Re: [PATCH 1/3] perf: Add macros to specify onchip L2/L3 accesses

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Sep 09, 2021 at 10:45:54PM +1000, Michael Ellerman wrote:

> > The 'new' composite doesnt have a hops field because the hardware that
> > nessecitated that change doesn't report it, but we could easily add a
> > field there.
> >
> > Suppose we add, mem_hops:3 (would 6 hops be too small?) and the
> > corresponding PERF_MEM_HOPS_{NA, 0..6}
> 
> It's really 7 if we use remote && hop = 0 to mean the first hop.

I don't think we can do that, becaus of backward compat. Currently:

  lvl_num=DRAM, remote=1

denites: "Remote DRAM of any distance". Effectively it would have the new
hops field filled with zeros though, so if you then decode with the hops
field added it suddenly becomes:

 lvl_num=DRAM, remote=1, hops=0

and reads like: "Remote DRAM of 0 hops" which is quite daft. Therefore 0
really must denote a 'N/A'.

> If we're wanting to use some of the hop levels to represent
> intra-chip/package hops then we could possibly use them all on a really
> big system.
> 
> eg. you could imagine something like:
> 
>  L2 | 		        - local L2
>  L2 | REMOTE | HOPS_0	- L2 of neighbour core
>  L2 | REMOTE | HOPS_1	- L2 of near core on same chip (same 1/2 of chip)
>  L2 | REMOTE | HOPS_2	- L2 of far core on same chip (other 1/2 of chip)
>  L2 | REMOTE | HOPS_3	- L2 of sibling chip in same package
>  L2 | REMOTE | HOPS_4	- L2 on separate package 1 hop away
>  L2 | REMOTE | HOPS_5	- L2 on separate package 2 hops away
>  L2 | REMOTE | HOPS_6	- L2 on separate package 3 hops away
> 
> 
> Whether it's useful to represent all those levels I'm not sure, but it's
> probably good if we have the ability.

I'm thinking we ought to keep hops as steps along the NUMA fabric, with
0 hops being the local node. That only gets us:

 L2, remote=0, hops=HOPS_0 -- our L2
 L2, remote=1, hops=HOPS_0 -- L2 on the local node but not ours
 L2, remote=1, hops!=HOPS_0 -- L2 on a remote node

> I guess I'm 50/50 on whether that's enough levels, or whether we want
> another bit to allow for future growth.

Right, possibly safer to add one extra bit while we can.... I suppose.