Re: [PATCH RFC 04/10] perf: Introduce deferred user callchains

[Peter Zijlstra]

On Fri, Sep 13, 2024 at 06:08:34AM -0700, Josh Poimboeuf wrote:
> On Mon, Nov 20, 2023 at 03:03:34PM +0100, Peter Zijlstra wrote:
> > On Wed, Nov 15, 2023 at 08:13:31AM -0800, Namhyung Kim wrote:
> > 
> > > ---8<---
> > > diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> > > index 39c6a250dd1b..a3765ff59798 100644
> > > --- a/include/uapi/linux/perf_event.h
> > > +++ b/include/uapi/linux/perf_event.h
> > > @@ -456,7 +456,8 @@ struct perf_event_attr {
> > >  				inherit_thread :  1, /* children only inherit if cloned with CLONE_THREAD */
> > >  				remove_on_exec :  1, /* event is removed from task on exec */
> > >  				sigtrap        :  1, /* send synchronous SIGTRAP on event */
> > > -				__reserved_1   : 26;
> > > +				defer_callchain:  1, /* generate DEFERRED_CALLCHAINS records for userspace */
> > > +				__reserved_1   : 25;
> > >  
> > >  	union {
> > >  		__u32		wakeup_events;	  /* wakeup every n events */
> > > @@ -1207,6 +1208,20 @@ enum perf_event_type {
> > >  	 */
> > >  	PERF_RECORD_AUX_OUTPUT_HW_ID		= 21,
> > >  
> > > +	/*
> > > +	 * Deferred user stack callchains (for SFrame).  Previous samples would
> > 
> > Possibly also useful for ShadowStack based unwinders. And by virtue of
> > it possibly saving work when multiple consecutive samples hit
> > the same kernel section, for everything.
> 
> [ necroing old thread as I'm finally working on a v2 ]
> 
> Peter, can you elaborate?  What did you mean by "same kernel section"?
> 
> Like if there's a duplicate kernel callchain?  Or something else?

Yeah, multiple samples hitting the same syscall invocation will, by
necessity, have the same user callchain.