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Date: Wed, 19 Aug 2009 13:16:48 +0200
Message-ID: <c384c5ea0908190416r4e0b5508x4765381888516484@mail.gmail.com>
Subject: Re: spin_is_locked() broken for uniprocessor?
From: Leon Woestenberg <leon.woestenberg@gmail.com>
To: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: David Howells <dhowells@redhat.com>, Thomas Gleixner <tglx@linutronix.de>,
       Kumar Gala <galak@kernel.crashing.org>,
       Peter Zijlstra <peterz@infradead.org>,
       Linux-Kernel List <linux-kernel@vger.kernel.org>,
       Steven Rostedt <rostedt@goodmis.org>,
       linuxppc-dev list <linuxppc-dev@ozlabs.org>,
       Ingo Molnar <mingo@elte.hu>
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Hello,

On Wed, Aug 19, 2009 at 12:53 PM, Alan Cox<alan@lxorguk.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