To SMP or not to SMP

[John Baldwin]

On Monday, January 14, 2013 4:07:56 pm Konstantin Belousov wrote:
> On Mon, Jan 14, 2013 at 03:07:50PM -0500, John Baldwin wrote:
> > On Sunday, January 13, 2013 1:15:13 am Bryan Venteicher wrote:
> > > 
> > > ----- Original Message -----
> > > > From: "John Baldwin" <jhb at freebsd.org>
> > > > To: freebsd-net at freebsd.org
> > > > Cc: "Barney Cordoba" <barney_cordoba at yahoo.com>, "Peter Jeremy" 
<peter at rulingia.com>
> > > > Sent: Friday, January 11, 2013 9:39:17 AM
> > > > Subject: Re: To SMP or not to SMP
> > > > 
> > > > On Thursday, January 10, 2013 02:36:59 PM Peter Jeremy wrote:
> > > > > On 2013-Jan-07 18:25:58 -0800, Barney Cordoba
> > > > > <barney_cordoba at yahoo.com>
> > > > wrote:
> > > > > >I have a situation where I have to run 9.1 on an old single core
> > > > > >box. Does anyone have a handle on whether it's better to build a
> > > > > >non
> > > > > >SMP kernel or to just use a standard SMP build with just the one
> > > > > >core?
> > > > > 
> > > > > Another input for this decision is kern/173322.  Currently on x86,
> > > > > atomic operations within kernel modules are implemented using calls
> > > > > to code in the kernel, which do or don't use lock prefixes
> > > > > depending
> > > > > on whethur the kernel was built as SMP.  My proposed change changes
> > > > > kernel modules to inline atomic operations but always include lock
> > > > > prefixes (effectively reverting r49999).  I'm appreciate anyone who
> > > > > feels like testing the impact of this change.
> > > > 
> > > > Presumably a locked atomic op is cheaper than a function call then?
> > > >  The
> > > > current setup assumes the opposite.
> > > > 
> > > > I think we should actually do this for atomics in modules on x86:
> > > > 
> > > > 1) If a module is built standalone, it should do whichever is cheaper:
> > > >    a function call or always use "LOCK".
> > > > 
> > > > 2) If a module is built as part of the kernel build, it should use 
inlined
> > > >    atomics that match what the kernel does.  Thus, modules built with 
a
> > > >    non-SMP kernel would use inlined atomic ops that do not use LOCK.  
We
> > > >    have a way to detect this now (some HAVE_FOO #define added in the 
past
> > > >    few years) that we didn't back when this bit of atomic.h was
> > > >    written.
> > > >
> > > 
> > > It would be nice to have the LOCK variants available even on UP
> > > kernels in non-hackish way. For VirtIO, we need to handle an guest
> > > UP kernel running on an SMP host. Whether this is an #define that
> > > forces the SMP atomics to be inlined, or if they're exposed with
> > > an _smp suffix. 
> Could you please, clarify why does UP kernel needs it ?
> Shouldn't the hypervisor context switching provide neccessary serialization
> anyway ?

I thought this, too, but in the case of virtio you are presumably sychronizing 
with other threads in the hypervisor itself which might be running 
concurrently on another physical CPU.

-- 
John Baldwin