linux-kernel - Re: [RFC PATCH 2/3] x86,smp: proportional backoff for ticket spinlocks

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Date:	Fri, 21 Dec 2012 22:14:34 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-kernel@...r.kernel.org, aquini@...hat.com, walken@...gle.com,
	lwoodman@...hat.com, jeremy@...p.org,
	Jan Beulich <JBeulich@...ell.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC PATCH 2/3] x86,smp: proportional backoff for ticket
 spinlocks

On Fri, Dec 21, 2012 at 10:07:56PM -0500, Steven Rostedt wrote:
> > diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c
> > index 20da354..4e44840 100644
> > --- a/arch/x86/kernel/smp.c
> > +++ b/arch/x86/kernel/smp.c
> > @@ -118,9 +118,11 @@ static bool smp_no_nmi_ipi = false;
> >  void ticket_spin_lock_wait(arch_spinlock_t *lock, struct __raw_tickets inc)
> >  {
> >  	for (;;) {
> > -		cpu_relax();
> > -		inc.head = ACCESS_ONCE(lock->tickets.head);
> > +		int loops = 50 * (__ticket_t)(inc.tail - inc.head);
> > +		while (loops--)
> > +			cpu_relax();
> 
> -ENOCOMMENT
> 
> Please add a comment above to explain what it's doing. Don't expect
> people to check change logs. Also, explain why you picked 50.
> 

OK, I replied here before reading patch 3 (still reviewing it). Why have
this patch at all? Just to test if you broke something between this and
patch 3? Or perhaps patch 3 may not get accepted? In that case, you
would still need a comment.

Either explicitly state that this patch is just a stepping stone for
patch 3, and will either be accepted or rejected along with patch 3. Or
keep it as a stand alone patch and add comments as such. Or just get rid
of it all together.

Thanks,

-- Steve

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