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

[Thomas Gleixner <tglx@linutronix.de>]

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On Mon, 19 Feb 2018, Reinette Chatre wrote:
> Hi Thomas,
> 
> On 2/19/2018 12:35 PM, Thomas Gleixner wrote:
> > On Tue, 13 Feb 2018, Reinette Chatre wrote:
> >> +Cache Pseudo-Locking
> >> +--------------------
> >> +CAT enables a user to specify the amount of cache space into which 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.
> > 
> > This lacks explanation how that preloading works.
> 
> Following this text you quote there is a brief explanation starting with
> "Pseudo-locking is accomplished in two stages:" - I'll add more details
> to that area.
> 
> > 
> >> 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.
> > 
> > Well, you only enforce in pseudo_lock_dev_mmap() that the caller is affine
> > to the right CPUs. But that's not a guarantee that the task stays there.
> 
> It is required that the user space application self sets affinity to
> cores associated with the cache. This is also highlighted in the example
> application code (later in this patch) within the comments as well as
> the example usage of sched_setaffinity(). The enforcement done in the
> kernel code is done as a check that the user space application did so,
> no the actual affinity management.

Right, but your documentation claims it's enforced. There is no enforcement
aside of the initial sanity check.

Thanks,

	tglx