linux-kernel - Re: [RFC PATCH V2 01/22] x86/intel_rdt: Documentation for Cache Pseudo-Locking

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Message-ID: <ae8252f6-83b4-d8d6-8eaf-ecbae78339aa@infradead.org>
Date:   Mon, 19 Feb 2018 13:27:33 -0800
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Reinette Chatre <reinette.chatre@...el.com>, tglx@...utronix.de,
        fenghua.yu@...el.com, tony.luck@...el.com
Cc:     gavin.hindman@...el.com, vikas.shivappa@...ux.intel.com,
        dave.hansen@...el.com, mingo@...hat.com, hpa@...or.com,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH V2 01/22] x86/intel_rdt: Documentation for Cache
 Pseudo-Locking

On 02/13/18 07:46, Reinette Chatre wrote:
> Add description of Cache Pseudo-Locking feature, its interface,
> as well as an example of its usage.
> 
> Signed-off-by: Reinette Chatre <reinette.chatre@...el.com>
> ---
>  Documentation/x86/intel_rdt_ui.txt | 229 ++++++++++++++++++++++++++++++++++++-
>  1 file changed, 228 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/x86/intel_rdt_ui.txt b/Documentation/x86/intel_rdt_ui.txt
> index 756fd76b78a6..bb3d6fe0a3e4 100644
> --- a/Documentation/x86/intel_rdt_ui.txt
> +++ b/Documentation/x86/intel_rdt_ui.txt

> @@ -329,6 +332,149 @@ L3CODE:0=fffff;1=fffff;2=fffff;3=fffff
>  L3DATA:0=fffff;1=fffff;2=3c0;3=fffff
>  L3CODE:0=fffff;1=fffff;2=fffff;3=fffff
>  
> +Cache Pseudo-Locking
> +--------------------
> +CAT enables a user to specify the amount of cache space into which an

                                                     space that an

> +application can fill. Cache pseudo-locking builds on the fact that a
> +CPU can still read and write data pre-allocated outside its current
> +allocated area on a cache hit. With cache pseudo-locking, data can be
> +preloaded into a reserved portion of cache that no application can
> +fill, and from that point on will only serve cache hits. The cache
> +pseudo-locked memory is made accessible to user space where an
> +application can map it into its virtual address space and thus have
> +a region of memory with reduced average read latency.
> +
> +Cache pseudo-locking increases the probability that data will remain
> +in the cache via carefully configuring the CAT feature and controlling
> +application behavior. There is no guarantee that data is placed in
> +cache. Instructions like INVD, WBINVD, CLFLUSH, etc. can still evict
> +“locked” data from cache. Power management C-states may shrink or
> +power off cache. It is thus recommended to limit the processor maximum
> +C-state, for example, by setting the processor.max_cstate kernel parameter.
> +
> +It is required that an application using a pseudo-locked region runs
> +with affinity to the cores (or a subset of the cores) associated
> +with the cache on which the pseudo-locked region resides. This is
> +enforced by the implementation.
> +
> +Pseudo-locking is accomplished in two stages:
> +1) During the first stage the system administrator allocates a portion
> +   of cache that should be dedicated to pseudo-locking. At this time an
> +   equivalent portion of memory is allocated, loaded into allocated
> +   cache portion, and exposed as a character device.
> +2) During the second stage a user-space application maps (mmap()) the
> +   pseudo-locked memory into its address space.
> +
> +Cache Pseudo-Locking Interface
> +------------------------------
> +Platforms supporting cache pseudo-locking will expose a new
> +"/sys/fs/restrl/pseudo_lock" directory after successful mount of the
> +resctrl filesystem. Initially this directory will contain a single file,
> +"avail" that contains the schemata, one line per resource, of cache region
> +available for pseudo-locking.

uh, sysfs is supposed to be one value per file.

> +A pseudo-locked region is created by creating a new directory within
> +/sys/fs/resctrl/pseudo_lock. On success two new files will appear in
> +the directory:
> +
> +"schemata":
> +	Shows the schemata representing the pseudo-locked cache region.
> +	User writes schemata of requested locked area to file.

	use complete sentences, please. E.g.:

	The user writes the schemata of the requested locked area to the file.

> +	Only one id of single resource accepted - can only lock from

	            of a single resource is accepted -

> +	single cache instance. Writing of schemata to this file will
> +	return success on successful pseudo-locked region setup.
> +"size":
> +	After successful pseudo-locked region setup this read-only file
> +	will contain the size in bytes of pseudo-locked region.


-- 
~Randy