linux-kernel - Re: [PATCH v3 1/2] rcu/tree: Add basic support for kfree

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Message-Id: <20190814184429.GV28441@linux.ibm.com>
Date:   Wed, 14 Aug 2019 11:44:29 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org, Rao Shoaib <rao.shoaib@...cle.com>,
        max.byungchul.park@...il.com, byungchul.park@....com,
        kernel-team@...roid.com, kernel-team@....com,
        Andrew Morton <akpm@...ux-foundation.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Jonathan Corbet <corbet@....net>,
        Josh Triplett <josh@...htriplett.org>,
        Kees Cook <keescook@...omium.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-doc@...r.kernel.org,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        rcu@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v3 1/2] rcu/tree: Add basic support for kfree_rcu batching

On Wed, Aug 14, 2019 at 01:22:33PM -0400, Joel Fernandes wrote:
> On Wed, Aug 14, 2019 at 10:38:17AM -0400, Joel Fernandes wrote:
> > On Tue, Aug 13, 2019 at 12:07:38PM -0700, Paul E. McKenney wrote:
>  [snip]
> > > > - * Queue an RCU callback for lazy invocation after a grace period.
> > > > - * This will likely be later named something like "call_rcu_lazy()",
> > > > - * but this change will require some way of tagging the lazy RCU
> > > > - * callbacks in the list of pending callbacks. Until then, this
> > > > - * function may only be called from __kfree_rcu().
> > > > + * Maximum number of kfree(s) to batch, if this limit is hit then the batch of
> > > > + * kfree(s) is queued for freeing after a grace period, right away.
> > > >   */
> > > > -void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
> > > > +struct kfree_rcu_cpu {
> > > > +	/* The rcu_work node for queuing work with queue_rcu_work(). The work
> > > > +	 * is done after a grace period.
> > > > +	 */
> > > > +	struct rcu_work rcu_work;
> > > > +
> > > > +	/* The list of objects being queued in a batch but are not yet
> > > > +	 * scheduled to be freed.
> > > > +	 */
> > > > +	struct rcu_head *head;
> > > > +
> > > > +	/* The list of objects that have now left ->head and are queued for
> > > > +	 * freeing after a grace period.
> > > > +	 */
> > > > +	struct rcu_head *head_free;
> > > 
> > > So this is not yet the one that does multiple batches concurrently
> > > awaiting grace periods, correct?  Or am I missing something subtle?
> > 
> > Yes, it is not. I honestly, still did not understand that idea. Or how it
> > would improve things. May be we can discuss at LPC on pen and paper? But I
> > think that can also be a follow-up optimization.
> 
> I got it now. Basically we can benefit a bit more by having another list
> (that is have multiple kfree_rcu batches in flight). I will think more about
> it - but hopefully we don't need to gate this patch by that.

I am willing to take this as a later optimization.

> It'll be interesting to see what rcuperf says about such an improvement :)

Indeed, no guarantees either way.  The reason for hope assumes a busy
system where each grace period is immediately followed by another
grace period.  On such a system, the current setup allows each CPU to
make use only of every second grace period for its kfree_rcu() work.
The hope would therefore be that this would reduce the memory footprint
substantially with no increase in overhead.

But no way to know without trying it!  ;-)

							Thanx, Paul