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Re: [Qemu-devel] [PATCH 2/2] cpu-exec: remove tb_lock from the hot-path
From: |
Alex Bennée |
Subject: |
Re: [Qemu-devel] [PATCH 2/2] cpu-exec: remove tb_lock from the hot-path |
Date: |
Mon, 04 Jul 2016 12:45:52 +0100 |
User-agent: |
mu4e 0.9.17; emacs 25.0.95.7 |
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?
>
> From a correctness point of view, we're reading tb->page_addr[1]
> without holding a lock. This field is set after qht_insert(tb),
> so we might read a yet-uninitialized value.
>
> I propose to just extend the critical section, like we used to
> do with tcg_lock_reset.
>
> Emilio
--
Alex Bennée
- [Qemu-devel] [PATCH 0/2] Reduce lock contention on TCG hot-path, Alex Bennée, 2016/07/01
- [Qemu-devel] [PATCH 2/2] cpu-exec: remove tb_lock from the hot-path, Alex Bennée, 2016/07/01
- Re: [Qemu-devel] [PATCH 2/2] cpu-exec: remove tb_lock from the hot-path, Richard Henderson, 2016/07/01
- Re: [Qemu-devel] [PATCH 2/2] cpu-exec: remove tb_lock from the hot-path, Emilio G. Cota, 2016/07/01
- Re: [Qemu-devel] [PATCH 2/2] cpu-exec: remove tb_lock from the hot-path,
Alex Bennée <=
- Re: [Qemu-devel] [PATCH 2/2] cpu-exec: remove tb_lock from the hot-path, Emilio G. Cota, 2016/07/04
- Re: [Qemu-devel] [PATCH 2/2] cpu-exec: remove tb_lock from the hot-path, Alex Bennée, 2016/07/05
- Re: [Qemu-devel] [PATCH 2/2] cpu-exec: remove tb_lock from the hot-path, Paolo Bonzini, 2016/07/05
- Re: [Qemu-devel] [PATCH 2/2] cpu-exec: remove tb_lock from the hot-path, Alex Bennée, 2016/07/05
- Re: [Qemu-devel] [PATCH 2/2] cpu-exec: remove tb_lock from the hot-path, Paolo Bonzini, 2016/07/05
- Re: [Qemu-devel] [PATCH 2/2] cpu-exec: remove tb_lock from the hot-path, Sergey Fedorov, 2016/07/05
- Re: [Qemu-devel] [PATCH 2/2] cpu-exec: remove tb_lock from the hot-path, Alex Bennée, 2016/07/05
- Re: [Qemu-devel] [PATCH 2/2] cpu-exec: remove tb_lock from the hot-path, Sergey Fedorov, 2016/07/05
[Qemu-devel] [PATCH 1/2] tcg: Ensure safe tb_jmp_cache lookup out of 'tb_lock', Alex Bennée, 2016/07/01