linux-kernel - Re: [PATCH v6 1/4] rcu/tree: Make rcu_do_batch count how many callbacks were executed

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Message-ID: <CAEXW_YQPFM81EXi+7c6ed9b2ULw4YmtfGzmT1GKUGVu5v6CyGg@mail.gmail.com>
Date:   Sun, 11 Oct 2020 09:35:37 -0700
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Madhuparna Bhowmik <madhuparnabhowmik10@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Neeraj upadhyay <neeraj.iitr10@...il.com>,
        "Paul E. McKenney" <paulmck@...nel.org>, rcu <rcu@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        "Uladzislau Rezki (Sony)" <urezki@...il.com>
Subject: Re: [PATCH v6 1/4] rcu/tree: Make rcu_do_batch count how many
 callbacks were executed

On Fri, Oct 9, 2020 at 4:14 PM Frederic Weisbecker <frederic@...nel.org> wrote:
>
> On Wed, Sep 23, 2020 at 11:22:08AM -0400, Joel Fernandes (Google) wrote:
> > Currently, rcu_do_batch() depends on the unsegmented callback list's len field
> > to know how many CBs are executed. This fields counts down from 0 as CBs are
> > dequeued.  It is possible that all CBs could not be run because of reaching
> > limits in which case the remaining unexecuted callbacks are requeued in the
> > CPU's segcblist.
> >
> > The number of callbacks that were not requeued are then the negative count (how
> > many CBs were run) stored in the rcl->len which has been counting down on every
> > dequeue. This negative count is then added to the per-cpu segmented callback
> > list's to correct its count.
> >
> > Such a design works against future efforts to track the length of each segment
> > of the segmented callback list. The reason is because
> > rcu_segcblist_extract_done_cbs() will be populating the unsegmented callback
> > list's length field (rcl->len) during extraction.
> > Also, the design of counting down from 0 is confusing and error-prone IMHO.
>
> Right :)

:)

> > This commit therefore explicitly counts have many callbacks were executed in
>
> s/have/how
>
> > rcu_do_batch() itself, and uses that to update the per-CPU segcb list's ->len
> > field, without relying on the negativity of rcl->len.
> >
> > Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
>
> Reviewed-by: Frederic Weisbecker <frederic@...nel.org>

Thanks! Paul would be Ok to make the minor fixup s/have/how/ that
Frederic pointed?

- Joel
(Due to COVID issues at home, I'm intermittently working so advance
apologies for slow replies.)