Patchwork Re: Role of qemu_fair_mutex

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Submitter Jan Kiszka
Date Jan. 3, 2011, 10:03 a.m.
Message ID <4D219F06.7040305@web.de>
Download mbox | patch
Permalink /patch/77242/
State New
Headers show

Comments

Jan Kiszka - Jan. 3, 2011, 10:03 a.m.
Am 03.01.2011 11:01, Avi Kivity wrote:
> On 01/03/2011 11:46 AM, Jan Kiszka wrote:
>> Hi,
>>
>> at least in kvm mode, the qemu_fair_mutex seems to have lost its
>> function of balancing qemu_global_mutex access between the io-thread and
>> vcpus. It's now only taken by the latter, isn't it?
>>
>> This and the fact that qemu-kvm does not use this kind of lock made me
>> wonder what its role is and if it is still relevant in practice. I'd
>> like to unify the execution models of qemu-kvm and qemu, and this lock
>> is the most obvious difference (there are surely more subtle ones as
>> well...).
>>
> 
> IIRC it was used for tcg, which has a problem that kvm doesn't have: a
> tcg vcpu needs to hold qemu_mutex when it runs, which means there will
> always be contention on qemu_mutex.  In the absence of fairness, the tcg
> thread could dominate qemu_mutex and starve the iothread.
> 
> This doesn't happen with kvm since kvm vcpus drop qemu_mutex when running.
> 

I see. Then I guess we should do this:


Jan
Avi Kivity - Jan. 3, 2011, 10:08 a.m.
On 01/03/2011 12:03 PM, Jan Kiszka wrote:
> Am 03.01.2011 11:01, Avi Kivity wrote:
> >  On 01/03/2011 11:46 AM, Jan Kiszka wrote:
> >>  Hi,
> >>
> >>  at least in kvm mode, the qemu_fair_mutex seems to have lost its
> >>  function of balancing qemu_global_mutex access between the io-thread and
> >>  vcpus. It's now only taken by the latter, isn't it?
> >>
> >>  This and the fact that qemu-kvm does not use this kind of lock made me
> >>  wonder what its role is and if it is still relevant in practice. I'd
> >>  like to unify the execution models of qemu-kvm and qemu, and this lock
> >>  is the most obvious difference (there are surely more subtle ones as
> >>  well...).
> >>
> >
> >  IIRC it was used for tcg, which has a problem that kvm doesn't have: a
> >  tcg vcpu needs to hold qemu_mutex when it runs, which means there will
> >  always be contention on qemu_mutex.  In the absence of fairness, the tcg
> >  thread could dominate qemu_mutex and starve the iothread.
> >
> >  This doesn't happen with kvm since kvm vcpus drop qemu_mutex when running.
> >
>
> I see. Then I guess we should do this:
>
> diff --git a/cpus.c b/cpus.c
> index 9bf5224..0de8552 100644
> --- a/cpus.c
> +++ b/cpus.c
> @@ -734,9 +734,7 @@ static sigset_t block_io_signals(void)
>   void qemu_mutex_lock_iothread(void)
>   {
>       if (kvm_enabled()) {
> -        qemu_mutex_lock(&qemu_fair_mutex);
>           qemu_mutex_lock(&qemu_global_mutex);
> -        qemu_mutex_unlock(&qemu_fair_mutex);
>       } else {
>           qemu_mutex_lock(&qemu_fair_mutex);
>           if (qemu_mutex_trylock(&qemu_global_mutex)) {

I think so, though Anthony or Marcelo should confirm my interpretation 
first.

Patch

diff --git a/cpus.c b/cpus.c
index 9bf5224..0de8552 100644
--- a/cpus.c
+++ b/cpus.c
@@ -734,9 +734,7 @@  static sigset_t block_io_signals(void)
 void qemu_mutex_lock_iothread(void)
 {
     if (kvm_enabled()) {
-        qemu_mutex_lock(&qemu_fair_mutex);
         qemu_mutex_lock(&qemu_global_mutex);
-        qemu_mutex_unlock(&qemu_fair_mutex);
     } else {
         qemu_mutex_lock(&qemu_fair_mutex);
         if (qemu_mutex_trylock(&qemu_global_mutex)) {