linux-kernel - Re: [PATCH 1/4, v2] x86: enlightenment for ticket spin locks

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Message-ID: <4C2B3E42.1070808@goop.org>
Date:	Wed, 30 Jun 2010 14:53:22 +0200
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Jan Beulich <JBeulich@...ell.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	Ky Srinivasan <KSrinivasan@...ell.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hpa@...or.com" <hpa@...or.com>
Subject: Re: [PATCH 1/4, v2] x86: enlightenment for ticket spin locks -	 base
 implementation

On 06/30/2010 01:52 PM, Jan Beulich wrote:
> I fail to see that: Depending on the hypervisor's capabilities, the
> two main functions could be much smaller (potentially there wouldn't
> even be a need for the unlock hook in some cases),

What mechanism are you envisaging in that case?

>> That appears to be a mechanism to allow it to take interrupts while
>> spinning on the lock, which is something that stock ticket locks don't
>> allow.  If that's a useful thing to do, it should happen in the generic
>> ticketlock code rather than in the per-hypervisor backend (otherwise we
>> end up with all kinds of subtle differences in lock behaviour depending
>> on the exact environment, which is just going to be messy).  Even if
>> interrupts-while-spinning isn't useful on native hardware, it is going
>> to be equally applicable to all virtual environments.
>>     
> While we do interrupt re-enabling in our pv kernels, I intentionally
> didn't do this here - it complicates the code quite a bit further, and
> that did seem right for an initial submission.
>   

Ah, I was confused by this:
> +	/*
> +	 * If we interrupted another spinlock while it was blocking, make
> +	 * sure it doesn't block (again) without re-checking the lock.
> +	 */
> +	if (spinning.prev)
> +		sync_set_bit(percpu_read(poll_evtchn),
> +			     xen_shared_info->evtchn_pending);
> +
> +

> The list really juts is needed to not pointlessly tickle CPUs that
> won't own the just released lock next anyway (or would own
> it, but meanwhile went for another one where they also decided
> to go into polling mode).

Did you measure that it was a particularly common case which was worth
optimising for?

    J
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