Re: [Qemu-devel] [PATCH 0/3] rcu: add option to use upstream liburcu

[Paolo Bonzini <pbonzini@redhat.com>]

On 03/02/2015 23:08, Emilio G. Cota wrote:
> * The first two patches bring back the RCU API to exactly
>   match that of liburcu.

Bringing over rcu_dereference/rcu_assign_pointer is unnecessary, I
think.  The names do not conflict.

As to call_rcu/call_rcu1, I understand there is a problem.  Maybe call
the QEMU versions rcu_call/rcu_call1?  Then you can add simple wrappers
if you want to use liburcu-mb.

> * The third patch adds a configure flag to choose from either
>   liburcu or QEMU's RCU.
> 
> Apart from this, I wonder what to do about other valuable bits in
> liburcu, particularly in liburcu-cds, which I'm using currently
> off-tree.  I see three ways of eventually doing this:
> 
> a) Add Windows support in liburcu, thereby eliminating the
>    de facto fork in QEMU.

This would be certainly doable.

Note that liburcu is not widely packaged, so we would have to add it as
a submodule.  What is the advantage of using liburcu?

> b) Bring (fork) liburcu-cds to QEMU, just like liburcu-mb was.

To some extent this is what we're doing.  cds is very much inspired by
the Linux kernel, but QEMU is already using both BSD (intrusive) and
GLib (non-intrusive) lists, and I didn't like the idea of adding yet
another API.  I like the simplicity of the Linux hlist/list APIs, but
two kinds of lists are already one too many.

So, part 2 of the RCU work has an API for RCU lists based on BSD lists
(that QEMU is already using).  These are not the lock-free data
structures available in CDS, just the usual RCU-based lists with
blocking write side and wait-free read side.

QEMU has very limited support for (non-RCU-based) lock-free data
structures in qemu/queue.h: see QSLIST_INSERT_HEAD_ATOMIC and
QSLIST_MOVE_ATOMIC.  The call_rcu implementation in util/rcu.c is based
on wfqueue from liburcu-cds, but it would not be hard to change it to
use QSLIST_INSERT_HEAD_ATOMIC/QSLIST_MOVE_ATOMIC instead.  In both cases
the data structure is multi-producer/single-consumer.

QEMU hardly uses hash tables at all.

Another thing to note is that I didn't envision a huge use of RCU in
QEMU; I'm employing it in decidedly central places where it can bring
great benefit, but I wouldn't be surprised if RCU only found a handful
of users in the end.

Coarse locks with very low contention (such as AioContext) have great
performance, and most data structures in QEMU fit this model: data
structures for separate devices are more or less independent, and the
lock is only needed for the rare cases when the main thread (for example
the monitor) interacts with the device.

> c) Add a compile-time flag (say CONFIG_LIBURCU_CDS), and then only
>    use data structures from liburcu-cds where appropriate, falling
>    back to traditional locked structures when !CONFIG_LIBURCU_CDS.
> 
> Would c) be acceptable for upstream, provided the gains (say in
> scalability/memory footprint) are significant?

I think if there were a killer use of liburcu-cds, we would just port
liburcu (the mb, bp and qsbr variants) to Windows, and switch to
liburcu-mb/liburcu-cds.

This brings the most important question of all: what are you doing with
QEMU? :)

Paolo