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@xxxxxxx> 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.
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.
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.
> 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.
thanks for your time!
--
Oscar Salvador
SUSE Labs
