Re: [PATCH RFC tip/core/rcu 14/41] rcu: Limit lazy-callback duration

[Josh Triplett]

On Thu, Feb 02, 2012 at 09:13:42AM -0800, Paul E. McKenney wrote:
> On Wed, Feb 01, 2012 at 06:03:56PM -0800, Josh Triplett wrote:
> > On Wed, Feb 01, 2012 at 11:41:32AM -0800, Paul E. McKenney wrote:
> > > Currently, a given CPU is permitted to remain in dyntick-idle mode
> > > indefinitely if it has only lazy RCU callbacks queued.  This is vulnerable
> > > to corner cases in NUMA systems, so limit the time to six seconds by
> > > default.  (Currently controlled by a cpp macro.)
> > 
> > I wonder: should this scale with the number of callbacks, or do we not
> > want to make estimates about memory usage based on that?
> 
> Interesting.  Which way would you scale it?  ;-)

Heh, I'd figured "don't wait too long if you have a giant pile of
callbacks", but I can see how the other direction could make sense as
well. :)

> > Interestingly, with kfree_rcu, we actually know at callback queuing time
> > *exactly* how much memory we'll get back by calling the callback, and we
> > could sum up those numbers.
> 
> We can indeed calculate for kfree_rcu(), but we won't be able to for
> call_rcu_lazy(), which is my current approach for cases where you cannot
> use kfree_rcu() due to (for example) freeing up a linked structure.
> A very large fraction of the call_rcu()s in the kernel could become
> call_rcu_lazy().

So, doing anything other than freeing memory makes a callback non-lazy?
Based on that, I'd find it at least somewhat surprising if any of the
current callers of call_rcu (other than synchronize_rcu() and similar)
had non-lazy callbacks.

> At some point in the future, it might make sense to tie into the
> low-memory notifier, which could potentially allow the longer timeout
> to be omitted.

Exactly the kind of thing that made me wonder about tracking the actual
amount of memory to free.  Still seems like a potentially useful
statistic to track on its own.

> My current guess is that the recent change allowing idle CPUs to
> exhaust their callback lists will make this kind of fine-tuning
> unnecessary, but we will see!

Good point; given that fix, idle CPUs should never need to wake up for
callbacks at all.

- Josh Triplett
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