Re: [PATCH v4 29/39] unwind_user/deferred: Add unwind cache

On Wed, Jan 22, 2025 at 02:57:00PM +0100, Peter Zijlstra wrote:
> On Tue, Jan 21, 2025 at 06:31:21PM -0800, Josh Poimboeuf wrote:
> > Cache the results of the unwind to ensure the unwind is only performed
> > once, even when called by multiple tracers.
> > 
> > Signed-off-by: Josh Poimboeuf <[email protected]>
> > ---
> >  include/linux/unwind_deferred_types.h |  8 +++++++-
> >  kernel/unwind/deferred.c              | 26 ++++++++++++++++++++------
> >  2 files changed, 27 insertions(+), 7 deletions(-)
> > 
> > diff --git a/include/linux/unwind_deferred_types.h b/include/linux/unwind_deferred_types.h
> > index 9749824aea09..6f71a06329fb 100644
> > --- a/include/linux/unwind_deferred_types.h
> > +++ b/include/linux/unwind_deferred_types.h
> > @@ -2,8 +2,14 @@
> >  #ifndef _LINUX_UNWIND_USER_DEFERRED_TYPES_H
> >  #define _LINUX_UNWIND_USER_DEFERRED_TYPES_H
> >  
> > -struct unwind_task_info {
> > +struct unwind_cache {
> >  	unsigned long		*entries;
> > +	unsigned int		nr_entries;
> > +	u64			cookie;
> > +};
> 
> If you make the return to user path clear nr_entries you don't need a
> second cookie field I think.

But if the NMI happens late in the exit-to-user path, with IRQs
disabled, right before nr_entries gets cleared, the cache won't get
used in the task work.

However I think we can clear it on entry-from-user.

-- 
Josh