Messages in this thread | | | Date | Wed, 10 Jul 2013 16:52:38 +0200 | From | Peter Zijlstra <> | Subject | Re: sched: add notifier for cross-cpu migrations |
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On Wed, Jul 10, 2013 at 04:39:02PM +0200, Ingo Molnar wrote: > > * Peter Zijlstra <peterz@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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