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Subject: Re: [RFC PATCH 1/3] sched/fair: Call newidle_balance() from
 finish_task_switch()
From:   Scott Wood <swood@redhat.com>
To:     Peter Zijlstra <peterz@infradead.org>
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>
Date:   Tue, 28 Apr 2020 18:20:32 -0500
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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.

> > 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?

-Scott