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Date: Mon, 4 Jul 2016 18:30:01 -0400
From: "Emilio G. Cota" <cota@braap.org>
Message-ID: <20160704223001.GA29624@flamenco>
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Subject: Re: [Qemu-devel] [PATCH 2/2] cpu-exec: remove tb_lock from the
 hot-path
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To: Alex =?iso-8859-1?Q?Benn=E9e?= <alex.bennee@linaro.org>
Cc: mttcg@listserver.greensocs.com, qemu-devel@nongnu.org, fred.konrad@greensocs.com, a.rigo@virtualopensystems.com, serge.fdrv@gmail.com, bobby.prani@gmail.com, rth@twiddle.net, mark.burton@greensocs.com, pbonzini@redhat.com, jan.kiszka@siemens.com, peter.maydell@linaro.org, claudio.fontana@huawei.com, Peter Crosthwaite <crosthwaite.peter@gmail.com>

On Mon, Jul 04, 2016 at 12:45:52 +0100, Alex Bennée wrote:
> 
> Emilio G. Cota <cota@braap.org> 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.