patch series also implements a patch that converts v9fs_stat() call
to make use of the asynchrnous threading framework. This is an example of how
the other calls could be converted eventually.

I had a doubt with respect to the original code where the communication between
the asynchronous aio threads and the io-thread occurs.

In posix-aio-compat.c, we do not do a write(wfd) from the context of the
asynchrnous thread, but instead send a SIGUSR2, the handler of which
does a write(wfd), thereby causing the iothread blocked on select() to unblock.

Why do we send a signal, and not execute whatever aio_signal_handler()
is from the context of the asynchronous thread ? Is it to ensure that the
io-thread does not miss any write(wfd) ?

In this patch series, I have made the virtio-9p make use of SIGUSR1 to
mimic similar behaviour. This is just a temporary piece of code to get
virtio-9p use the async framework for now.

The patch series passed fsstress test without any issues.

Awaiting your comments.
---

Aneesh Kumar K.V (1):
      qemu: Generic asynchronous threading framework to offload tasks

Gautham R Shenoy (3):
      qemu: Convert AIO code to use the generic threading infrastructure.
      virtio-9p: Add async helper functions
      virtio-9p: convert lstat to use async infrastructure.


 Makefile.objs      |    2 +
 async-work.c       |  152 +++++++++++++++++++++++++++++++++++++++++
 async-work.h       |   85 +++++++++++++++++++++++
 hw/virtio-9p.c     |  195 ++++++++++++++++++++++++++++++++++++++++++++++++++--
 hw/virtio-9p.h     |    4 +
 posix-aio-compat.c |  155 ++++++++---------------------------------
 6 files changed, 458 insertions(+), 135 deletions(-)
 create mode 100644 async-work.c
 create mode 100644 async-work.h

-- 
Thanks and Regards
gautham.