linux-kernel - Re: [PATCH RFC 4/4] xen: implement Xen-specific spinlocks

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Date:	Tue, 08 Jul 2008 09:30:00 +0200
From:	Johannes Weiner <hannes@...urebad.de>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Jens Axboe <axboe@...nel.dk>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Lameter <clameter@...ux-foundation.org>,
	Petr Tesarik <ptesarik@...e.cz>,
	Virtualization <virtualization@...ts.linux-foundation.org>,
	Xen devel <xen-devel@...ts.xensource.com>,
	Thomas Friebel <thomas.friebel@....com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
Subject: Re: [PATCH RFC 4/4] xen: implement Xen-specific spinlocks

Hi,

Jeremy Fitzhardinge <jeremy@...p.org> writes:

> Johannes Weiner wrote:
>>> +static DEFINE_PER_CPU(int, lock_kicker_irq) = -1;
>>> +static DEFINE_PER_CPU(struct xen_spinlock *, lock_spinners);
>>>     
>>
>> The plural is a bit misleading, as this is a single pointer per CPU.
>>   
>
> Yeah.  And it's wrong because it's specifically *not* spinning, but
> blocking.

I thought of it as `virtually spinning', so had no problems with the
naming itself :)

>>> +static noinline void xen_spin_unlock_slow(struct xen_spinlock *xl)
>>> +{
>>> +	int cpu;
>>> +
>>> +	for_each_online_cpu(cpu) {
>>>     
>>
>> Would it be feasible to have a bitmap for the spinning CPUs in order to
>> do a for_each_spinning_cpu() here instead?  Or is setting a bit in
>> spinning_lock() and unsetting it in unspinning_lock() more overhead than
>> going over all CPUs here?
>>   
>
> Not worthwhile, I think.  This is a very rare path: it will only
> happen if 1) there's lock contention, that 2) wasn't resolved within
> the timeout.  In practice, this gets called a few thousand times per
> cpu over a kernbench, which is nothing.

Okay, I agree.

	Hannes
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