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Date:   Wed, 29 Apr 2020 01:02:04 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Scott Wood <swood@redhat.com>
Cc:     Valentin Schneider <valentin.schneider@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Rik van Riel <riel@surriel.com>,
        Mel Gorman <mgorman@suse.de>, linux-kernel@vger.kernel.org,
        linux-rt-users <linux-rt-users@vger.kernel.org>
Subject: Re: [RFC PATCH 1/3] sched/fair: Call newidle_balance() from
 finish_task_switch()
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On Tue, Apr 28, 2020 at 05:55:03PM -0500, Scott Wood wrote:
> On Wed, 2020-04-29 at 00:09 +0200, Peter Zijlstra wrote:

> > Also, if you move it this late, this is entirely the wrong place. If you
> > do it after the context switch either use the balance_callback or put it
> > in the idle path.
> > 
> > But what Valentin said; this needs a fair bit of support, the whole
> > reason we've never done this is to avoid that double context switch...
> > 
> 
> balance_callback() enters with the rq lock held but BH not separately

BH? softirqs you mean? Pray tell more.

> disabled, which interferes with the ability to enable interrupts but not BH.
> It also gets called from rt_mutex_setprio() and __sched_setscheduler(), and
> I didn't want the caller of those to be stuck with the latency.

You're not reading it right.