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

Peter Zijlstra
2016-09-10T16:22:17Z

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@google.com>
> > Signed-off-by: Todd Kjos <tkjos@google.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