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Date:   Wed, 29 Apr 2020 11:05:33 +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 06:20:32PM -0500, Scott Wood wrote:
> On Wed, 2020-04-29 at 01:02 +0200, Peter Zijlstra wrote:
> > 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.
> 
> In https://lore.kernel.org/lkml/5122CD9C.9070702@oracle.com/ the need to
> keep softirqs disabled during rebalance was brought up, but simply wrapping
> the lock dropping in local_bh_enable()/local_bh_disable() meant that
> local_bh_enable() would be called with interrupts disabled, which isn't
> allowed.

That thread, nor your explanation make any sense -- why do we care about
softirqs?, nor do I see how placing it in finish_task_switch() helps
with any of this.

> > > 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.
> 
> Could you elaborate?

If you were to do a queue_balance_callback() from somewhere in the
pick_next_task() machinery, then the balance_callback() at the end of
__schedule() would run it, and it'd be gone. How would
rt_mutex_setprio() / __sched_setscheduler() be affected?