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

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Message-ID: <48731409.9070304@goop.org>
Date:	Tue, 08 Jul 2008 00:15:21 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Johannes Weiner <hannes@...urebad.de>
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

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.

>> +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.

My very original version of this code kept a bitmask of interested CPUs 
within the lock, but there's only space for 24 cpus if we still use a 
byte for the lock itself.  It all turned out fairly awkward, and this 
version is a marked improvement.

    J
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