linux-kernel - Re: [PATCH 4/7] node: Add memory caching attributes

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Message-ID: <91698cef-cdcd-5143-884f-3da5536e156f@arm.com>
Date:   Mon, 19 Nov 2018 09:44:00 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Keith Busch <keith.busch@...el.com>, linux-kernel@...r.kernel.org,
        linux-acpi@...r.kernel.org, linux-mm@...ck.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rafael Wysocki <rafael@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH 4/7] node: Add memory caching attributes

On 11/15/2018 04:19 AM, Keith Busch wrote:
> System memory may have side caches to help improve access speed. While
> the system provided cache is transparent to the software accessing
> these memory ranges, applications can optimize their own access based
> on cache attributes.

Cache is not a separate memory attribute. It impacts how the real attributes
like bandwidth, latency e.g which are already captured in the previous patch.
What is the purpose of adding this as a separate attribute ? Can you explain
how this is going to help the user space apart from the hints it has already
received with bandwidth, latency etc properties.

> 
> In preparation for such systems, provide a new API for the kernel to
> register these memory side caches under the memory node that provides it.

Under target memory node interface /sys/devices/system/node/nodeY/target* ?

> 
> The kernel's sysfs representation is modeled from the cpu cacheinfo
> attributes, as seen from /sys/devices/system/cpu/cpuX/cache/. Unlike CPU
> cacheinfo, though, a higher node's memory cache level is nearer to the
> CPU, while lower levels are closer to the backing memory. Also unlike
> CPU cache, the system handles flushing any dirty cached memory to the
> last level the memory on a power failure if the range is persistent.

Lets assume that a CPU has got four levels of caches L1, L2, L3, L4 before
reaching memory. L4 is the backing cache for the memory and L1-L3 is from
CPU till the system bus. Hence some of them will be represented as CPU
caches and some of them will be represented as memory caches ?

/sys/devices/system/cpu/cpuX/cache/ --> L1, L2, L3
/sys/devices/system/node/nodeY/target --> L4 

L4 will be listed even if the node is memory only ?