On Wed, Jul 10, 2013 at 04:39:02PM +0200, Ingo Molnar wrote:
>
> * Peter Zijlstra <pet...@infradead.org> wrote:
>
> > Hi,
> >
> > I just stumbled over commit 582b336ec2 and had a massive WTF moment.
> >
> > The Changelog -- empty!
> > The Implementation -- complete crap!
> >
> > fail^2.
> >
> > A git grep later I find its x86_64 paravirt only.. for this we add
> > unconditional crap to the scheduler?
> >
> > At the _very_ least this should have been wrapped in a static_key and
> > the changelog should have given some clue as to the what and why of this
> > code.
>
> My bad for letting it slip through...
>
> Marcelo, mind implementing the suggestions from Peter?
So ideally we'd kill the entire notifier, notifiers make it far too easy for
others to use -- and I don't want silent users of stuff like this.
Ideally I'd see a direct callback:
pvclock_migration_callback(t, new_cpu);
That compiles to 'do { } while (0);' for kernels without CONFIG_PARAVIRT. Then
for paravirt it would use a static_key to get a real callback only if there's a
pvclock user.
Something like:
#define pvclock_migration_callback(_t, _cpu) \
do { \
if (static_key_false(&pvclock_key)) \
__pvclock_migration_callback((_t), (_cpu)); \
} while (0)
NOTE: static_key_false() doesn't test false, it assumes the default is false
and makes the function call the out-of-line jump -- horridly confusing function
name.
Your pvclock muck would do:
static_key_slow_inc() -- for every new user and,
static_key_slow_dec() -- for every user gone.
Its a reference count scheme, so that when there's no users there is only a 5
byte nop.
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