Re: [Qemu-devel] [PATCH 2/2] cpu-exec: remove tb_lock from the hot-path

[Emilio G. Cota]

On Mon, Jul 04, 2016 at 12:45:52 +0100, Alex Bennée wrote:
> 
> Emilio G. Cota <address@hidden> writes:
> 
> > On Fri, Jul 01, 2016 at 17:16:10 +0100, Alex Bennée wrote:
> >> Lock contention in the hot path of moving between existing patched
> >> TranslationBlocks is the main drag in multithreaded performance. This
> >> patch pushes the tb_lock() usage down to the two places that really need
> >> it:
> >>
> >>   - code generation (tb_gen_code)
> >>   - jump patching (tb_add_jump)
> >>
> >> The rest of the code doesn't really need to hold a lock as it is either
> >> using per-CPU structures, atomically updated or designed to be used in
> >> concurrent read situations (qht_lookup).
> >>
> >> To keep things simple I removed the #ifdef CONFIG_USER_ONLY stuff as the
> >> locks become NOPs anyway until the MTTCG work is completed.
> >
> > From a scalability point of view it would be better to have a single
> > critical section.
> 
> You mean merge the critical region for patching and code-generation?

Yes, I'd keep the lock held and drop it (if it was held) after the patching
is done, like IIRC we used to do:
(snip)
> > I propose to just extend the critical section, like we used to
> > do with tcg_lock_reset.

                E.