----- Original Message ----- > From: "John Baldwin" <j...@freebsd.org> > To: freebsd-net@freebsd.org > Cc: "Konstantin Belousov" <kostik...@gmail.com>, "Bryan Venteicher" > <bry...@daemoninthecloset.org>, "Peter Jeremy" > <pe...@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" <j...@freebsd.org> > > > > > To: freebsd-net@freebsd.org > > > > > Cc: "Barney Cordoba" <barney_cord...@yahoo.com>, "Peter > > > > > Jeremy" > <pe...@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_cord...@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 > _______________________________________________ freebsd-net@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-net To unsubscribe, send any mail to "freebsd-net-unsubscr...@freebsd.org"
