linux-kernel - Re: [PATCH v2 2/2] perf mem: Support HITM for when mem_lvl

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Date:   Sun, 13 Mar 2022 19:19:33 +0000
From:   Ali Saidi <alisaidi@...zon.com>
To:     <leo.yan@...aro.org>
CC:     <acme@...nel.org>, <alexander.shishkin@...ux.intel.com>,
        <alisaidi@...zon.com>, <andrew.kilroy@....com>,
        <benh@...nel.crashing.org>, <german.gomez@....com>,
        <james.clark@....com>, <john.garry@...wei.com>, <jolsa@...nel.org>,
        <kjain@...ux.ibm.com>, <lihuafei1@...wei.com>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <linux-perf-users@...r.kernel.org>,
        <mark.rutland@....com>, <mathieu.poirier@...aro.org>,
        <mingo@...hat.com>, <namhyung@...nel.org>, <peterz@...radead.org>,
        <will@...nel.org>, <yao.jin@...ux.intel.com>
Subject: Re: [PATCH v2 2/2] perf mem: Support HITM for when mem_lvl_num is used

Hi Leo,

On Sun, 13 Mar 2022 12:46:02 +0000, Leo Yan wrote:
> 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@...zon.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.

I agree here. The more we talk about this, the more I'm wondering if we're
spending too much code solving a problem that doesn't exist. I know of no
Neoverse systems that actually have 4 cache levels, they all actually have three
even though it's technically possible to have four.  I have some doubts anyone
will actually build four levels of cache and perhaps the most prudent path here
is to assume only three levels (and adjust the previous patch) until someone 
actually produces a system with four levels instead of a lot of code that is
never actually exercised?

Thanks,
Ali