linux-kernel - Re: [PATCH -tip] x86,perf: P4 PMU -- protect sensible procedures from preemption

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Date:	Thu, 6 May 2010 19:26:46 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH -tip] x86,perf: P4 PMU -- protect sensible procedures
	from preemption

On Thu, May 06, 2010 at 06:48:54PM +0400, Cyrill Gorcunov wrote:
> On Thu, May 06, 2010 at 09:45:24AM -0400, Steven Rostedt wrote:
> ...
> > > > We want the one with the least runtime overhead. These are instrumentation
> > > > routines, so we want to optimize them as much as possible.
> > 
> > 
> > Yeah, my point was either disable preemption or keep the checks. In
> > other words, if you don't disable preemption, do not use
> > raw_smp_procesor_id(), because then we will not catch it if it changes
> > in the future.
> > 
> > > ok, Ingo, dont apply this patch then for a while.
> > 
> > Send another patch, I'll test it again ;-)
> > 
> > -- Steve
> > 
> > 
> 
> Ingo, Steven, it seems we have potential preemtion available
> in perf_event.c:validate_group:x86_pmu.schedule_events() which
> is reached via syscall from userspace perf_event_open() call,
> so get_cpu is still needed. But I'm a bit messed with call
> graph at the moment :(
> 
> 	-- Cyrill

Steve, while I'm diving through call graph could you give this
patch a shot? If preemtion happens -- it'll trigger it fast.

	-- Cyrill
---
 arch/x86/kernel/cpu/perf_event_p4.c |    8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

Index: linux-2.6.git/arch/x86/kernel/cpu/perf_event_p4.c
=====================================================================
--- linux-2.6.git.orig/arch/x86/kernel/cpu/perf_event_p4.c
+++ linux-2.6.git/arch/x86/kernel/cpu/perf_event_p4.c
@@ -421,7 +421,7 @@ static u64 p4_pmu_event_map(int hw_event
 
 static int p4_hw_config(struct perf_event *event)
 {
-	int cpu = raw_smp_processor_id();
+	int cpu = get_cpu();
 	u32 escr, cccr;
 
 	/*
@@ -440,7 +440,7 @@ static int p4_hw_config(struct perf_even
 		event->hw.config = p4_set_ht_bit(event->hw.config);
 
 	if (event->attr.type != PERF_TYPE_RAW)
-		return 0;
+		goto out;
 
 	/*
 	 * We don't control raw events so it's up to the caller
@@ -455,6 +455,8 @@ static int p4_hw_config(struct perf_even
 		(p4_config_pack_escr(P4_ESCR_MASK_HT) |
 		 p4_config_pack_cccr(P4_CCCR_MASK_HT));
 
+out:
+	put_cpu();
 	return 0;
 }
 
@@ -741,7 +743,7 @@ static int p4_pmu_schedule_events(struct
 {
 	unsigned long used_mask[BITS_TO_LONGS(X86_PMC_IDX_MAX)];
 	unsigned long escr_mask[BITS_TO_LONGS(ARCH_P4_TOTAL_ESCR)];
-	int cpu = raw_smp_processor_id();
+	int cpu = smp_processor_id();
 	struct hw_perf_event *hwc;
 	struct p4_event_bind *bind;
 	unsigned int i, thread, num;
--
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