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

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Date:   Mon, 12 Sep 2016 08:49:58 -0700
From:   Todd Kjos <tkjos@...roid.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        devel@...verdev.osuosl.org, Riley Andrews <riandrews@...roid.com>,
        linux-kernel@...r.kernel.org,
        Christoph Hellwig <hch@...radead.org>,
        "Arve Hj??nnev??g" <arve@...roid.com>, Todd Kjos <tkjos@...gle.com>
Subject: Re: [PATCH] android: binder: Disable preemption while holding the
 global binder lock

Thanks for the reviews. We'll come up with a different solution.

On Sat, Sep 10, 2016 at 10:28 AM, Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
> On Sat, Sep 10, 2016 at 06:37:29PM +0200, Thomas Gleixner wrote:
>> On Sat, 10 Sep 2016, Peter Zijlstra wrote:
>>
>> > On Sat, Sep 10, 2016 at 09:16:59AM -0700, Christoph Hellwig wrote:
>> > > On Thu, Sep 08, 2016 at 09:12:50AM -0700, Todd Kjos wrote:
>> > > > In Android systems, the display pipeline relies on low
>> > > > latency binder transactions and is therefore sensitive to
>> > > > delays caused by contention for the global binder lock.
>> > > > Jank is siginificantly reduced by disabling preemption
>> > > > while the global binder lock is held.
>> > >
>> > > That's now how preempt_disable is supposed to use.  It is for critical
>> >
>> > not, that's supposed to be _not_. Just to be absolutely clear, this is
>> > NOT how you're supposed to use preempt_disable().
>> >
>> > > sections that use per-cpu or similar resources.
>> > >
>> > > >
>> > > > Originally-from: Riley Andrews <riandrews@...gle.com>
>> > > > Signed-off-by: Todd Kjos <tkjos@...gle.com>
>> >
>> > > > @@ -389,7 +390,11 @@ static int task_get_unused_fd_flags(struct
>> > > > binder_proc *proc, int flags)
>> > > >   rlim_cur = task_rlimit(proc->tsk, RLIMIT_NOFILE);
>> > > >   unlock_task_sighand(proc->tsk, &irqs);
>> > > >
>> > > > - return __alloc_fd(files, 0, rlim_cur, flags);
>> > > > + preempt_enable_no_resched();
>> > > > + ret = __alloc_fd(files, 0, rlim_cur, flags);
>> > > > + preempt_disable();
>> >
>> > And the fact that people want to use preempt_enable_no_resched() shows
>> > that they're absolutely clueless.
>> >
>> > This is so broken its not funny.
>> >
>> > NAK NAK NAK
>>
>> Indeed. Sprinkling random preempt_enabe/disable() pairs all over the place
>> documents clearly that this is tinkering and not proper software
>> engineering.
>
> I have pointed out in the other thread for this patch (the one that had
> a patch that could be applied) that the single lock in the binder code
> is the main problem here, it should be solved instead of this messing
> around with priorities.
>
> So don't worry, I'm not taking this change :)
>
> thanks,
>
> greg k-h