linux-kernel - Re: [PATCH v2 1/9] perf: Add EVENT_ATTR

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Message-ID: <9b764f1c-3c69-b825-5d5e-640d26be79e4@huawei.com>
Date:   Wed, 2 Jun 2021 18:49:57 +0800
From:   "liuqi (BA)" <liuqi115@...wei.com>
To:     Will Deacon <will@...nel.org>
CC:     Linuxarm <linuxarm@...wei.com>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Ingo Molnar" <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        "Mark Rutland" <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Subject: Re: [PATCH v2 1/9] perf: Add EVENT_ATTR_ID to simplify event
 attributes


Hi Will,
On 2021/6/2 17:49, Will Deacon wrote:
> On Wed, Jun 02, 2021 at 04:45:23PM +0800, liuqi (BA) wrote:
>>
>> Hi Will,
>>
>> Thanks for reviewing this patch.
>>
>> On 2021/6/1 21:10, Will Deacon wrote:
>>> On Wed, May 19, 2021 at 05:51:51PM +0800, Qi Liu wrote:
>>>> Similar EVENT_ATTR macros are defined in many PMU drivers,
>>>> like HiSilicon PMU driver, Arm PMU driver, Arm SMMU PMU
>>>> driver. So Add a generic macro to simplify code.
>>>>
>>>> Cc: Peter Zijlstra <peterz@...radead.org>
>>>> Cc: Ingo Molnar <mingo@...hat.com>
>>>> Cc: Arnaldo Carvalho de Melo <acme@...nel.org>
>>>> Cc: Mark Rutland <mark.rutland@....com>
>>>> Cc: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
>>>> Signed-off-by: Qi Liu <liuqi115@...wei.com>
>>>> ---
>>>>    include/linux/perf_event.h | 6 ++++++
>>>>    kernel/events/core.c       | 2 ++
>>>>    2 files changed, 8 insertions(+)
>>>>
>>>> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
>>>> index f5a6a2f..d0aa74e 100644
>>>> --- a/include/linux/perf_event.h
>>>> +++ b/include/linux/perf_event.h
>>>> @@ -1576,6 +1576,12 @@ static struct perf_pmu_events_attr _var = {				    \
>>>>    	.event_str	= _str,						    \
>>>>    };
>>>> +#define PMU_EVENT_ATTR_ID(_name, _id)					     \
>>>> +	(&((struct perf_pmu_events_attr[]) {				     \
>>>> +		{ .attr = __ATTR(_name, 0444, perf_event_sysfs_show, NULL),  \
>>>> +		  .id = _id, }						     \
>>>> +	})[0].attr.attr)
>>>> +
>>>>    #define PMU_FORMAT_ATTR(_name, _format)					\
>>>>    static ssize_t								\
>>>>    _name##_show(struct device *dev,					\
>>>> diff --git a/kernel/events/core.c b/kernel/events/core.c
>>>> index 0ac818b..330d9cc 100644
>>>> --- a/kernel/events/core.c
>>>> +++ b/kernel/events/core.c
>>>> @@ -13295,6 +13295,8 @@ ssize_t perf_event_sysfs_show(struct device *dev, struct device_attribute *attr,
>>>>    	if (pmu_attr->event_str)
>>>>    		return sprintf(page, "%s\n", pmu_attr->event_str);
>>>> +	else
>>>> +		return sprintf(page, "config=%#llx\n", pmu_attr->id);
>>>
>>> I think it's a really bad idea to hardcode this here. For example, I think
>>> this patch series breaks user ABI for the SMMU PMU which used to print:
>>>
>>> 	"event=0x%02llx\n"
>>>
>>> and by the looks of it many of the other conversions are unsound too.
>>>
>> Got it, so I'll use pmu_attr->event_str here, for example,
>> SMMU_EVENT_ATTR(cycles, "event=0x00")
> 
> You could, but honestly I don't really see this being any better than the
> current code. The advantage of using "event=0x%02llx\n" is that things are
> consistent by construction and therefore userspace can parse the information
> easily. We lose that if we just hardcode the string and it's error-prone to
> extend.
> 
> Will
Got it, thanks.
So how about adding a common macro like this:

#define PMU_EVENT_ATTR_ID(_name,_func, _id)					     \
(&((struct perf_pmu_events_attr[]) {				     \
	{ .attr = __ATTR(_name, 0444, _func, NULL),  \
	  .id = _id, }						     \
})[0].attr.attr)

Drivers could define thir private macro, like SMMU_PMU_EVENT_ATTR, and 
use their private event_show() functions if they need, or use the common 
function perf_event_sysfs_show().

Thanks,
Qi
> .
>