linux-kernel - Re: spin_is_locked() broken for uniprocessor?

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Message-ID: <c384c5ea0908190416r4e0b5508x4765381888516484@mail.gmail.com>
Date:	Wed, 19 Aug 2009 13:16:48 +0200
From:	Leon Woestenberg <leon.woestenberg@...il.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	David Howells <dhowells@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Kumar Gala <galak@...nel.crashing.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Linux-Kernel List <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	linuxppc-dev list <linuxppc-dev@...abs.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: spin_is_locked() broken for uniprocessor?

Hello,

On Wed, Aug 19, 2009 at 12:53 PM, Alan Cox<alan@...rguk.ukuu.org.uk> wrote:
> On Wed, 19 Aug 2009 10:38:06 +0100
>
> in drivers because there is driver code that uses spin_is_locked() in
> fairly sensible fashion when dealing with locking.
>
One use is to measure lock contention hits on a particular spin lock.


However I wonder if there are tracing capabilities to measure lock
contention on a particular lock?

Currently I have inserted code much like this to get a feeling on the
contention:

this_cpu = get_cpu();
put_cpu();
contention = spin_is_locked(&lock);
spin_lock*(&lock);
if (contention) {
  /* spin lock was contended, prev_cpu, this_cpu */
  /* no hard guarantee, as we had a possible race inbetween
is_locked() and lock(), but works for driver/irq spin lock */
}

/* critical section */

prev_cpu = this_cpu;
spin_unlock*(&lock);

Regards,
-- 
Leon
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