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

[Namhyung Kim]

On Mon, Nov 13, 2023 at 08:56:39AM -0800, Namhyung Kim wrote:
> On Sat, Nov 11, 2023 at 10:49 AM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> >
> > On Fri, Nov 10, 2023 at 10:57:58PM -0800, Namhyung Kim wrote:
> > > Anyway I'm not sure it can support these additional samples for
> > > deferred callchains without breaking the existing perf tools.
> > > Anyway it doesn't know PERF_CONTEXT_USER_DEFERRED at least.
> > > I think this should be controlled by a new feature bit in the
> > > perf_event_attr.
> > >
> > > Then we might add a breaking change to have a special
> > > sample record for the deferred callchains and sample ID only.
> >
> > Sounds like a good idea.  I'll need to study the code to figure out how
> > to do that on the perf tool side.  Or would you care to write a patch?
> 
> Sure, I'd be happy to write one.

I think we can start with something like the below.
The sample id (attr.sample_type) should have
IDENTIFIER | TID | TIME to enable defer_callchain
in order to match sample and callchain records.

Thanks,
Namhyung


---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
+	 * have kernel callchains only and they need to be stitched with this
+	 * to make full callchains.
+	 *
+	 * struct {
+	 *	struct perf_event_header	header;
+	 *	u64				nr;
+	 *	u64				ips[nr];
+	 *	struct sample_id		sample_id;
+	 * };
+	 */
+	PERF_RECORD_DEFERRED_CALLCHAINS		= 22,
+
 	PERF_RECORD_MAX,			/* non-ABI */
 };