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Re: [Qemu-devel] [PATCH v3.1 00/31] NUMA series, and hostmem improvement


From: Hu Tao
Subject: Re: [Qemu-devel] [PATCH v3.1 00/31] NUMA series, and hostmem improvements
Date: Tue, 13 May 2014 10:12:24 +0800
User-agent: Mutt/1.5.21 (2010-09-15)

On Fri, May 09, 2014 at 02:54:15PM -0300, Eduardo Habkost wrote:
> On Fri, May 09, 2014 at 04:29:49PM +0800, Hu Tao wrote:
> > On Thu, May 08, 2014 at 04:51:56PM +0200, Paolo Bonzini wrote:
> > > Il 06/05/2014 11:27, Hu Tao ha scritto:
> > > > This series includes work on QOMifying the memory backends.
> > > > the idea is to delegate all properties of the memory backend to
> > > > a new QOM class hierarchy, in which the concrete classes
> > > > are hostmem-ram and hostmem-file.  The backend is passed to the
> > > > machine via "-numa node,memdev=foo" where "foo" is the id of the
> > > > backend object.
> > > 
> > > Hello,
> > > 
> > > I noticed now that if you have the host-nodes property set Linux
> > > requires you to set a policy other than "default" too.  If you don't,
> > > the mbind system call fails.
> > > 
> > > What about squashing something like this?
> > > 
> > > Paolo
> > > 
> > > diff --git a/backends/hostmem.c b/backends/hostmem.c
> > > index d3f8476..a0a3111 100644
> > > --- a/backends/hostmem.c
> > > +++ b/backends/hostmem.c
> > > @@ -299,12 +299,23 @@ host_memory_backend_memory_init(UserCreatable *uc, 
> > > Error **errp)
> > >  
> > >  #ifdef CONFIG_NUMA
> > >      unsigned long maxnode = find_last_bit(backend->host_nodes, 
> > > MAX_NODES);
> > > +    unsigned policy = backend->policy;
> > > +
> > > +    /* Linux does not accept MPOL_DEFAULT with nonzero bitmap, but
> > > +     * "-object memory-ram,size=128M,hostnodes=0,policy=bind" is a
> > > +     * bit of a mouthful.  So if the host_nodes bitmap is nonzero,
> > > +     * pick the BIND policy.
> 
> Are we sure MPOL_BIND is a better default than MPOL_INTERLEAVE or
> MPOL_PREFERRED?
> 
> 
> > > +     */
> > > +    if (find_first_bit(backend->host_nodes, MAX_NODES) != MAX_NODES &&
> > > +        policy == MPOL_DEFAULT) {
> > > +        policy = MPOL_BIND;
> > > +    }
> > 
> > This only changes the policy applied to memory. But info memdev still
> > shows the policy as 'default'(the 'default' here is interpreted as
> > 'MPOL_DEFAULT', right?).
> 
> That's what I would expect. With this, the user can't even check what is
> the actual result of the command-line they used.
> 
> > 
> > So this is related to how we deal with cases where 'policy' is not
> > specified in qemu:
> > 
> > 1. if possible, the policy is set to MPOL_DEFAULT.
> > 2. if host-nodes is not zero, the policy is set to MPOL_BIND.(set
> >    backend->policy too, other than just apply it)
> 
> You mean, only if policy is omitted, right? host-nodes may be set if the
> user wants policy=preferred or policy=interleave, too.

Yes.

> 
> > 
> > Opinions?
> 
> I find it confusing that an explicit policy=default option won't always
> mean MPOL_DEFAULT.
> 
> So, what about:
> 
>  * If policy=default is set, it is always going to be MPOL_DEFAULT.
>  * If policy=bind is set, it is always going to be MPOL_BIND.
>  * if policy=preferred is set, it is always going to be MPOL_PREFERRED.
>  * If policy is omitted, it will be be MPOL_DEFAULT is host-nodes is
>    unset, and MPOL_BIND if host_nodes is set.

Agreed.


Regards,
Hu Tao



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