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

From: "Paul E. McKenney" <paulmck@linux.ibm.com>
To: Byungchul Park <byungchul.park@lge.com>
Cc: Joel Fernandes <joel@joelfernandes.org>,
	Byungchul Park <max.byungchul.park@gmail.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Rao Shoaib <rao.shoaib@oracle.com>,
	kernel-team@android.com, kernel-team <kernel-team@lge.com>,
	Davidlohr Bueso <dave@stgolabs.net>,
	Josh Triplett <josh@joshtriplett.org>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	rcu <rcu@vger.kernel.org>, Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH RFC v1 1/2] rcu/tree: Add basic support for kfree_rcu batching
Date: Wed, 14 Aug 2019 09:59:27 -0700	[thread overview]
Message-ID: <20190814165927.GT28441@linux.ibm.com> (raw)
In-Reply-To: <20190814034310.GA31272@X58A-UD3R>

On Wed, Aug 14, 2019 at 12:43:10PM +0900, Byungchul Park wrote:
> On Tue, Aug 13, 2019 at 07:53:49PM -0700, Paul E. McKenney wrote:
> > On Wed, Aug 14, 2019 at 09:11:03AM +0900, Byungchul Park wrote:
> > > 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!
> > 
> > Didn't you say earlier that you were getting OOM on your system even
> > with the patches?  Or did I miss the resolution of that issue?
> 
> I said I saw OOM with a *larger* value of KFREE_DRAIN_JIFFIES. It was
> fine with the patch itself.

Got it, thank you!

> Anyway I'm sorry I expressed it in a confusing way.

But what is life without a little confusion?  ;-)

							Thanx, Paul