Re: [PATCH v4 28/39] unwind_user/deferred: Add deferred unwinding interface

Josh Poimboeuf
2025-01-22T21:52:44Z

On Wed, Jan 22, 2025 at 02:44:20PM +0100, Peter Zijlstra wrote:
> On Tue, Jan 21, 2025 at 06:31:20PM -0800, Josh Poimboeuf wrote:
>
> > +/* entry-from-user counter */
> > +static DEFINE_PER_CPU(u64, unwind_ctx_ctr);
>
> AFAICT from the below, this thing does *not* count entry-from-user. It
> might count a subset, but I need to stare longer.

Right, it's a subset. Something like so:

/*
* This is a unique percpu identifier for a given task entry context.
* Conceptually, it's incremented every time the CPU enters the kernel from
* user space, so that each "entry context" on the CPU gets a unique ID. In
* reality, as an optimization, it's only incremented on demand for the first
* deferred unwind request after a given entry-from-user.
*
* It's combined with the CPU id to make a systemwide-unique "context cookie".
*/
static DEFINE_PER_CPU(u64, unwind_ctx_ctr);

--
Josh