Re: [PATCH v2 1/3] lib/stackdepot: Add a refcount field in stack_record

[Andrey Konovalov <andreyknvl@gmail.com>]

On Tue, Sep 6, 2022 at 5:54 AM Oscar Salvador <osalvador@suse.de> wrote:
>
> On Mon, Sep 05, 2022 at 10:57:20PM +0200, Andrey Konovalov wrote:
> > On Mon, Sep 5, 2022 at 5:10 AM Oscar Salvador <osalvador@suse.de> wrote:
> > > +enum stack_depot_action {
> > > +       STACK_DEPOT_ACTION_NONE,
> > > +       STACK_DEPOT_ACTION_COUNT,
> > > +};
> >
> > Hi Oscar,
>
> Hi Andrey
>
> > Why do we need these actions? Why not just increment the refcount on
> > each stack trace save?
>
> Let me try to explain it.
>
> Back in RFC, there were no actions and the refcount
> was incremented/decremented in __set_page_ownwer()
> and __reset_page_owner() functions.
>
> This lead to a performance "problem", where you would
> look for the stack twice, one when save it
> and one when increment it.

I don't get this. If you increment the refcount at the same moment
when you save a stack trace, why do you need to do the lookup twice?

> We figured we could do better and, at least, for the __set_page_owner()
> we could look just once for the stacktrace when calling __stack_depot_save,
> and increment it directly there.

Exactly.

> We cannot do that for __reset_page_owner(), because the stack we are
> saving is the freeing stacktrace, and we are not interested in that.
> That is why __reset_page_owner() does:
>
>  <---
>  depot_stack_handle_t alloc_handle;
>
>  ...
>  alloc_handle = page_owner->handle;
>  handle = save_stack(GFP_NOWAIT | __GFP_NOWARN, STACK_DEPOT_ACTION_NONE);
>  page_owner->free_handle = handle
>  stack_depot_dec_count(alloc_handle);
>  --->
>
> alloc_handle contains the handle for the allocation stacktrace, which was set
> in __set_page_owner(), and page_owner->free handle contains the handle for the
> freeing stacktrace.
> But we are only interested in the allocation stack and we only want to increment/decrement
> that on allocation/free.

But what is the problem with incrementing the refcount for free
stacktrace in __reset_page_owner? You save the stack there anyway.

Or is this because you don't want to see free stack traces when
filtering for bigger refcounts? I would argue that this is not a thing
stack depot should care about. If there's a refcount, it should
reflect the true number of references.

Perhaps, what you need is some kind of per-stack trace user metadata.
And the details of what format this metadata takes shouldn't be
present in the stack depot code.

> > Could you split out the stack depot and the page_owner changes into
> > separate patches?
>
> I could, I am not sure if it would make the review any easier though,
> as you could not match stackdepot <-> page_owner changes.
>
> And we should be adding a bunch of code that would not be used till later on.
> But I can try it out if there is a strong opinion.

Yes, splitting would be quite useful. It allows backporting stack
depot changes without having to backport any page_owner code. As long
as there are not breaking interface changes, of course.

Thanks!