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

From: Paul E. McKenney
Date: Fri Feb 03 2012 - 00:55:22 EST

On Thu, Feb 02, 2012 at 08:07:51PM -0800, Josh Triplett wrote:
> 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.

Yep! But the caller has to tell me.

Something like 90% of the call_rcu()s could be call_rcu_lazy(), but there
are a significant number that wake someone up, manipulate a reference
counter that someone else is paying attention to, etc.

> > 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.

There is the qlen statistic in the debugfs tracing, tracked on a per-CPU
basis. But unless it is kfree_rcu(), I have no way to tell how much
memory a given callback frees.

> > 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.

Here is hoping! ;-)

Thanx, Paul

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