Re: [Qemu-devel] [PATCH v6 72/73] cpu: add async_run_on_cpu_no_bql

["Emilio G. Cota" <cota@braap.org>]

On Fri, Feb 08, 2019 at 14:58:40 +0000, Alex Bennée wrote:
> 
> Emilio G. Cota <cota@braap.org> writes:
> 
> > Some async jobs do not need the BQL.
> >
> > Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
> > Signed-off-by: Emilio G. Cota <cota@braap.org>
(snip)
> So we now have a locking/scheduling hierarchy that goes:
> 
>   - run_on_cpu - synchronously wait until target cpu has done the thing
>   - async_run_on_cpu - schedule work on cpu at some point (soon) resources protected by BQL
>   - async_run_on_cpu_no_bql - as above but only protected by cpu_lock

This one doesn't hold any locks when calling the passed function,
though. The CPU lock is just to serialise the queueing/dequeueing.

>   - async_safe_run_on_cpu - as above but locking (probably) not required as everything else asleep
> 
> So the BQL is only really needed to manipulate data that is shared
> across multiple vCPUs like device emulation or other state shared across
> multiple vCPUS. For all "just do it over there" cases we should be able
> to stick to cpu locks.
> 
> It would be nice if we could expand the documentation in
> multi-thread-tcg.txt to cover this in long form for people trying to
> work out the best thing to use.

I think the documentation in the functions is probably enough -- they
point to each other, so figuring out what to use should be pretty easy.

Besides, these functions aren't MTTCG-only, they're QEMU-wide, so
perhaps their documentation should go in a different file?

Since this can be done later, I'll post a v7 with the other
changes you suggested.

> Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

Thanks!

		Emilio