linux-kernel - Re: [PATCH RFC v1 1/2] rcu/tree: Add basic support for kfree

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Message-ID: <20190813154145.GE28441@linux.ibm.com>
Date:   Tue, 13 Aug 2019 08:41:45 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Byungchul Park <byungchul.park@....com>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        Byungchul Park <max.byungchul.park@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Rao Shoaib <rao.shoaib@...cle.com>, kernel-team@...roid.com,
        kernel-team <kernel-team@....com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        rcu <rcu@...r.kernel.org>, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH RFC v1 1/2] rcu/tree: Add basic support for kfree_rcu
 batching

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
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.

							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]
>