Messages in this thread | | | Date | Tue, 2 Jun 2009 16:35:15 +0900 | From | Paul Mundt <> | Subject | Re: [PATCH] sched: sched_clock() clocksource handling. |
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On Tue, Jun 02, 2009 at 09:25:02AM +0200, Peter Zijlstra wrote: > On Tue, 2009-06-02 at 16:17 +0900, Paul Mundt wrote: > > > @@ -362,6 +364,9 @@ static struct clocksource *select_clocksource(void) > > if (next == curr_clocksource) > > return NULL; > > > > + if (next->flags & CLOCK_SOURCE_USE_FOR_SCHED_CLOCK) > > + sched_clocksource = next; > > + > > return next; > > } > > > > @@ -440,7 +445,21 @@ void clocksource_unregister(struct clocksource *cs) > > list_del(&cs->list); > > if (clocksource_override == cs) > > clocksource_override = NULL; > > + > > next_clocksource = select_clocksource(); > > + > > + /* > > + * If select_clocksource() fails to find another suitable > > + * clocksource for sched_clocksource and we are unregistering > > + * it, switch back to jiffies. > > + */ > > + if (sched_clocksource == cs) { > > + rcu_assign_pointer(sched_clocksource, &clocksource_jiffies); > > + spin_unlock_irqrestore(&clocksource_lock, flags); > > + synchronize_rcu(); > > + return; > > + } > > + > > spin_unlock_irqrestore(&clocksource_lock, flags); > > } > > > What if there's multiple CLOCK_SOURCE_USER_FOR_SCHED_CLOCK [ damn, thats > a long name to type :-) ] ? > > That is, should we have logic in select_clocksource that does: > > if ((next->flags & ..) && next->prio > sched_clocksource->prio) > > or whatever, so that it picks the best one? > > Same for unregister, should we re-evaluate all clocksources before > falling back to basic?
We already do via select_clocksource(), if we are unregistering the current one then a new one with the flag set is selected. Before that, the override is likewise given preference, and we fall back on jiffies if there is nothing else. I suppose we could try and find the "best" one, but I think the override and manual clocksource selection should be fine for this.
Now that you mention it though, the sched_clocksource() assignment within select_clocksource() happens underneath the clocksource_lock, but is not using rcu_assign_pointer(). If the assignment there needs to use rcu_assign_pointer() then presumably all of the unlock paths that do select_clocksource() will have to synchronize_rcu()?
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