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From: "K.Prasad" <prasad@linux.vnet.ibm.com>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Ingo Molnar <mingo@redhat.com>,
	Arnaldo Carvalho de Melo <acme@ghostprotocols.net>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Edjunior Barbosa Machado <emachado@linux.vnet.ibm.com>,
	"Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>
Subject: Re: [Patch][perf] Invoke __perf_event_disable without an IPI
Date: Tue, 12 Jun 2012 11:36:26 +0530	[thread overview]
Message-ID: <20120612060626.GB2538@in.ibm.com> (raw)
In-Reply-To: <1339413213.30462.11.camel@twins>

On Mon, Jun 11, 2012 at 01:13:33PM +0200, Peter Zijlstra wrote:
> On Mon, 2012-06-11 at 11:32 +0530, K.Prasad wrote:
> 
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index fd126f8..0e2c1eb 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -1302,6 +1302,7 @@ static int __perf_event_disable(void *info)
> >   */
> >  void perf_event_disable(struct perf_event *event)
> >  {
> > +	int ret;
> >  	struct perf_event_context *ctx = event->ctx;
> >  	struct task_struct *task = ctx->task;
> >  
> > @@ -1314,6 +1315,17 @@ void perf_event_disable(struct perf_event *event)
> >  	}
> >  
> >  retry:
> > +	/*
> > +	 * perf_event_disable may be called when interrupts are disabled.
> > +	 * For e.g. hw_breakpoint_handler exception in PowerPC. Hence using
> > +	 * IPIs to invoke __perf_event_disable is not always suitable. When
> > +	 * possible invoke __perf_event_disable directly.
> > +	 */
> > +	if ((task_cpu(task) == smp_processor_id()) && irqs_disabled()) {
> 
> Urgh.. 
> 
> So what's the callchain for the ppc->hw_bp->perf that triggers this?

Hardware breakpoints for user-space have traditionally operated in a
one-shot mode i.e. breakpoint is disabled after the first hit by
invoking perf_event_disable from hw_breakpoint_handler.

PowerPC server class processors follow 'trigger-before-execute', wherein
the breakpoint exception is triggered before the instruction performing
the memory access is executed. So the one-shot mode prevents the
processor from entering an infinite loop and the debugging tools like
GDB re-enable the breakpoints explicitly after a SIGTRAP.

Thanks,
K.Prasad


  reply	other threads:[~2012-06-12  6:06 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-06-11  6:02 K.Prasad
2012-06-11 11:13 ` Peter Zijlstra
2012-06-12  6:06   ` K.Prasad [this message]
2012-06-12  9:12     ` Peter Zijlstra
2012-07-06  9:52       ` [PATCH v2] Hardware breakpoints: Invoke __perf_event_disable() if interrupts are already disabled Naveen N. Rao
2012-07-06 10:18         ` Naveen N. Rao
2012-07-18 10:30           ` [PATCH v2 RESEND] " Naveen N. Rao
2012-07-18 11:57             ` Frederic Weisbecker
2012-07-19 11:16               ` Naveen N. Rao
2012-07-25 11:32                 ` Naveen N. Rao
2012-07-31 13:41                   ` Frederic Weisbecker
2012-08-02  8:16                     ` [PATCH v3] " Naveen N. Rao
2012-08-15 17:37                       ` Naveen N. Rao
2012-08-15 18:42                         ` Frederic Weisbecker
2012-08-16  8:16                           ` Peter Zijlstra
2012-08-29  2:45                             ` Naveen N. Rao
2012-09-04 18:53                       ` [tip:perf/urgent] perf/hwpb: " tip-bot for K.Prasad

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