Re: user mode backtrace

[David Boreham <david_list@boreham.org>]

Vara Prasad wrote:

> If i read your above message correctly you would like to put probes in 
> the kernel but you want to get a full stack which includes kernel mode 
> stack and as well as user mode stack, correct.

ack

> David, if you want to put probes in the user space itself and do back 
> trace we are working on user space probing patch that will let you  
> see the stack just in the application space only. We may be able to 
> give some thing for you to play within couple of weeks if you are 
> interested in user space probing.

I'm not sure if this would work. It depends on whether I would need to 
identify the
processes subject to probing in advance. The application spawns 
processes left and right,
many of which are sort-lived. If I could place a probe in the glibc 
layer above
the system calls that might work, but said probes would need to magically
apply to any process that called that code. Is that the feature you have 
in mind ?

But even so, I can imagine cases where it would still be useful to probe 
in the
kernel but show user stack : e.g. I probe physical read from disk 
because I am
not interested in reads served from filesystem cache. But I'd like to 
find out
where the read calls that hit non-cached data are coming from in the 
application.
Hope that makes sense.

> My next question to you is we are looking to put good load on the 
> system while running probes, is this filesystem application is 
> something that you can share and it is easy for us to run outside of 
> your environment. If this is an application that you can not share 
> outside can you help us run this application while putting lots of 
> probes touching various parts of the system. We can give you scripts, 
> if you like, that will place probes in many parts of the system or 
> specifically in the areas this application touches the most. Please 
> let us know.

The application is not supersecret. In fact in binary form it can be
downloaded from a public web site. I have been loading it with
an open source benchmark tool. So it would be possible for
someone else to reproduce my setup. To see source you'd need
to wait for the application to be open source'ed, which it will be soon
but not right now.

For now however I want to be a little cautious about the details
of what I'm working on. Let me go find out how much I am able
to share...