Re: [PATCH RFC v1 1/2] rcu/tree: Add basic support for kfree_rcu batching

[Byungchul Park]

On Tue, Aug 13, 2019 at 08:41:45AM -0700, Paul E. McKenney wrote:
> On Tue, Aug 13, 2019 at 02:29:54PM +0900, Byungchul Park wrote:
> > On Mon, Aug 12, 2019 at 09:12:34AM -0400, Joel Fernandes wrote:
> > > On Mon, Aug 12, 2019 at 07:10:52PM +0900, Byungchul Park wrote:
> > > > On Sun, Aug 11, 2019 at 04:49:39PM -0700, Paul E. McKenney wrote:
> > > > > Maybe.  Note well that I said "potential issue".  When I checked a few
> > > > > years ago, none of the uses of rcu_barrier() cared about kfree_rcu().
> > > > > They cared instead about call_rcu() callbacks that accessed code or data
> > > > > that was going to disappear soon, for example, due to module unload or
> > > > > filesystem unmount.
> > > > > 
> > > > > So it -might- be that rcu_barrier() can stay as it is, but with changes
> > > > > as needed to documentation.
> > > 
> > > Right, we should update the docs. Byungchul, do you mind sending a patch that
> > > documents the rcu_barrier() behavior?
> > 
> > Are you trying to give me the chance? I feel thankful. It doens't matter
> > to try it at the moment though, I can't follow-up until September. I'd
> > better do that in Septamber or give it up this time.
> 
> Which reminds me...  I recall your asking if the kfree_rcu() patch
> might be sensitive to the exact hardware, but I cannot locate that
> email right off-hand.  This is an excellent question!  When faced with
> floods of kfree_rcu() calls, I would expect some hardware, compiler,
> and kernel-configuration sensitivity.  Which is why it will likely be

Yes.

> necessary to do a few more improvements over time -- for but one example,
> accumulating callbacks into vectors in order to reduce the number of
> kfree()-time cache misses.

Yes. That would be a pretty good way to mitigate the problem. I hope
the simple way we've done works well enough so it would never happen
though.

Or I would check the condition of all system resourses e.g. CPU and
memory and control the bandwith of them, of course only if that actually
happens.

Thanks a lot for sharing your opinion on it!

Thanks,
Byungchul

> 							Thanx, Paul
> 
> > Thanks,
> > Byungchul
> > 
> > > > > It also -might- be, maybe now or maybe some time in the future, that
> > > > > there will need to be a kfree_rcu_barrier() or some such.  But if so,
> > > > > let's not create it until it is needed.  For one thing, it is reasonably
> > > > > likely that something other than a kfree_rcu_barrier() would really
> > > > > be what was needed.  After all, the main point would be to make sure
> > > > > that the old memory really was freed before allocating new memory.
> > > > 
> > > > Now I fully understand what you meant thanks to you. Thank you for
> > > > explaining it in detail.
> > > > 
> > > > > But if the system had ample memory, why wait?  In that case you don't
> > > > > really need to wait for all the old memory to be freed, but rather for
> > > > > sufficient memory to be available for allocation.
> > > > 
> > > > Agree. Totally make sense.
> > > 
> > > Agreed, all makes sense.
> > > 
> > > thanks,
> > > 
> > >  - Joel
> > > 
> > > [snip]
> >