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

On Wed, Jan 22, 2025 at 03:16:16PM +0100, Peter Zijlstra wrote:
> On Wed, Jan 22, 2025 at 02:37:30PM +0100, Peter Zijlstra wrote:
> > On Tue, Jan 21, 2025 at 06:31:20PM -0800, Josh Poimboeuf wrote:
> > > +/*
> > > + * The context cookie is a unique identifier which allows post-processing to
> > > + * correlate kernel trace(s) with user unwinds.  The high 12 bits are the CPU
> > 
> > s/12/16/ ?
> > 
> > > + * id; the lower 48 bits are a per-CPU entry counter.
> > > + */
> > > +static u64 ctx_to_cookie(u64 cpu, u64 ctx)
> > > +{
> > > +	BUILD_BUG_ON(NR_CPUS > 65535);
> > > +	return (ctx & ((1UL << 48) - 1)) | (cpu << 48);
> > > +}
> 
> Also, I have to note that 0 is a valid return value here, which will
> give a ton of fun.

The ctx_ctr is always incremented before calling this, so 0 isn't a
valid cookie.

-- 
Josh