From: David Gibson <david@gibson.dropbear.id.au>
To: Nicholas Piggin <npiggin@gmail.com>
Cc: "Greg Kurz" <groug@kaod.org>,
qemu-ppc@nongnu.org, qemu-devel@nongnu.org,
"Cédric Le Goater" <clg@kaod.org>
Subject: Re: [Qemu-devel] [RFC PATCH] Implement qemu_thread_yield for posix, use it in mttcg to handle EXCP_YIELD
Date: Wed, 17 Jul 2019 21:48:54 +1000 [thread overview]
Message-ID: <20190717114854.GA8468@umbus.fritz.box> (raw)
In-Reply-To: <20190717054655.14104-1-npiggin@gmail.com>
[-- Attachment #1: Type: text/plain, Size: 4123 bytes --]
On Wed, Jul 17, 2019 at 03:46:55PM +1000, Nicholas Piggin wrote:
> This is a bit of proof of concept in case mttcg becomes more important
> yield could be handled like this. You can have by accident or deliberately
> force vCPUs onto the same physical CPU and cause inversion issues when the
> lock holder was preempted by the waiter. This is lightly tested but not
> to the point of measuring performance difference.
>
> I really consider the previous confer/prod patches more important just to
> provide a more complete guest environment and better test coverage, than
> performance, but maybe someone wants to persue this.
>
> Thanks,
> Nick
> ---
> cpus.c | 6 ++++++
> hw/ppc/spapr_hcall.c | 14 +++++++-------
> include/qemu/thread.h | 1 +
> util/qemu-thread-posix.c | 5 +++++
> util/qemu-thread-win32.c | 4 ++++
> 5 files changed, 23 insertions(+), 7 deletions(-)
>
> diff --git a/cpus.c b/cpus.c
> index 927a00aa90..f036e062d9 100644
> --- a/cpus.c
> +++ b/cpus.c
> @@ -1760,6 +1760,12 @@ static void *qemu_tcg_cpu_thread_fn(void *arg)
> qemu_mutex_unlock_iothread();
> cpu_exec_step_atomic(cpu);
> qemu_mutex_lock_iothread();
> + break;
> + case EXCP_YIELD:
> + qemu_mutex_unlock_iothread();
> + qemu_thread_yield();
> + qemu_mutex_lock_iothread();
> + break;
> default:
> /* Ignore everything else? */
> break;
> diff --git a/hw/ppc/spapr_hcall.c b/hw/ppc/spapr_hcall.c
> index 57c1ee0fe1..9c24a64dfe 100644
> --- a/hw/ppc/spapr_hcall.c
> +++ b/hw/ppc/spapr_hcall.c
> @@ -1162,13 +1162,13 @@ static target_ulong h_confer(PowerPCCPU *cpu, SpaprMachineState *spapr,
> return H_SUCCESS;
> }
>
> - /*
> - * The targeted confer does not do anything special beyond yielding
> - * the current vCPU, but even this should be better than nothing.
> - * At least for single-threaded tcg, it gives the target a chance to
> - * run before we run again. Multi-threaded tcg does not really do
> - * anything with EXCP_YIELD yet.
> - */
> + /*
> + * The targeted confer does not do anything special beyond yielding
> + * the current vCPU, but even this should be better than nothing.
> + * For single-threaded tcg, it gives the target a chance to run
> + * before we run again, multi-threaded tcg will yield the CPU to
> + * another vCPU.
> + */
Uh.. this looks like a whitespace fixup leaked in from your other patches.
> }
>
> cs->exception_index = EXCP_YIELD;
> diff --git a/include/qemu/thread.h b/include/qemu/thread.h
> index 55d83a907c..8525b0a70a 100644
> --- a/include/qemu/thread.h
> +++ b/include/qemu/thread.h
> @@ -160,6 +160,7 @@ void qemu_thread_get_self(QemuThread *thread);
> bool qemu_thread_is_self(QemuThread *thread);
> void qemu_thread_exit(void *retval);
> void qemu_thread_naming(bool enable);
> +void qemu_thread_yield(void);
>
> struct Notifier;
> /**
> diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c
> index 1bf5e65dea..91b12a1082 100644
> --- a/util/qemu-thread-posix.c
> +++ b/util/qemu-thread-posix.c
> @@ -573,3 +573,8 @@ void *qemu_thread_join(QemuThread *thread)
> }
> return ret;
> }
> +
> +void qemu_thread_yield(void)
> +{
> + pthread_yield();
> +}
> diff --git a/util/qemu-thread-win32.c b/util/qemu-thread-win32.c
> index 572f88535d..72fe406bef 100644
> --- a/util/qemu-thread-win32.c
> +++ b/util/qemu-thread-win32.c
> @@ -442,3 +442,7 @@ bool qemu_thread_is_self(QemuThread *thread)
> {
> return GetCurrentThreadId() == thread->tid;
> }
> +
> +void qemu_thread_yield(void)
> +{
> +}
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 833 bytes --]
next prev parent reply other threads:[~2019-07-17 13:24 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-17 5:46 [Qemu-devel] [RFC PATCH] Implement qemu_thread_yield for posix, use it in mttcg to handle EXCP_YIELD Nicholas Piggin
2019-07-17 11:48 ` David Gibson [this message]
2019-12-20 13:11 ` Alex Bennée
2020-01-21 11:20 ` Nicholas Piggin
2020-01-21 14:37 ` Alex Bennée
2020-01-22 3:26 ` David Gibson
2020-01-22 18:01 ` Richard Henderson
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=20190717114854.GA8468@umbus.fritz.box \
--to=david@gibson.dropbear.id.au \
--cc=clg@kaod.org \
--cc=groug@kaod.org \
--cc=npiggin@gmail.com \
--cc=qemu-devel@nongnu.org \
--cc=qemu-ppc@nongnu.org \
/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 a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).