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