linux-kernel - Re: [PATCH v5 18/20] perf: Allocate ring buffers for inherited per-task kernel events

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Date:	Thu, 30 Oct 2014 12:20:33 +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,
	Robert Richter <rric@...nel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mike Galbraith <efault@....de>,
	Paul Mackerras <paulus@...ba.org>,
	Stephane Eranian <eranian@...gle.com>,
	Andi Kleen <ak@...ux.intel.com>, kan.liang@...el.com,
	adrian.hunter@...el.com, acme@...radead.org
Subject: Re: [PATCH v5 18/20] perf: Allocate ring buffers for inherited per-task kernel events

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

> On Fri, Oct 24, 2014 at 10:44:54AM +0300, Alexander Shishkin wrote:
>> Peter Zijlstra <peterz@...radead.org> writes:
>> 
>> > On Mon, Oct 13, 2014 at 04:45:46PM +0300, Alexander Shishkin wrote:
>> >> Normally, per-task events can't be inherited parents' ring buffers to
>> >> avoid multiple events contending for the same buffer. And since buffer
>> >> allocation is typically done by the userspace consumer, there is no
>> >> practical interface to allocate new buffers for inherited counters.
>> >> 
>> >> However, for kernel users we can allocate new buffers for inherited
>> >> events as soon as they are created (and also reap them on event
>> >> destruction). This pattern has a number of use cases, such as event
>> >> sample annotation and process core dump annotation.
>> >> 
>> >> When a new event is inherited from a per-task kernel event that has a
>> >> ring buffer, allocate a new buffer for this event so that data from the
>> >> child task is collected and can later be retrieved for sample annotation
>> >> or core dump inclusion. This ring buffer is released when the event is
>> >> freed, for example, when the child task exits.
>> >> 
>> >
>> > This causes a pinned memory explosion, not at all nice that.
>> >
>> > I think I see why and all, but it would be ever so good to not have to
>> > allocate so much memory.
>> 
>> Are there any controls we could use to limit such memory usage?
>
> I'd say the same limit we're already accounting the mmap()s against. But
> the question is; what do we do when we run out?
>
> Will we fail clone()? That might 'surprise' quite a few people, that
> their application won't work when profiled.

Or we just don't allocate any more buffers for this user; if there's a
perf stream involved, we can output a record saying that.

> In any case, lets focus on the other parts of this work and delay this
> feature till later.

Agreed.

Regards,
--
Alex
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