linux-kernel - Re: [PATCH v0 04/71] itrace: Infrastructure for instruction flow tracing units

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Message-ID: <87ob4drsww.fsf@ashishki-desk.ger.corp.intel.com>
Date:	Thu, 19 Dec 2013 13:17:51 +0200
From:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Jiri Olsa <jolsa@...hat.com>, Mike Galbraith <efault@....de>,
	Namhyung Kim <namhyung@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	Stephane Eranian <eranian@...gle.com>,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH v0 04/71] itrace: Infrastructure for instruction flow tracing units

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

> On Thu, Dec 19, 2013 at 09:53:44AM +0200, Alexander Shishkin wrote:
>> Yes and some implementations of PT have the same issue, but you can do a
>> sufficiently large high order allocation and map it to userspace and
>> still no copying (or parsing/decoding) in kernel space required.
>
> What's sufficiently large? The largest we could possibly allocate is
> something like 4k^11 which is 8M or so. That's not all that big given
> you keep saying it generates in the order of 100 MB/s.

One chunk is 8M. You can have as many as the buddy allocator permits you
to have. When you get a PMI, you simply switch one chunk for another and
on the tracing goes.

> Also, 'some implementations', that sounds like a fail right there. Why
> are there already different implementations, and some which such stupid
> design, of something this new?
>
> How about just saying NO to the ones that requires physically contiguous
> allocations?

No reason to leave those out, because they are still extremely useful
for tracing and fit perfectly fine in a model with two buffers.

Regards,
--
Alex
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