linux-kernel - Re: [PATCH 00/40] Memory allocation profiling

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Date:   Wed, 3 May 2023 11:07:43 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Michal Hocko <mhocko@...e.com>, akpm@...ux-foundation.org,
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Subject: Re: [PATCH 00/40] Memory allocation profiling

On Wed, May 3, 2023 at 11:03 AM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Wed, 3 May 2023 10:40:42 -0700
> Suren Baghdasaryan <surenb@...gle.com> wrote:
>
> > > This approach is actually quite common, especially since tagging every
> > > instance is usually overkill, as if you trace function calls in a running
> > > kernel, you will find that only a small percentage of the kernel ever
> > > executes. It's possible that you will be allocating a lot of tags that will
> > > never be used. If run time allocation is possible, that is usually the
> > > better approach.
> >
> > True but the memory overhead should not be prohibitive here. As a
> > ballpark number, on my machine I see there are 4838 individual
> > allocation locations and each codetag structure is 32 bytes, so that's
> > 152KB.
>
> If it's not that big, then allocating at runtime should not be an issue
> either. If runtime allocation can make it less intrusive to the code, that
> would be more rationale to do so.

As I noted, this issue is minor since we can be smart about how we
allocate these entries. The main issue is the performance overhead.
The kmalloc path is extremely fast and very hot. Even adding a per-cpu
increment in our patchset has a 35% overhead. Adding an additional
lookup here would prevent us from having it enabled all the time in
production.

>
> -- Steve