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

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Message-ID: <20160914161103.GC5016@twins.programming.kicks-ass.net>
Date:   Wed, 14 Sep 2016 18:11:03 +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:
> > 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.

Thing is, for FIFO/DL the important parameters (prio and deadline resp.)
are not cgroup dependent.

For CFS you're right, and as per usual, cgroups will be a royal pain.
While we can compute the total weight in the block chain, getting that
back to a task which is stuck in a cgroup is just not going to work
well.

The only 'solution' I can come up with in a hurry is, when the task is
boosted, move it to the root cgroup. That of course has a whole heap of
problems all on its own.

/me curses @ cgroups.. bloody stupid things.