| Date | Thu, 2 Feb 2012 08:34:35 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH RFC tip/core/rcu 05/41] rcu: Avoid waking up CPUs having only kfree_rcu() callbacks |
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On Wed, Feb 01, 2012 at 05:15:52PM -0800, Josh Triplett wrote: > On Wed, Feb 01, 2012 at 11:41:23AM -0800, Paul E. McKenney wrote: > > From: "Paul E. McKenney" <paul.mckenney@linaro.org> > > > > When CONFIG_RCU_FAST_NO_HZ is enabled, RCU will allow a given CPU to > > enter dyntick-idle mode even if it still has RCU callbacks queued. > > RCU avoids system hangs in this case by scheduling a timer for several > > jiffies in the future. However, if all of the callbacks on that CPU > > are from kfree_rcu(), there is no reason to wake the CPU up, as it is > > not a problem to defer freeing of memory. > > > > This commit therefore tracks the number of callbacks on a given CPU > > that are from kfree_rcu(), and avoids scheduling the timer if all of > > a given CPU's callbacks are from kfree_rcu(). > > Minor nit: I think it would make much more sense to track the number of > "strict" callbacks *not* from kfree_rcu, and check for that number != 0, > rather than tracking the number of "lazy" callbacks from kfree_rcu and > checking for all != lazy. You can always compute one number from the > other, but since you only ever need to know the strict count, not the > lazy count, why not directly track the thing you care about?
I will think about this. My guess is that when I add call_rcu_lazy(), the naming will outweigh the slowpath subtraction, but will see.
> Also, any way this could hide the new kfree_call_rcu internally rather > than adding it as a new export? In particular, why introduce a new > exported API only suitable for internal use or foot-shooting?
It is called from __kfree_rcu, which is a static inline in include/linux/rcupdate.h, so needs to be exported so that modules can use kfree_rcu().
Of course, if I take the earlier patch that removes the first BUILD_BUG_ON() from __kfree_rcu(), then I could move __kfree_rcu() to rcupdate.c, and then I could move the export from kfree_call_rcu() to __kfree_rcu(). But either way I would be exporting a function that should not be invoked directly.
My concern with the earlier patch is that someone might have something silly like a structure with an array with an rcu_head in each element. Calling kfree_rcu() on such a beast would be ill-advised. This might just be me being overly paranoid, though, hence my lack of additional response to that patch.
Thanx, Paul
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