Re: [PATCH v4 34/39] perf: Skip user unwind if !current->mm

[Peter Zijlstra]

On Wed, Jan 22, 2025 at 03:08:38PM -0800, Josh Poimboeuf wrote:
> On Wed, Jan 22, 2025 at 03:29:10PM +0100, Peter Zijlstra wrote:
> > On Tue, Jan 21, 2025 at 06:31:26PM -0800, Josh Poimboeuf wrote:
> > > If the task doesn't have any memory, there's no stack to unwind.
> > > 
> > > Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> > > ---
> > >  kernel/events/core.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > > index 99f0f28feeb5..a886bb83f4d0 100644
> > > --- a/kernel/events/core.c
> > > +++ b/kernel/events/core.c
> > > @@ -7792,7 +7792,7 @@ struct perf_callchain_entry *
> > >  perf_callchain(struct perf_event *event, struct pt_regs *regs)
> > >  {
> > >  	bool kernel = !event->attr.exclude_callchain_kernel;
> > > -	bool user   = !event->attr.exclude_callchain_user;
> > > +	bool user   = !event->attr.exclude_callchain_user && current->mm;
> > 
> > What about things like io_uring helpers, don't they keep ->mm but are
> > never in userspace?
> 
> Hm, that's news to me.  At least this patch doesn't make things worse in
> that regard.
> 
> IIRC, user_regs(task_pt_regs(current)) returns false for kthreads
> because the regs area is cleared.  We could add in a user_regs() check?

Possibly, I've not yet dug into the io_worker thing in much detail yet.