linux-kernel - Re: [patch 1/3] performance counters: core code

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Date:	Fri, 5 Dec 2008 12:20:52 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>, linux-arch@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Stephane Eranian <eranian@...glemail.com>,
	Eric Dumazet <dada1@...mosbay.com>,
	Robert Richter <robert.richter@....com>,
	Arjan van de Veen <arjan@...radead.org>,
	Peter Anvin <hpa@...or.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Steven Rostedt <rostedt@...dmis.org>,
	David Miller <davem@...emloft.net>
Subject: Re: [patch 1/3] performance counters: core code


* Paul Mackerras <paulus@...ba.org> wrote:

> Thomas Gleixner writes:
> 
> > +static void
> > +perf_install_in_context(struct perf_counter_context *ctx,
> > +			struct perf_counter *counter,
> > +			int cpu)
> > +{
> > +	struct task_struct *task = ctx->task;
> > +
> 
> [...]
> 
> > +	if (task) {
> > +		task_oncpu_function_call(task, __perf_install_in_context,
> > +					 counter);
> > +	} else {
> > +		smp_call_function_single(cpu, __perf_install_in_context,
> > +					 counter, 1);
> > +	}
> 
> What happens if we send an IPI to the cpu where the task is running, 
> but by the time the IPI arrives, the task has been migrated to another 
> cpu and is now running there?  Do you chase after it and send another 
> IPI to its new cpu, or is there some reason why it can't migrate? If 
> it's the former, where is that code?  I haven't seen it so far (at 
> least, task_oncpu_function_call doesn't seem to do it).

in that case the schedule-in method will install the perf counter 
automatically. No need to chase after it. The smp call is only for the 
case where the task does not schedule at all.

	Ingo
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