Re: [PATCH V11 06/10] arm64/perf: Enable branch stack events via FEAT_BRBE

[Mark Rutland <mark.rutland@arm.com>]

On Fri, Jun 09, 2023 at 01:47:18PM +0100, Mark Rutland wrote:
> On Fri, Jun 09, 2023 at 10:52:37AM +0530, Anshuman Khandual wrote:
> > On 6/5/23 19:13, Mark Rutland wrote:
> > > Looking at <linux/perf_event.h> I see:
> > > 
> > > | /*
> > > |  * branch stack layout:
> > > |  *  nr: number of taken branches stored in entries[]
> > > |  *  hw_idx: The low level index of raw branch records
> > > |  *          for the most recent branch.
> > > |  *          -1ULL means invalid/unknown.
> > > |  *
> > > |  * Note that nr can vary from sample to sample
> > > |  * branches (to, from) are stored from most recent
> > > |  * to least recent, i.e., entries[0] contains the most
> > > |  * recent branch.
> > > |  * The entries[] is an abstraction of raw branch records,
> > > |  * which may not be stored in age order in HW, e.g. Intel LBR.
> > > |  * The hw_idx is to expose the low level index of raw
> > > |  * branch record for the most recent branch aka entries[0].
> > > |  * The hw_idx index is between -1 (unknown) and max depth,
> > > |  * which can be retrieved in /sys/devices/cpu/caps/branches.
> > > |  * For the architectures whose raw branch records are
> > > |  * already stored in age order, the hw_idx should be 0.
> > > |  */
> > > | struct perf_branch_stack {
> > > |         __u64                           nr;  
> > > |         __u64                           hw_idx;
> > > |         struct perf_branch_entry        entries[];
> > > | };
> > > 
> > > ... which seems to indicate we should be setting hw_idx to 0, since IIUC our
> > > records are in age order.
> > Branch records are indeed in age order, sure will change hw_idx as 0. Earlier
> > figured that there was no need for hw_idx and hence marked it as -1UL similar
> > to other platforms like powerpc.
> 
> That's fair enough; looking at power_pmu_bhrb_read() in
> arch/powerpc/perf/core-book3s.c, I see a comment:
> 
> 	Branches are read most recent first (ie. mfbhrb 0 is
> 	the most recent branch).
> 
> ... which suggests that should be 0 also, or that the documentation is wrong.
> 
> Do you know how the perf tool consumes this?


Thinking about this some more, if what this is saying is that if entries[0]
must be strictly the last branch, and we've lost branches due to interrupt
latency, then we clearly don't meet that requirement and must report -1ULL
here.

So while it'd be nice to figure this out, I'm happy using -1ULL, and would be a
bit concerned using 0.

Sorry for flip-flopping on this.

Thanks,
Mark.