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Date:      Mon, 14 Jan 2013 16:12:09 -0600 (CST)
From:      Bryan Venteicher <bryanv@daemoninthecloset.org>
To:        John Baldwin <jhb@freebsd.org>
Cc:        Konstantin Belousov <kostikbel@gmail.com>, freebsd-net@freebsd.org, Peter Jeremy <peter@rulingia.com>
Subject:   Re: To SMP or not to SMP
Message-ID:  <1893331462.132.1358201529856.JavaMail.root@daemoninthecloset.org>
In-Reply-To: <201301141657.58727.jhb@freebsd.org>

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----- Original Message -----
> From: "John Baldwin" <jhb@freebsd.org>
> To: freebsd-net@freebsd.org
> Cc: "Konstantin Belousov" <kostikbel@gmail.com>, "Bryan Venteicher" <bryanv@daemoninthecloset.org>, "Peter Jeremy"
> <peter@rulingia.com>
> Sent: Monday, January 14, 2013 3:57:58 PM
> Subject: Re: To SMP or not to SMP
> 
> 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@freebsd.org>
> > > > > To: freebsd-net@freebsd.org
> > > > > Cc: "Barney Cordoba" <barney_cordoba@yahoo.com>, "Peter
> > > > > Jeremy"
> <peter@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@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.
> 

Yes, that is the case to be concerned about. Although, thinking
about this a bit more, in VirtIO (at least the current spec), all
the shared fields are updated by either the host or guest, not
both, so a UP kernel can get by without the LOCK, correct?


> --
> John Baldwin
> 



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