linux-kernel - [patch/rfc] perf on raspberry-pi without overflow interrupt

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Date:	Wed, 8 Jan 2014 16:28:20 -0500 (EST)
From:	Vince Weaver <vincent.weaver@...ne.edu>
To:	linux-kernel@...r.kernel.org
cc:	Will Deacon <will.deacon@....com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Stephane Eranian <eranian@...il.com>
Subject: [patch/rfc] perf on raspberry-pi without overflow interrupt

I'm working on getting the hardware performance counters working on a 
Raspberry-Pi (BCM2835/ARM1176).  

The counters are there, but the overflow interrupt is not hooked up so the 
init code disables perf_event.

The following patch enables perf_event and it works fine for simple 
"perf stat" type workloads.  perf record and anything requiring sampling 
doesn't work (as expected).

I thought I would have to add a periodic timer to catch counter overflows, 
but it turns out that's unnecessary.  From what I can tell even though the
nPMUIRQ interrupt is not hooked up, the overflows are marked in the status 
register and this is noticed and handled at context-switch time.  So as 
long as the counters overflow less frequently than the context switch 
interval the registers don't overflow.

So my question, is a patch like this acceptable?

Should the perf_event interface handle setups like this better and work 
fine in aggregate mode but return ENOTSUP if a sampled or overflow event 
is attempted?

Vince

Signed-off-by: Vince Weaver <vincent.weaver@...ne.edu>

diff --git a/arch/arm/kernel/perf_event_cpu.c b/arch/arm/kernel/perf_event_cpu.c
index d85055c..ff1a752 100644
--- a/arch/arm/kernel/perf_event_cpu.c
+++ b/arch/arm/kernel/perf_event_cpu.c
@@ -97,8 +97,8 @@ static int cpu_pmu_request_irq(struct arm_pmu *cpu_pmu, irq_handler_t handler)

 	irqs = min(pmu_device->num_resources, num_possible_cpus());
 	if (irqs < 1) {
-		pr_err("no irqs for PMUs defined\n");
-		return -ENODEV;
+		printk_once("no irqs for PMUs defined, enabling anyway\n");
+		return 0;
 	}

 	for (i = 0; i < irqs; ++i) {
--
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