linux-kernel - Re: [PATCH 2/3] utrace core

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Date:	Mon, 23 Mar 2009 10:57:11 +0000
From:	Will Newton <will.newton@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Roland McGrath <roland@...hat.com>, utrace-devel@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] utrace core

On Sat, Mar 21, 2009 at 8:49 AM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
> On Fri, 20 Mar 2009 18:41:40 -0700 (PDT) Roland McGrath <roland@...hat.com> wrote:
>
>> This adds the utrace facility, a new modular interface in the kernel for
>> implementing user thread tracing and debugging.  This fits on top of the
>> tracehook_* layer, so the new code is well-isolated.
>>
>> The new interface is in <linux/utrace.h> and the DocBook utrace book
>> describes it.  It allows for multiple separate tracing engines to work in
>> parallel without interfering with each other.  Higher-level tracing
>> facilities can be implemented as loadable kernel modules using this layer.
>>
>> The new facility is made optional under CONFIG_UTRACE.
>> When this is not enabled, no new code is added.
>> It can only be enabled on machines that have all the
>> prerequisites and select CONFIG_HAVE_ARCH_TRACEHOOK.
>>
>> In this initial version, utrace and ptrace do not play together at all.
>> If ptrace is attached to a thread, the attach calls in the utrace kernel
>> API return -EBUSY.  If utrace is attached to a thread, the PTRACE_ATTACH
>> or PTRACE_TRACEME request will return EBUSY to userland.  The old ptrace
>> code is otherwise unchanged and nothing using ptrace should be affected
>> by this patch as long as utrace is not used at the same time.  In the
>> future we can clean up the ptrace implementation and rework it to use
>> the utrace API.
>
> I'd be interested in seeing a bit of discussion regarding the overall value
> of utrace - it has been quite a while since it floated past.
>
> I assume that redoing ptrace to be a client of utrace _will_ happen, and
> that this is merely a cleanup exercise with no new user-visible features?
>
> The "prototype utrace-ftrace interface" seems to be more a cool toy rather
> than a serious new kernel feature (yes?)
>
> If so, what are the new killer utrace clients which would justify all these
> changes?

It looks like utrace could provide a nice way to do low latency
tracing of userspace processes via a hardware interface (e.g. JTAG or
custom trace hardware). The only way to do that at present is to
scatter bits of instrumentation throughout the kernel.

I would like to see utrace merged so I can work on that type of feature.
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