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

[Josh Poimboeuf <jpoimboe@kernel.org>]

On Fri, Sep 13, 2024 at 03:36:29PM +0200, Peter Zijlstra wrote:
> 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.

Ah, I thought you were talking about kernel callchains for some reason.
n/m...

-- 
Josh