linux-kernel - Re: [PATCH] perf, x86: P4 PMU - Fix unflagged overflows handling v4

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Message-ID: <AANLkTimgMOi2VkwM4O1jVrWCb9aU7CvHovUTCJhBEC_u@mail.gmail.com>
Date:	Tue, 7 Dec 2010 10:28:39 +0300
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Lin Ming <ming.m.lin@...el.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Don Zickus <dzickus@...hat.com>,
	Jason Wessel <jason.wessel@...driver.com>,
	Stephane Eranian <eranian@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] perf, x86: P4 PMU - Fix unflagged overflows handling v4

as far as i can tell you should disable nmi watchdog at bootstrap, ie
set nmi_wachdog=0 in kernel params. same time perf statistics for
other than cpu cycles should work. the problem is that cpu cycles
event cant be moved between counters and as result kernel consumes it
for own needs so the userspace can't obtain it anymore. i dontknow at
moment how to workaround this feature. the patch purpose is to fix
issue with kgdb rather than perf top.

On 12/7/10, Lin Ming <ming.m.lin@...el.com> wrote:
> On Mon, 2010-12-06 at 06:34 +0800, Cyrill Gorcunov wrote:
>> Don found that P4 PMU reads CCCR register instead of counter
>> itself (in attempt to catch unflagged event) this makes P4
>> NMI handler to consume all NMIs it observes. So the other
>> NMI users such as kgdb simply have no chance to get NMI
>> on their hands.
>>
>> v2: Call checking_wrmsrl only if needed.
>> v3: Check the valuable bits.
>> v4: Leave p4_pmu_clear_cccr_ovf early if P4_CCCR_OVF flag is set.
>>
>> Reported-by: Jason Wessel <jason.wessel@...driver.com>
>> Reported-by: Don Zickus <dzickus@...hat.com>
>> Signed-off-by: Cyrill Gorcunov <gorcunov@...nvz.org>
>> CC: Lin Ming <ming.m.lin@...el.com>
>> CC: Stephane Eranian <eranian@...gle.com>
>> CC: Peter Zijlstra <peterz@...radead.org>
>> ---
>>
>> Ming, Jason, mind to give this patch a shot? Note that it
>> seems you have to disable nmi-watchdog at bootstrap to be
>> able to run perf top. I believe it's a different issue
>> unrelated to this particular nit.
>
> perf top gets nothing with current -tip/master(93edb453), both with and
> without this patch.
>
> I'm checking where the problem is.
>
> Lin Ming
>
>>
>>  arch/x86/include/asm/perf_event_p4.h |    3 +++
>>  arch/x86/kernel/cpu/perf_event_p4.c  |   28 +++++++++++++++-------------
>>  2 files changed, 18 insertions(+), 13 deletions(-)
>>
>> Index: linux-2.6.git/arch/x86/include/asm/perf_event_p4.h
>> =====================================================================
>> --- linux-2.6.git.orig/arch/x86/include/asm/perf_event_p4.h
>> +++ linux-2.6.git/arch/x86/include/asm/perf_event_p4.h
>> @@ -20,6 +20,9 @@
>>  #define ARCH_P4_MAX_ESCR	(ARCH_P4_TOTAL_ESCR - ARCH_P4_RESERVED_ESCR)
>>  #define ARCH_P4_MAX_CCCR	(18)
>>
>> +#define ARCH_P4_CNTRVAL_BITS	(40)
>> +#define ARCH_P4_CNTRVAL_MASK	((1ULL << ARCH_P4_CNTRVAL_BITS) - 1)
>> +
>>  #define P4_ESCR_EVENT_MASK	0x7e000000U
>>  #define P4_ESCR_EVENT_SHIFT	25
>>  #define P4_ESCR_EVENTMASK_MASK	0x01fffe00U
>> 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
>> @@ -753,19 +753,21 @@ out:
>>
>>  static inline int p4_pmu_clear_cccr_ovf(struct hw_perf_event *hwc)
>>  {
>> -	int overflow = 0;
>> -	u32 low, high;
>> +	u64 v;
>>
>> -	rdmsr(hwc->config_base + hwc->idx, low, high);
>> -
>> -	/* we need to check high bit for unflagged overflows */
>> -	if ((low & P4_CCCR_OVF) || !(high & (1 << 31))) {
>> -		overflow = 1;
>> -		(void)checking_wrmsrl(hwc->config_base + hwc->idx,
>> -			((u64)low) & ~P4_CCCR_OVF);
>> +	/* an official way for overflow indication */
>> +	rdmsrl(hwc->config_base + hwc->idx, v);
>> +	if (v & P4_CCCR_OVF) {
>> +		wrmsrl(hwc->config_base + hwc->idx, v & ~P4_CCCR_OVF);
>> +		return 1;
>>  	}
>>
>> -	return overflow;
>> +	/* it might be unflagged overflow */
>> +	rdmsrl(hwc->event_base + hwc->idx, v);
>> +	if (!(v & ARCH_P4_CNTRVAL_MASK))
>> +		return 1;
>> +
>> +	return 0;
>>  }
>>
>>  static void p4_pmu_disable_pebs(void)
>> @@ -1152,9 +1154,9 @@ static __initconst const struct x86_pmu
>>  	 */
>>  	.num_counters		= ARCH_P4_MAX_CCCR,
>>  	.apic			= 1,
>> -	.cntval_bits		= 40,
>> -	.cntval_mask		= (1ULL << 40) - 1,
>> -	.max_period		= (1ULL << 39) - 1,
>> +	.cntval_bits		= ARCH_P4_CNTRVAL_BITS,
>> +	.cntval_mask		= ARCH_P4_CNTRVAL_MASK,
>> +	.max_period		= (1ULL << (ARCH_P4_CNTRVAL_BITS - 1)) - 1,
>>  	.hw_config		= p4_hw_config,
>>  	.schedule_events	= p4_pmu_schedule_events,
>>  	/*
>
>
>
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