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: <20090225095814.GC12352@elte.hu>
Date:	Wed, 25 Feb 2009 10:58:14 +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:

> > 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.

	Ingo
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