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Date:   Thu, 12 Oct 2023 16:22:50 +0100
From:   Mel Gorman <mgorman@techsingularity.net>
To:     "Huang, Ying" <ying.huang@intel.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Arjan Van De Ven <arjan@linux.intel.com>,
        Sudeep Holla <sudeep.holla@arm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        David Hildenbrand <david@redhat.com>,
        Johannes Weiner <jweiner@redhat.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Michal Hocko <mhocko@suse.com>,
        Pavel Tatashin <pasha.tatashin@soleen.com>,
        Matthew Wilcox <willy@infradead.org>,
        Christoph Lameter <cl@linux.com>
Subject: Re: [PATCH 02/10] cacheinfo: calculate per-CPU data cache size
Message-ID: <20231012152250.xuu5mvghwtonpvp2@techsingularity.net>
References: <20230920061856.257597-1-ying.huang@intel.com>
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 <87h6mwf3gf.fsf@yhuang6-desk2.ccr.corp.intel.com>
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On Thu, Oct 12, 2023 at 09:12:00PM +0800, Huang, Ying wrote:
> Mel Gorman <mgorman@techsingularity.net> writes:
> 
> > On Thu, Oct 12, 2023 at 08:08:32PM +0800, Huang, Ying wrote:
> >> Mel Gorman <mgorman@techsingularity.net> writes:
> >> 
> >> > On Wed, Sep 20, 2023 at 02:18:48PM +0800, Huang Ying wrote:
> >> >> Per-CPU data cache size is useful information.  For example, it can be
> >> >> used to determine per-CPU cache size.  So, in this patch, the data
> >> >> cache size for each CPU is calculated via data_cache_size /
> >> >> shared_cpu_weight.
> >> >> 
> >> >> A brute-force algorithm to iterate all online CPUs is used to avoid
> >> >> to allocate an extra cpumask, especially in offline callback.
> >> >> 
> >> >> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
> >> >
> >> > It's not necessarily relevant to the patch, but at least the scheduler
> >> > also stores some per-cpu topology information such as sd_llc_size -- the
> >> > number of CPUs sharing the same last-level-cache as this CPU. It may be
> >> > worth unifying this at some point if it's common that per-cpu
> >> > information is too fine and per-zone or per-node information is too
> >> > coarse. This would be particularly true when considering locking
> >> > granularity,
> >> >
> >> >> Cc: Sudeep Holla <sudeep.holla@arm.com>
> >> >> Cc: Andrew Morton <akpm@linux-foundation.org>
> >> >> Cc: Mel Gorman <mgorman@techsingularity.net>
> >> >> Cc: Vlastimil Babka <vbabka@suse.cz>
> >> >> Cc: David Hildenbrand <david@redhat.com>
> >> >> Cc: Johannes Weiner <jweiner@redhat.com>
> >> >> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> >> >> Cc: Michal Hocko <mhocko@suse.com>
> >> >> Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
> >> >> Cc: Matthew Wilcox <willy@infradead.org>
> >> >> Cc: Christoph Lameter <cl@linux.com>
> >> >> ---
> >> >>  drivers/base/cacheinfo.c  | 42 ++++++++++++++++++++++++++++++++++++++-
> >> >>  include/linux/cacheinfo.h |  1 +
> >> >>  2 files changed, 42 insertions(+), 1 deletion(-)
> >> >> 
> >> >> diff --git a/drivers/base/cacheinfo.c b/drivers/base/cacheinfo.c
> >> >> index cbae8be1fe52..3e8951a3fbab 100644
> >> >> --- a/drivers/base/cacheinfo.c
> >> >> +++ b/drivers/base/cacheinfo.c
> >> >> @@ -898,6 +898,41 @@ static int cache_add_dev(unsigned int cpu)
> >> >>  	return rc;
> >> >>  }
> >> >>  
> >> >> +static void update_data_cache_size_cpu(unsigned int cpu)
> >> >> +{
> >> >> +	struct cpu_cacheinfo *ci;
> >> >> +	struct cacheinfo *leaf;
> >> >> +	unsigned int i, nr_shared;
> >> >> +	unsigned int size_data = 0;
> >> >> +
> >> >> +	if (!per_cpu_cacheinfo(cpu))
> >> >> +		return;
> >> >> +
> >> >> +	ci = ci_cacheinfo(cpu);
> >> >> +	for (i = 0; i < cache_leaves(cpu); i++) {
> >> >> +		leaf = per_cpu_cacheinfo_idx(cpu, i);
> >> >> +		if (leaf->type != CACHE_TYPE_DATA &&
> >> >> +		    leaf->type != CACHE_TYPE_UNIFIED)
> >> >> +			continue;
> >> >> +		nr_shared = cpumask_weight(&leaf->shared_cpu_map);
> >> >> +		if (!nr_shared)
> >> >> +			continue;
> >> >> +		size_data += leaf->size / nr_shared;
> >> >> +	}
> >> >> +	ci->size_data = size_data;
> >> >> +}
> >> >
> >> > This needs comments.
> >> >
> >> > It would be nice to add a comment on top describing the limitation of
> >> > CACHE_TYPE_UNIFIED here in the context of
> >> > update_data_cache_size_cpu().
> >> 
> >> Sure.  Will do that.
> >> 
> >
> > Thanks.
> >
> >> > The L2 cache could be unified but much smaller than a L3 or other
> >> > last-level-cache. It's not clear from the code what level of cache is being
> >> > used due to a lack of familiarity of the cpu_cacheinfo code but size_data
> >> > is not the size of a cache, it appears to be the share of a cache a CPU
> >> > would have under ideal circumstances.
> >> 
> >> Yes.  And it isn't for one specific level of cache.  It's sum of per-CPU
> >> shares of all levels of cache.  But the calculation is inaccurate.  More
> >> details are in the below reply.
> >> 
> >> > However, as it appears to also be
> >> > iterating hierarchy then this may not be accurate. Caches may or may not
> >> > allow data to be duplicated between levels so the value may be inaccurate.
> >> 
> >> Thank you very much for pointing this out!  The cache can be inclusive
> >> or not.  So, we cannot calculate the per-CPU slice of all-level caches
> >> via adding them together blindly.  I will change this in a follow-on
> >> patch.
> >> 
> >
> > Please do, I would strongly suggest basing this on LLC only because it's
> > the only value you can be sure of. This change is the only change that may
> > warrant a respin of the series as the history will be somewhat confusing
> > otherwise.
> 
> I am still checking whether it's possible to get cache inclusive
> information via cpuid.
> 

cpuid may be x86-specific so that potentially leads to different behaviours
on different architectures.

> If there's no reliable way to do that.  We can use the max value of
> per-CPU share of each level of cache.  For inclusive cache, that will be
> the value of LLC.  For non-inclusive cache, the value will be more
> accurate.  For example, on Intel Sapphire Rapids, the L2 cache is 2 MB
> per core, while LLC is 1.875 MB per core according to [1].
> 

Be that as it may, it still opens the possibility of significantly different
behaviour depending on the CPU family. I would strongly recommend that you
start with LLC only because LLC is also the topology level of interest used
by the scheduler and it's information that is generally available. Trying
to get accurate information on every level and the complexity of dealing
with inclusive vs exclusive cache or write-back vs write-through should
be a separate patch, with separate justification and notes on how it can
lead to behaviour specific to the CPU family or architecture.

-- 
Mel Gorman
SUSE Labs