linux-kernel - Re: [PATCH 1/8] ARC: perf: support RAW events

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Date:	Wed, 17 Jun 2015 19:40:37 +0530
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Alexey Brodkin <Alexey.Brodkin@...opsys.com>
CC:	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"arnd@...db.de" <arnd@...db.de>,
	"arc-linux-dev@...opsys.com" <arc-linux-dev@...opsys.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Vineet Gupta <Vineet.Gupta1@...opsys.com>
Subject: Re: [PATCH 1/8] ARC: perf: support RAW events

On Wednesday 17 June 2015 07:03 PM, Peter Zijlstra wrote:
>> But in case of raw events we need to set a counter with index of a
>> > particular event. For example we need to count "myevnt0" events. For
>> > this we need first to find-out what's an index in events list of
>> > "myevnt0" event and then set event counter to count event #x.
>> > 
>> > Even though in theory we may set raw even passing an index of desired
>> > event 
> That is what I was thinking of, until I read:
> 
>> > but given my explanation above each particular CPU may have the
>> > same event with different index in events list.
> ...
> 
>> > Fortunately there's already a patch series floating in LKML that
>> > attempts to simplify usage of architecture-specific events, see
>> > http://www.spinics.net/lists/kernel/msg2010232.html
> So you _can_ know a priory in which order the events are for a
> particular CPU ? Say through /proc/cpuinfo.
> 
> I would much prefer the raw thing to remain a number; it puts you in
> pretty much the same place as most other archs, including x86.

Sure, but that doesn't mean we start using the index of event as a raw ID. There
are zillion ways to configure cores and the index will certainly start varying.
Which means that user has to determine at run time (using whatever /proc/xxx) what
crun corresponds to. Why inflict such pain on poor guy.

The current raw event, despite representing an ASCII string is still a u64 number.
So it is not more "raw" than what others have. We can get rid of the swapping
business when setting up a raw event, by making sure that cached values from probe
are already in same format.

But using index as a event id is no go sorry...

> 
> On x86 we have a huge event list (see the Intel SDM for example) and the
> only way to access those (currently, without the above patch series) is
> to look in the PDF under the right model and enter the numbers by hand.

It is not much different for us !

> The only way in which your hardware appears different is in that it
> seems to include these names in hardware; which to me seems a waste of
> perfectly fine silicon, but hey not my call.

Yeah sins of the past.... :-)


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