To SMP or not to SMP

[Konstantin Belousov]

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 ?

> > 
> > VirtIO currently uses mb() to enforce ordering. I have a patch
> > to change to use atomic(9), but can only do so when VirtIO is
> > included in the an SMP kernel (among other constraints - must
> > have 16-bit atomic operations too).
> > 
> > (FreeBSD's VirtIO is x86 only for now - but that will be changing
> > soon; I haven't looked if other arch's atomic(9) behave differently
> > for UP/SMP.)
> 
> Only x86 does this weirdness.  The simplest workaround might be to require
> guest kernels to be compiled with SMP for now.
> 
> -- 
> John Baldwin
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