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

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Date:	Thu, 19 Dec 2013 13:58:42 +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 01:17:51PM +0200, Alexander Shishkin wrote:
>> 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.
>
> This document you referred me to looks to specify something with a
> proper s/g implementation; called ToPA. There doesn't appear to be a
> limit to the linked entries and you can specify a size per entry, and I
> don't see anywhere why 4k would be bad.

JFYI, 11.2.4.1, "Single Output Region ToPA Implementation".

Regards,
--
Alex
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