Messages in this thread | | | Date | Thu, 20 Jun 2019 23:10:19 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] time/tick-broadcast: Fix tick_broadcast_offline() lockdep complaint |
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On Thu, Jun 20, 2019 at 09:01:18AM -0700, Paul E. McKenney wrote:
> > > +#define TICK_SCHED_REMOTE_OFFLINE 0 > > > +#define TICK_SCHED_REMOTE_RUNNING 1 > > > +#define TICK_SCHED_REMOTE_OFFLINING 2 > > > > That seems a daft set of values; consider { RUNNING, OFFLINING, OFFLINE } > > and see below. > > As in make it an enum? I could do that.
Enum or define, I don't much care, but the 'natural' ordering of the states is either: running -> offlining -> offline, or the other way around, the given order in the patch just didn't make sense.
The one with running=0 just seems to work out nicer.
> > > + > > > +// State diagram for ->state: > > > +// > > > +// > > > +// +----->OFFLINE--------------------------+ > > > +// | | > > > +// | | > > > +// | | sched_tick_start() > > > +// | sched_tick_remote() | > > > +// | | > > > +// | V > > > +// | +---------->RUNNING > > > +// | | | > > > +// | | | > > > +// | | | > > > +// | sched_tick_start() | | sched_tick_stop() > > > +// | | | > > > +// | | | > > > +// | | | > > > +// +--------------------OFFLINING<---------+ > > > +// > > > +// > > > +// Other transitions get WARN_ON_ONCE(), except that sched_tick_remote() > > > +// and sched_tick_start() are happy to leave the state in RUNNING.
> > Also, I find it harder to read that needed, maybe a little something > > like so: > > > > /* > > * OFFLINE > > * | ^ > > * | | tick_remote() > > * | | > > * +--OFFLINING > > * | ^ > > * tick_start() | | tick_stop() > > * v | > > * RUNNING > > */ > > As in remove the leading "sched_" from the function names? (The names > were already there, so I left them be.)
That was just me being lazy, the main part being getting the states in a linear order, instead of spread around a 2d grid.
> > While not wrong, it seems overly complicated; can't we do something > > like: > > > > tick: > > As in sched_tick_remote(), right? > > > state = atomic_read(->state); > > if (state) { > > You sure you don't want "if (state != RUNNING)"? But I guess you need > to understand that RUNNING==0 to understand the atomic_inc_not_zero().
Right..
> > > WARN_ON_ONCE(state != OFFLINING); > > if (atomic_inc_not_zero(->state)) > > This assumes that there cannot be concurrent calls to sched_tick_remote(), > otherwise, you can end up with ->state==3. Which is a situation that > my version does a WARN_ON_ONCE() for, so I guess the only difference is > that mine would be guaranteed to complain and yours would complain with > high probability. So fair enough!
I was assuming there was only a single work per CPU and there'd not be concurrency on this path.
> > return; > > } > > queue_delayed_work(); > > > > > > stop: > > /* > > * This is hotplug; even without stop-machine, there cannot be > > * concurrency on offlining specific CPUs. > > */ > > state = atomic_read(->state); > > There cannot be a sched_tick_stop() or sched_tick_stop(), but there really > can be a sched_tick_remote() right here in the absence of stop-machine, > can't there? Or am I missing something other than stop-machine that > prevents this?
There can be a remote tick, indeed.
> Now, you could argue that concurrency is safe: Either sched_tick_remote() > sees RUNNING and doesn't touch the value, or it sees offlining and > sched_tick_stop() makes no further changes,
That was indeed the thinking.
> but I am not sure that this qualifies as simpler...
There is that I suppose. I think my initial version was a little more complicated, but after a few passes this happened :-)
> > WARN_ON_ONCE(state != RUNNING); > > atomic_set(->state, OFFLINING); > > Another option would be to use atomic_xchg() as below instead of the > atomic_read()/atomic_set() pair. Would that work for you?
Yes, that works I suppose. Is more expensive, but I don't think we particularly care about that here.
> > start: > > state = atomic_xchg(->state, RUNNING); > > WARN_ON_ONCE(state == RUNNING); > > if (state == OFFLINE) { > > // ... > > queue_delayed_work(); > > } > > This one looks to be an improvement on mine regardless of the other two.
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