On Wed, May 3, 2023 at 11:03 AM Steven Rostedt [off-list ref] wrote:

On Wed, 3 May 2023 10:40:42 -0700
Suren Baghdasaryan [off-list ref] wrote:

quoted

quoted

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