Re: [RFC PATCH v4] Unified trace buffer

[Mathieu Desnoyers]

* Peter Zijlstra (peterz@xxxxxxxxxxxxx) wrote:
> On Thu, 2008-09-25 at 23:20 -0400, Mathieu Desnoyers wrote:
> > * Steven Rostedt (rostedt@xxxxxxxxxxx) wrote:
> > > 
> > > On Thu, 25 Sep 2008, Masami Hiramatsu wrote:
> > > 
> > > > Hi Steven,
> > > > 
> > > > Steven Rostedt wrote:
> > > > > This version has been cleaned up a bit. I've been running it as
> > > > > a back end to ftrace, and it has been handling pretty well.
> > > > 
> > > > Thank you for your great work.
> > > > It seems good to me(especially, encapsulating events :)).
> > > 
> > > Thanks!
> > > 
> > > > 
> > > > I have one request of enhancement.
> > > > 
> > > > > +static struct ring_buffer_per_cpu *
> > > > > +ring_buffer_allocate_cpu_buffer(struct ring_buffer *buffer, int cpu)
> > > > > +{
> > > > [...]
> > > > > +	cpu_buffer->pages = kzalloc_node(ALIGN(sizeof(void *) * pages,
> > > > > +					       cache_line_size()), GFP_KERNEL,
> > > > > +					 cpu_to_node(cpu));
> > > > 
> > > > Here, you are using a slab object for page managing array,
> > > > the largest object size is 128KB(x86-64), so it can contain
> > > > 16K pages = 64MB.
> > > > 
> > > > As I had improved relayfs, in some rare case(on 64bit arch),
> > > > we'd like to use larger buffer than 64MB.
> > > > 
> > > > http://sourceware.org/ml/systemtap/2008-q2/msg00103.html
> > > > 
> > > > So, I think similar hack can be applicable.
> > > > 
> > > > Would it be acceptable for the next version?
> > > 
> > > I would like to avoid using vmalloc as much as possible, but I do see the 
> > > limitation here. Here's my compromise.
> > > 
> > > Instead of using vmalloc if the page array is greater than one page, 
> > > how about using vmalloc if the page array is greater than 
> > > KMALLOC_MAX_SIZE?
> > > 
> > > This would let us keep the vmap area free unless we have no choice.
> > > 
> > > -- Steve
> > > 
> > 
> > You could also fallback on a 2-level page array when buffer size is >
> > 64MB. The cost is mainly a supplementary pointer dereference, but one
> > more should not make sure a big difference overall.
> 
> I'm still not sure why we don't just link the pages using the page
> frames, we don't need the random access, do we?
> 

Yes, that's a brilliant idea :)

Mathieu

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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