linux-kernel - Re: [PATCH v2 1/3] lib/stackdepot: Add a refcount field in stack

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Date:   Tue, 6 Sep 2022 05:54:51 +0200
From:   Oscar Salvador <osalvador@...e.de>
To:     Andrey Konovalov <andreyknvl@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Eric Dumazet <edumazet@...gle.com>,
        Waiman Long <longman@...hat.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Marco Elver <elver@...gle.com>,
        Alexander Potapenko <glider@...gle.com>
Subject: Re: [PATCH v2 1/3] lib/stackdepot: Add a refcount field in
 stack_record

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@...e.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.

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