On Tue, Oct 06, 2015 at 06:24:23PM +0200, Peter Zijlstra wrote:
> On Tue, Oct 06, 2015 at 06:04:50PM +0200, Peter Zijlstra wrote:
> > On Mon, Sep 21, 2015 at 07:46:11PM +0200, Oleg Nesterov wrote:
> > > On 09/18, Peter Zijlstra wrote:
> > > >
> > > > the text is correct, right?
> > >
> > > Yes, it looks good to me and helpful.
> > >
> > > But damn. I forgot why exactly try_to_wake_up() needs rmb() after
> > > ->on_cpu check... It looks reasonable in any case, but I do not
> > > see any strong reason immediately.
> >
> > I read it like the smp_rmb() we have for
> > acquire__after_spin_is_unlocked. Except, as you note below, we need to
> > need an smp_read_barrier_depends for control barriers as well....
>
> > Yes, but I'm not sure we should go write:
> >
> > while (READ_ONCE_CTRL(p->on_cpu))
> > cpu_relax();
> >
> > Or:
> >
> > while (p->on_cpu)
> > cpu_relax();
> >
> > smp_read_barrier_depends();
> >
> > It seems to me that doing the smp_mb() (for Alpha) inside the loop might
> > be sub-optimal.
>
> And also referring to:
>
> lkml.kernel.org/r/20150812133109.ga8...@redhat.com
>
> Do we want something like this?
>
> #define smp_spin_acquire(cond) do { \
> while (cond) \
> cpu_relax(); \
> smp_read_barrier_depends(); /* ctrl */ \
> smp_rmb(); /* ctrl + rmb := acquire */ \
> } while (0)
>
> And use it like:
>
> smp_spin_acquire(raw_spin_is_locked(&task->pi_lock));
>
> That might work for your task_work_run() and the scheduler case,
> although it might be somewhat awkward for sem_wait_array().
I could *really* use something like this for implementing power-saving
busy loops for arch/arm64 (i.e. in the qrwlock code). We have a WFE
instruction (wait for event) that can stop the processor clock and resume
it when the exclusive monitor is cleared (i.e. a cacheline migrates to
another CPU). That means we can implement a targetted wake-up when an
unlocker writes to a node in a queued lock, which isn't something
expressible with cpu_relax alone.
Will
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