linux-kernel - Re: Performance Events hangs with Intel P4 system

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Date:	Fri, 14 May 2010 17:52:40 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Lin Ming <ming.m.lin@...el.com>,
	Jaswinder Singh Rajput <jaswinderlinux@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: Performance Events hangs with Intel P4 system

On Fri, May 14, 2010 at 01:56:55PM +0200, Ingo Molnar wrote:
> 
> * Lin Ming <ming.m.lin@...el.com> wrote:
> 
> > p4_event_bind::cntr is "unsigned char".
> > But p4_next_cntr has return type of "int".
> > So the explicit conversion is needed to get the correct result. 
> 
> > @@ -780,7 +780,7 @@ static int p4_pmu_schedule_events(struct cpu_hw_events *cpuc, int n, int *assign
> >  		if (unlikely(escr_idx == -1))
> >  			goto done;
> >  
> > -		if (hwc->idx != -1 && !p4_should_swap_ts(hwc->config, cpu)) {
> > +		if (hwc->idx != (unsigned char)-1 && !p4_should_swap_ts(hwc->config, cpu)) {
> 
> That cast is _extremely_ ugly. Please fix the signedness of the types instead.
> 
> 	Ingo
> 

Ingo, what about this one? Jaswinder could you give it a shot (untested)?

	-- Cyrill
---
[PATCH -tip/master] x86,perf: P4 PMU - fix counters allocation logic and sign issue

Jaswinder reported GP:
|
| Message from syslogd@ht at May 14 09:39:32 ...
| kernel:[  314.908612] EIP: [<c100ccca>]
| x86_perf_event_set_period+0x19d/0x1b2 SS:ESP 0068:edac3d70
|

Ming has narrowed it down to comparation issue between signed/unsigned values.
As result event index reaches value 255 which in turn leads to GP fault.

Also it was found that p4_next_cntr has a broken logic and should return
counter index if only it was not yet borrowed for another event.

Reported-by: Jaswinder Singh Rajput <jaswinderlinux@...il.com>
Reported-by: Lin Ming <ming.m.lin@...el.com>
Bisected-by: Lin Ming <ming.m.lin@...el.com>
CC: Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC: Ingo Molnar <mingo@...e.hu>
CC: Frederic Weisbecker <fweisbec@...il.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@...nvz.org>
---
 arch/x86/kernel/cpu/perf_event_p4.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 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
@@ -18,7 +18,7 @@
 struct p4_event_bind {
 	unsigned int opcode;			/* Event code and ESCR selector */
 	unsigned int escr_msr[2];		/* ESCR MSR for this event */
-	unsigned char cntr[2][P4_CNTR_LIMIT];	/* counter index (offset), -1 on abscence */
+	char cntr[2][P4_CNTR_LIMIT];		/* counter index (offset), -1 on abscence */
 };
 
 struct p4_cache_event_bind {
@@ -747,11 +747,11 @@ static int p4_get_escr_idx(unsigned int 
 static int p4_next_cntr(int thread, unsigned long *used_mask,
 			struct p4_event_bind *bind)
 {
-	int i = 0, j;
+	int i, j;
 
 	for (i = 0; i < P4_CNTR_LIMIT; i++) {
-		j = bind->cntr[thread][i++];
-		if (j == -1 || !test_bit(j, used_mask))
+		j = (int)bind->cntr[thread][i];
+		if (j != -1 && !test_bit(j, used_mask))
 			return j;
 	}
 
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