Re: [PATCH 0/5] perf kmem: Add more functions and show more statistics

Ingo Molnar
2009-11-24T07:34:39Z

* Pekka Enberg <penberg@cs.helsinki.fi> wrote:

> Hi Li,
>
> On Tue, Nov 24, 2009 at 7:25 AM, Li Zefan <lizf@cn.fujitsu.com> wrote:
> > Pekka, do you think we can remove kmemtrace now?
>
> One more use case I forgot to mention: boot time tracing. Much of the
> persistent kernel memory footprint comes from the boot process which
> is why it's important to be able to trace memory allocations
> immediately after kmem_cache_init() has run. Can we make "perf kmem"
> do that? Eduard put most of his efforts into making that work for
> kmemtrace.

Would be lovely if someone looked at perf from that angle (and extended
it).

Another interesting area would be to allow a capture session without a
process context running immediately. (i.e. pre-allocate all the buffers,
use them, for a later 'perf save' to pick it up.)

The two are kind of the same thing conceptually: a boot time trace is a
preallocated 'process context less' recording, to be picked up after
bootup.

[ It also brings us 'stability/persistency of event logging' - i.e. a
capture session could be started and guaranteed by the kernel to be
underway, regardless of what user-space does. ]

Btw., Arjan is doing a _lot_ of boot time tracing for Moblin, and he
indicated it in the past that starting a perf recording session from an
initrd is a pretty practical substitute as well. (I've Cc:-ed Arjan.)

> On Tue, Nov 24, 2009 at 7:25 AM, Li Zefan <lizf@cn.fujitsu.com> wrote:
>
> > With kmem trace events, low-level analyzing can be done using
> > ftrace, and high-level analyzing can be done using perf-kmem.
> >
> > And chance is, more people may use and improve perf-kmem, and it
> > will be well-maintained within the perf infrastructure. On the other
> > hand, I guess few people use and contribute to kmemtrace-user.
>
> Sure, I think "perf kmem" is the way forward. I'd love to hear
> Eduard's comments on this before we remove the code from kernel. Do we
> need to do that for 2.6.33 or can we postpone that for 2.6.34?

Certainly we can postpone it, as long as there's rough strategic
consensus on the way forward. I'd hate to have two overlapping core
kernel facilities and friction between the groups pursuing them and
constant distraction from having two targets.

Such situations just rarely end with a good solution for the user - see
security modules for a horror story ...

[ I dont think it will occur here, just wanted to mention it out of
abundance of caution that 1.5 decades of kernel hacking experience
inflicts on me ;-) ]

Ingo