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

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Message-ID: <20190810035222.GA157218@google.com>
Date:   Fri, 9 Aug 2019 23:52:22 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     Byungchul Park <byungchul.park@....com>,
        linux-kernel@...r.kernel.org, Rao Shoaib <rao.shoaib@...cle.com>,
        max.byungchul.park@...il.com, kernel-team@...roid.com,
        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@...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 Fri, Aug 09, 2019 at 08:40:27PM -0700, Paul E. McKenney wrote:
[snip]
> > > In contrast, a heavy duty userspace-driven workload would transition to
> > > and from userspace for each kfree_rcu(), and that would increment the
> > > dyntick-idle count on each transition to and from userspace.  Adding the
> > > rcu_momentary_dyntick_idle() emulates a pair of such transitions.
> > 
> > But even if we're in kernel mode and not transitioning, I thought the FQS
> > loop (rcu_implicit_dynticks_qs() function) would set need_heavy_qs to true at
> > 2 * jiffies_to_sched_qs.
> > 
> > Hmm, I forgot that jiffies_to_sched_qs can be quite large I guess. You're
> > right, we could call rcu_momentary_dyntick_idle() in advance before waiting
> > for FQS loop to do the setting of need_heavy_qs.
> > 
> > Or, am I missing something with the rcu_momentary_dyntick_idle() point you
> > made?
> 
> The trick is that rcu_momentary_dyntick_idle() directly increments the
> CPU's dyntick counter, so that the next FQS loop will note that the CPU
> passed through a quiescent state.  No need for need_heavy_qs in this case.

Yes, that's what I also understand. Thanks for confirming,

 - Joel