linux-kernel - Re: [PATCH] android: binder: Disable preemption while holding the global binder lock

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Date:   Wed, 14 Sep 2016 09:10:01 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Arve Hjønnevåg <arve@...roid.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "devel@...verdev.osuosl.org" <devel@...verdev.osuosl.org>,
        Riley Andrews <riandrews@...roid.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Todd Kjos <tkjos@...gle.com>
Subject: Re: [PATCH] android: binder: Disable preemption while holding the
 global binder lock

On Tue, Sep 13, 2016 at 12:53:27PM -0700, Arve Hjønnevåg wrote:
> On Tue, Sep 13, 2016 at 12:32 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Mon, Sep 12, 2016 at 08:44:09PM -0700, Arve Hjønnevåg wrote:
> >
> >> A previous attempt to fix this problem, changed the lock to use
> >> rt_mutex instead of mutex, but this apparently did not work as well as
> >> this patch. I believe the added overhead was noticeable, and it did
> >> not work when the preempted thread was in a different cgroup (I don't
> >> know if this is still the case).
> >
> > Do you actually have RR/FIFO/DL tasks? Currently PI isn't
> > defined/implemented for OTHER.
> >
> 
> Most of the tasks here are not RR/FIFO/DL tasks. I don't see anything
> in the rtmutex code or documentation that indicates that they don't
> work for normal tasks. From what I can tell the priority gets boosted
> in every case. This may not work as well for CFS tasks as for realtime
> tasks, but it should at least help when there is a large priority
> difference.

It does something (it used to explicitly ignore OTHER) but its not
something well defined or usable.

> > cgroups should be irrelevant, PI is unaware of them.
> 
> I don't think cgroups are irrelevant. PI being unaware of them
> explains the problem I described. If the task that holds the lock is
> in a cgroup that has a low cpu.shares value, then boosting the task's
> priority within that group does necessarily make it any more likely to
> run.

See, the problem is that 'priority' is a meaningless concept for
OTHER/CFS.

In any case, typically only RT tasks care about PI, and the traditional
Priority Inheritance algorithm only really works correctly for FIFO. As
is RR has issues and DL is a crude hack, CFS is really just an accident
by not explicitly exempting it.

We could define a meaningful something for CFS and implement that, but
it isn't currently done.