linux-kernel - Re: [tip:tracing/hw-branch-tracing] tracing/hw-branch-tracing: convert bts-tracer mutex to a spinlock

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Message-ID: <20090225111116.GB15453@elte.hu>
Date:	Wed, 25 Feb 2009 12:11:16 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	"Metzger, Markus T" <markus.t.metzger@...el.com>
Cc:	"hpa@...or.com" <hpa@...or.com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>
Subject: Re: [tip:tracing/hw-branch-tracing] tracing/hw-branch-tracing:
	convert bts-tracer mutex to a spinlock


* Metzger, Markus T <markus.t.metzger@...el.com> wrote:

> >-----Original Message-----
> >From: Ingo Molnar [mailto:mingo@...e.hu]
> >Sent: Wednesday, February 25, 2009 10:58 AM
> >To: Metzger, Markus T
> 
> 
> >* Metzger, Markus T <markus.t.metzger@...el.com> wrote:
> >
> >> > static void trace_bts_prepare(struct trace_iterator *iter)
> >> > {
> >> >-    mutex_lock(&bts_tracer_mutex);
> >> >+    spin_lock(&bts_tracer_lock);
> >> >
> >> >     on_each_cpu(trace_bts_cpu, iter->tr, 1);
> >> >
> >> >-    mutex_unlock(&bts_tracer_mutex);
> >> >+    spin_unlock(&bts_tracer_lock);
> >> > }
> >>
> >> Whereas start/stop are relatively fast, the above operation is
> >> rather expensive. Would it make sense to use
> >> schedule_on_each_cpu() instead of on_each_cpu()?
> >
> >it's perfectly fine to do that on_each_cpu() under the spinlock.
> >schedule_on_each_cpu() would likely be more expensive - and for
> >no good reason.
> 
> OK. 
> 
> And I assume you like the spinlock better than the
> get/put_online_cpus(), as well.

yeah - and get/put_online_cpus is sleepable too, so it doesnt 
really help unless i'm missing something ...

	Ingo
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