linux-kernel - Re: [PATCH v0 3/5] perf: Introduce instruction trace filtering

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Date:	Fri, 11 Dec 2015 19:13:10 +0200
From:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	vince@...ter.net, eranian@...gle.com,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Mathieu Poirier <mathieu.poirier@...aro.org>
Subject: Re: [PATCH v0 3/5] perf: Introduce instruction trace filtering

Peter Zijlstra <peterz@...radead.org> writes:

> On Fri, Dec 11, 2015 at 04:28:54PM +0100, Peter Zijlstra wrote:
>> On Fri, Dec 11, 2015 at 03:36:36PM +0200, Alexander Shishkin wrote:
>> 
>> > @@ -9063,6 +9621,18 @@ inherit_event(struct perf_event *parent_event,
>> >  	get_ctx(child_ctx);
>> >  
>> >  	/*
>> > +	 * Clone itrace filters from the parent, if any
>> > +	 */
>> > +	if (has_itrace_filter(child_event)) {
>> > +		if (perf_itrace_filters_clone(child_event, parent_event,
>> > +					      child)) {
>> > +			put_ctx(child_ctx);
>> > +			free_event(child_event);
>> > +			return NULL;
>> 
>> So inherit_event()'s return policy is somewhat opaque, there's 3
>> possible returns:
>> 
>>  1) a valid struct perf_event pointer; the clone was successful
>>  2) ERR_PTR(err), the clone failed, abort inherit_group, fail fork()
>>  3) NULL, the clone failed, ignore, continue
>> 
>> We return NULL under two special cases:
>> 
>>  - the original event doesn't exist anymore, we're an orphan, do not make
>>    more orphans.
>> 
>>  - the parent event is dying
>> 
>> 
>> I'm fairly sure this return should be in the 2) category. If we cannot
>> fully clone the event something bad happened, we should not ignore it.
>
> On second thought; we should not inherit the filters at all.
>
> We should always use event->parent (if exists) for filters. Otherwise
> inherited events will get different filters if you change the filter
> after clone.

But children will have different mappings, so the actual filter
configurations will still differ between parents and children. I guess I
could split the filter in two parts: one that's defined by the user and
one that we calculated from vma addresses, that we later program into
hardware.

Regards,
--
Alex
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