Re: [PATCH v2 2/2] perf mem: Support HITM for when mem_lvl_num is used

Leo Yan
2022-03-13T12:44:42Z

On Wed, Mar 02, 2022 at 03:39:04PM +0000, German Gomez wrote:
>
> On 21/02/2022 22:48, Ali Saidi wrote:
> > Current code only support HITM statistics for last level cache (LLC)
> > when mem_lvl encodes the level. On existing Arm64 machines there are as
> > many as four levels cache and this change supports decoding l1, l2, and
> > llc hits from the mem_lvl_num data. Given that the mem_lvl namespace is
> > being deprecated take this opportunity to encode the neoverse data into
> > mem_lvl_num.
>
> Since Neoverse is mentioned in the commit message, I think there should be a comment somewhere in the code as well.
>

> > For loads that hit in a the LLC snoop filter and are fullfilled from a
> > higher level cache, it's not usually clear what the true level of the
> > cache the data came from (i.e. a transfer from a core could come from
> > it's L1 or L2). Instead of making an assumption of where the line came
> > from, add support for incrementing HITM if the source is CACHE_ANY.
> >
> > Since other architectures don't seem to populate the mem_lvl_num field
> > here there shouldn't be a change in functionality.
> >
> > Signed-off-by: Ali Saidi <alisaidi@amazon.com>
> > ---
> > tools/perf/util/mem-events.c | 14 ++++++++++----
> > 1 file changed, 10 insertions(+), 4 deletions(-)
> >
> > diff --git a/tools/perf/util/mem-events.c b/tools/perf/util/mem-events.c
> > index ed0ab838bcc5..6c3fd4aac7ae 100644
> > --- a/tools/perf/util/mem-events.c
> > +++ b/tools/perf/util/mem-events.c
> > @@ -485,6 +485,7 @@ int c2c_decode_stats(struct c2c_stats *stats, struct mem_info *mi)
> > u64 daddr = mi->daddr.addr;
> > u64 op = data_src->mem_op;
> > u64 lvl = data_src->mem_lvl;
> > + u64 lnum = data_src->mem_lvl_num;
> > u64 snoop = data_src->mem_snoop;
> > u64 lock = data_src->mem_lock;
> > u64 blk = data_src->mem_blk;
> > @@ -527,16 +528,18 @@ do { \
> > if (lvl & P(LVL, UNC)) stats->ld_uncache++;
> > if (lvl & P(LVL, IO)) stats->ld_io++;
> > if (lvl & P(LVL, LFB)) stats->ld_fbhit++;
> > - if (lvl & P(LVL, L1 )) stats->ld_l1hit++;
> > - if (lvl & P(LVL, L2 )) stats->ld_l2hit++;
> > - if (lvl & P(LVL, L3 )) {
> > + if (lvl & P(LVL, L1) || lnum == P(LVLNUM, L1))
> > + stats->ld_l1hit++;
> > + if (lvl & P(LVL, L2) || lnum == P(LVLNUM, L2))
> > + stats->ld_l2hit++;

It's good to split into two patches: one patch is to add statistics for
field 'mem_lvl_num', the second patch is to handle HITM tags.

> > + if (lvl & P(LVL, L3) || lnum == P(LVLNUM, L4)) {

It's a bit weird that we take either PERF_MEM_LVL_L3 or
PERF_MEM_LVLNUM_L4 as the last level local cache in the same condition
checking.

> According to a comment in the previous patch, using L4 is specific to Neoverse, right?
>
> Maybe we need to distinguish the Neoverse case from the generic one here as well
>
> if (is_neoverse)
> // treat L4 as llc
> else
> // treat L3 as llc

I personally think it's not good idea to distinguish platforms in the decoding code.

To make more more clear statistics, we can firstly increment hit values
for every level cache respectively; so we can consider to adde two
extra statistics items 'stats->ld_l3hit' and 'stats->ld_l4hit'.

if (lvl & P(LVL, L3) || lnum == P(LVLNUM, L3))
stats->ld_l3hit++;
if (lnum == P(LVLNUM, L4))
stats->ld_l4hit++;

> > if (snoop & P(SNOOP, HITM))
> > HITM_INC(lcl_hitm);
> > else
> > stats->ld_llchit++;

For the statistics of 'ld_llchit' and 'lcl_hitm', please see below comment.

> > }
> >
> > - if (lvl & P(LVL, LOC_RAM)) {
> > + if (lvl & P(LVL, LOC_RAM) || lnum == P(LVLNUM, RAM)) {
> > stats->lcl_dram++;
> > if (snoop & P(SNOOP, HIT))
> > stats->ld_shared++;
> > @@ -564,6 +567,9 @@ do { \
> > HITM_INC(rmt_hitm);
> > }
> >
> > + if (lnum == P(LVLNUM, ANY_CACHE) && snoop & P(SNOOP, HITM))
> > + HITM_INC(lcl_hitm);
> > +

The condition checking of "lnum == P(LVLNUM, ANY_CACHE)" is a bit
suspecious and it might be fragile for support multiple archs.

So I am just wandering if it's possible that we add a new field
'llc_level' in the structure 'mem_info', we can initialize this field
based on different memory hardware events (e.g. Intel mem event,
Arm SPE, etc). During the decoding phase, the local last level cache
is dynamically set to 'mem_info:: llc_level', we can base on it to
increment 'ld_llchit' and 'lcl_hitm', the code is like below:

if ((lvl & P(LVL, REM_CCE1)) ||
(lvl & P(LVL, REM_CCE2)) ||
mrem) {
if (snoop & P(SNOOP, HIT))
stats->rmt_hit++;
else if (snoop & P(SNOOP, HITM))
HITM_INC(rmt_hitm);
+ } else {
+ if ((snoop & P(SNOOP, HIT)) && (lnum == mi->llc_level))
+ stats->ld_llchit++;
+ else if (snoop & P(SNOOP, HITM))
+ HITM_INC(lcl_hitm);
}

Thanks,
Leo

> > if ((lvl & P(LVL, MISS)))
> > stats->ld_miss++;
> >