All of lore.kernel.org
 help / color / mirror / Atom feed
From: "Emilio G. Cota" <cota@braap.org>
To: "Alex Bennée" <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>
Subject: Re: [Qemu-devel] [PATCH 2/2] cpu-exec: remove tb_lock from the hot-path
Date: Fri, 1 Jul 2016 20:39:42 -0400	[thread overview]
Message-ID: <20160702003942.GB2295@flamenco> (raw)
In-Reply-To: <1467389770-9738-3-git-send-email-alex.bennee@linaro.org>

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.

>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

  parent reply	other threads:[~2016-07-02  0:39 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-07-01 16:16 [Qemu-devel] [PATCH 0/2] Reduce lock contention on TCG hot-path Alex Bennée
2016-07-01 16:16 ` [Qemu-devel] [PATCH 1/2] tcg: Ensure safe tb_jmp_cache lookup out of 'tb_lock' Alex Bennée
2016-07-01 23:14   ` Richard Henderson
2016-07-02  0:17   ` Emilio G. Cota
2016-07-02  0:32     ` Richard Henderson
2016-07-04 22:33       ` Emilio G. Cota
2016-07-02  7:09     ` Alex Bennée
2016-07-04 22:31       ` Emilio G. Cota
2016-07-05 12:49         ` Paolo Bonzini
2016-07-01 16:16 ` [Qemu-devel] [PATCH 2/2] cpu-exec: remove tb_lock from the hot-path Alex Bennée
2016-07-01 23:19   ` Richard Henderson
2016-07-02  0:39   ` Emilio G. Cota [this message]
2016-07-04 11:45     ` Alex Bennée
2016-07-04 22:30       ` Emilio G. Cota
2016-07-05 11:14         ` Alex Bennée
2016-07-05 12:47           ` Paolo Bonzini
2016-07-05 13:11             ` Alex Bennée
2016-07-05 13:42               ` Paolo Bonzini
2016-07-05 15:34                 ` Sergey Fedorov
2016-07-05 16:00                   ` Alex Bennée
2016-07-05 16:04                     ` Sergey Fedorov
2016-07-02  0:52 ` [Qemu-devel] [PATCH 0/2] Reduce lock contention on TCG hot-path Emilio G. Cota
2016-07-02  7:08   ` Alex Bennée
2016-07-02 16:03     ` Paolo Bonzini

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20160702003942.GB2295@flamenco \
    --to=cota@braap.org \
    --cc=a.rigo@virtualopensystems.com \
    --cc=alex.bennee@linaro.org \
    --cc=bobby.prani@gmail.com \
    --cc=claudio.fontana@huawei.com \
    --cc=crosthwaite.peter@gmail.com \
    --cc=fred.konrad@greensocs.com \
    --cc=jan.kiszka@siemens.com \
    --cc=mark.burton@greensocs.com \
    --cc=mttcg@listserver.greensocs.com \
    --cc=pbonzini@redhat.com \
    --cc=peter.maydell@linaro.org \
    --cc=qemu-devel@nongnu.org \
    --cc=rth@twiddle.net \
    --cc=serge.fdrv@gmail.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body. 
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.