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Date: Sun, 7 Aug 2011 22:54:42 -0700
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Subject: Re: [PATCH v2 6/6] perf tool: Parse general/raw events from sysfs
From: Stephane Eranian <eranian@google.com>
To: Lin Ming <ming.m.lin@intel.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>, Ingo Molnar <mingo@elte.hu>,
        Andi Kleen <andi@firstfloor.org>,
        Arnaldo Carvalho de Melo <acme@ghostprotocols.net>,
        linux-kernel <linux-kernel@vger.kernel.org>
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On Sun, Aug 7, 2011 at 6:08 PM, Lin Ming <ming.m.lin@intel.com> wrote:
> On Mon, 2011-08-08 at 07:47 +0800, Stephane Eranian wrote:
>> On Sat, Aug 6, 2011 at 4:38 PM, Lin Ming <ming.m.lin@intel.com> wrote:
>> > On Sun, 2011-08-07 at 04:10 +0800, Stephane Eranian wrote:
>> >> Hi,
>> >>
>> >> On Fri, Jul 15, 2011 at 7:35 AM, Lin Ming <ming.m.lin@intel.com> wrote:
>> >> > PMU can export general events to sysfs, for example,
>> >> >
>> >> > /sys/bus/event_source/devices/uncore/events
>> >> > └── cycle
>> >> >
>> >> > Then specify the event as <pmu>:<event>,
>> >> >
>> >> > $ sudo perf stat -a -C 0 -e uncore:cycle
>> >>
>> >> I think this event syntax should be adjusted a bit.
>> >>
>> >> How would the tool differentiate:
>> >>        perf stat -e uncore:cycle
>> >> form:
>> >>       perf stat -e cycle:u
>> >>
>> >> It would have to scan sysfs for a 'cycle' PMU and conclude
>> >> there is none,  then resolve the 'cycle' event name. And if
>> >> you're unlucky and you have a event name that matches
>> >> the PMU name, you get into troubles.
>> >>
>> >> I think, one could instead do:
>> >>
>> >>     perf stat -e uncore::cycle:k
>> >>
>> >> That way, by virtue of the '::' separator, the tool would know
>> >> that it needs to first look into sysfs for an 'uncore' PMU, then
>> >> it needs to look for the 'cycle' event.
>> >
>> > Yes, I like this '::' separator too.
>> > Will update to use it.
>> >
>> >>
>> >> I also use the '::' notation in libpfm4 to separate the PMU model
>> >> form the event+umask+modifiers.
>> >>
>> >> I also suspect that with this sysfs interface for PMU models, you
>> >> would simply add a number to differentiate each instance of a PMU.
>> >> So for GPU, you would do:
>> >>     perf stat -e  gfx0::cycles
>> >>
>> >> Is that right?
>> >
>> > A number or other thing is OK.
>> >
>> > int perf_pmu_register(struct pmu *pmu, char *name, int type)
>> > will be called to register a PMU.
>> >
>> > So I think any name that can differentiate each instance is OK.
>> >
>> > Adding a number looks like the easiest way.
>> >
>> Well, there is something I am still missing here.
>>
>> Based on the current patch, it seems that each instance
>> of a PMU needs to register to get an ID and an entry in
>> sysfs.
>>
>> Suppose you have a system with two graphics cards. Then,
>> you would need two IDs and two entries in sysfs to correctly
>> name each gfx card.
>>
>> That means that the kernel would have to iterate over each instance
>> of a PMU and create a name for it, e.g., something like:
>>    for_each_gfx_card(i) {
>>        sprintf(name, "gfx%d", i);
>>       register_pmu(&pmu, name);
>>   }
>>
>> Is that what you are proposing?
>
> Think this more closely. My previous reply was not correct.
>
> We only need to register one pmu with two same graphics cards.
> Then we can overload pid argument of sys_perf_event_open() to
> differentiate each instance of graphic card.
>
Ok, then you are predicting that no other PMU will ever be useful
in per-thread mode.