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

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Date:   Sat, 10 Oct 2020 01:14:09 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     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.iitr10@...il.com, "Paul E. McKenney" <paulmck@...nel.org>,
        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 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.