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Date:   Mon, 22 Mar 2021 11:41:06 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Barret Rhoden <brho@google.com>
Cc:     Ingo Molnar <mingo@redhat.com>, Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        linux-kernel@vger.kernel.org
Subject: Re: sched: allow resubmits to queue_balance_callback()
Message-ID: <YFh0QjnoGPmbykyE@hirez.programming.kicks-ass.net>
References: <20210318195734.3579799-1-brho@google.com>
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On Thu, Mar 18, 2021 at 03:57:34PM -0400, Barret Rhoden wrote:
> Prior to this commit, if you submitted the same callback_head twice, it
> would be enqueued twice, but only if it was the last callback on the
> list.  The first time it was submitted, rq->balance_callback was NULL,
> so head->next is NULL.  That defeated the check in
> queue_balance_callback().
> 
> This commit changes the callback list such that whenever an item is on
> the list, its head->next is not NULL.  The last element (first inserted)
> will point to itself.  This allows us to detect and ignore any attempt
> to reenqueue a callback_head.
> 
> Signed-off-by: Barret Rhoden <brho@google.com>

AFAICT you're patching dead code, please check a current tree.