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

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Date:   Sat, 10 Sep 2016 18:22:10 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Todd Kjos <tkjos@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arve Hj??nnev??g <arve@...roid.com>,
        Riley Andrews <riandrews@...roid.com>,
        devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] android: binder: Disable preemption while holding the
 global binder lock

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