Messages in this thread | | | Date | Fri, 1 Jul 2016 09:50:41 -0700 | From | "Luck, Tony" <> | Subject | Re: [PATCH] cacheinfo: Introduce cache id |
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On Fri, Jul 01, 2016 at 12:21:43PM +0200, Borislav Petkov wrote: > On Wed, Jun 29, 2016 at 06:56:10PM -0700, Fenghua Yu wrote: > > From: Fenghua Yu <fenghua.yu@intel.com> > > > > Each cache node is described by cacheinfo and is a unique node across > > What is a cache node?
Clearly not a good name for the concept we are trying to communicate here. Better suggestions welcome!
Here's the situation. We have lots of (mostly) independent caches on a system. The L3 cache (also called LLC - Last Level Cache - in some documentation) is usually shared by all cpus on a physical socket. The L1 and L2 caches are typically local to a core, so only shared by the hyperthreads on a core. But I say "usually" and "typically" because the architecture doesn't require that. We could have multiple separate L3 caches on a socket with a subset of cpus assigned to each of them. We could have an L2 cache that is shared by two or more cores.
/sys/devices/system/cpu/cpu*/cache/index*/shared_cpu_{list,map} allow for arbitrary sharing schemes and tell you which caches are shared among which groups of Linux logical cpus.
What we don't have is a *name* for each cache.
CAT (Intel Cache Allocation Technology) allows us to partition caches on a per task basis. E.g. process A can only use 20% of the L3 cache on socket0, process B can use 40% (that doesn't overlap with the 20% assigned to process A - or perhaps does overlap - the feature is quite versatile). But since caches aren't necessarily mapped to sockets or cores, we need a name for each cache.
> And since this patch adds the generic functionality and then enables it > on x86, it should be split into two patches.
Mea culpa. Fenghua had split this as you suggest, and I said "with just 23 lines added this can be one patch". Sorry Fenghua.
-Tony
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