linux-kernel - Re: [RFC] perf, x86: Segregate PMU workaraunds into x86_pmu_quirk

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Date:	Mon, 31 May 2010 20:45:53 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Robert Richter <robert.richter@....com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [RFC] perf, x86: Segregate PMU workaraunds into
	x86_pmu_quirk_ops structure

On Sat, May 29, 2010 at 08:33:10PM +0200, Peter Zijlstra wrote:
> 
> On Sat, 2010-05-29 at 22:24 +0400, Cyrill Gorcunov wrote:
> > @@ -924,7 +930,11 @@ x86_perf_event_set_period(struct perf_ev
> >          */
> >         atomic64_set(&hwc->prev_count, (u64)-left);
> >  
> > -       wrmsrl(hwc->event_base + idx,
> > +       if (x86_pmu.quirks.perfctr_write)
> > +               x86_pmu.quirks.perfctr_write(hwc->event_base + idx,
> > +                               (u64)(-left) & x86_pmu.cntval_mask);
> > +       else
> > +               wrmsrl(hwc->event_base + idx,
> >                         (u64)(-left) & x86_pmu.cntval_mask);
> 
> This bit is rather ugly,.. not quite sure how to clean it up though.
> Anybody got a bright idea?
> 

Yes, I know, only a bit lighter solution could be like in patch
below, alternative instructions bring mess (and considering we
may have paravirt turned on -- even more mess), jump labels...
I didn't find them in tree, in which file(s) they are? I mean,
are they under review now or merged in some place?

So I guess plain test may be more-less fine here, hmm?

	-- Cyrill
---
perf, x86: Make a second write to performance counter when needed

On Netburst cpu we need a second write to performance counter to
be sure it's updated properly.

Signed-off-by: Cyrill Gorcunov <gorcunov@...nvz.org>
---
 arch/x86/kernel/cpu/perf_event.c    |   10 ++++++++++
 arch/x86/kernel/cpu/perf_event_p4.c |    9 +++++++++
 2 files changed, 19 insertions(+)

Index: linux-2.6.git/arch/x86/kernel/cpu/perf_event.c
=====================================================================
--- linux-2.6.git.orig/arch/x86/kernel/cpu/perf_event.c
+++ linux-2.6.git/arch/x86/kernel/cpu/perf_event.c
@@ -220,6 +220,7 @@ struct x86_pmu {
 						 struct perf_event *event);
 	struct event_constraint *event_constraints;
 	void		(*quirks)(void);
+	int		perfctr_second_write;
 
 	int		(*cpu_prepare)(int cpu);
 	void		(*cpu_starting)(int cpu);
@@ -926,6 +927,15 @@ x86_perf_event_set_period(struct perf_ev
 	atomic64_set(&hwc->prev_count, (u64)-left);
 
 	wrmsrl(hwc->event_base + idx,
+		(u64)(-left) & x86_pmu.cntval_mask);
+
+	/*
+	 * Due to erratum on certan cpu we need
+	 * a second write to be sure the register
+	 * is updated properly
+	 */
+	if (x86_pmu.perfctr_second_write)
+		wrmsrl(hwc->event_base + idx,
 			(u64)(-left) & x86_pmu.cntval_mask);
 
 	perf_event_update_userpage(event);
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
@@ -829,6 +829,15 @@ static __initconst const struct x86_pmu 
 	.max_period		= (1ULL << 39) - 1,
 	.hw_config		= p4_hw_config,
 	.schedule_events	= p4_pmu_schedule_events,
+	/*
+	 * This handles erratum N15 in intel doc 249199-029,
+	 * the counter may not be updated correctly on write
+	 * so we need a second write operation to do the trick
+	 * (the official workaround didn't work)
+	 *
+	 * the former idea is taken from OProfile code
+	 */
+	.perfctr_second_write	= 1,
 };
 
 static __init int p4_pmu_init(void)
--
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