RE: About add an A64FX cache control function into resctrl

["tan.shaopeng@fujitsu.com" <tan.shaopeng@fujitsu.com>]

Hi Reinette,

> On 5/25/2021 1:45 AM, tan.shaopeng@fujitsu.com wrote:
> > Hi Reinette,
> >
> > Sorry, I have not explained A64FX's sector cache function well yet.
> > I think I need explain this function from different perspective.
> 
> You have explained the A64FX's sector cache function well. I have also read
> both specs to understand it better. It appears to me that you are not considering
> the resctrl architecture as part of your solution but instead just forcing your
> architecture onto the resctrl filesystem. For example, in resctrl the resource
> groups are not just a directory structure but has significance in what is being
> represented within the directory (a class of service). The files within a resource
> group's directory build on that. From your side I have not seen any effort in
> aligning the sector cache function with the resctrl architecture but instead you
> are just changing resctrl interface to match the A64FX architecture.
> 
> Could you please take a moment to understand what resctrl is and how it could
> be mapped to A64FX in a coherent way?

Thanks for your mail.
Sorry, I’m wrong in understanding how to use 
/sys/fs/resctrl/p0/cpus and /sys/fs/resctrl/tasks. 
I think I have not understood resctrl yet, and I will learn more about it.
If I have questions, please allow me to mail you.

> >> On 5/17/2021 1:31 AM, tan.shaopeng@fujitsu.com wrote:
> 
> > --------
> > A64FX NUMA-PE-Cache Architecture:
> > NUMA0:
> >    PE0:
> >      L1sector0,L1sector1,L1sector2,L1sector3
> >    PE1:
> >      L1sector0,L1sector1,L1sector2,L1sector3
> >    ...
> >    PE11:
> >      L1sector0,L1sector1,L1sector2,L1sector3
> >
> >    L2sector0,1/L2sector2,3
> > NUMA1:
> >    PE0:
> >      L1sector0,L1sector1,L1sector2,L1sector3
> >    ...
> >    PE11:
> >      L1sector0,L1sector1,L1sector2,L1sector3
> >
> >    L2sector0,1/L2sector2,3
> > NUMA2:
> >    ...
> > NUMA3:
> >    ...
> > --------
> > In A64FX processor, one L1 sector cache capacity setting register is
> > only for one PE and not shared among PEs. L2 sector cache maximum
> > capacity setting registers are shared among PEs in same NUMA, and it
> > is to be noted that changing these registers in one PE influences other PE.
> 
> Understood. cache affinity is familiar to resctrl. When a CPU becomes online it
> is discovered which caches/resources it has affinity to.
> Resources then have CPU mask associated with them to indicate on which
> CPU a register could be changed to configure the resource/cache. See
> domain_add_cpu() and struct rdt_domain.
> 
> > The number of ways for L2 Sector ID (0,1 or 2,3) can be set through
> > any PEs in same NUMA. The sector ID 0,1 and 2,3 are not available at
> > the same time in same NUMA.
> >
> >
> > I think, in your idea, a resource group will be created for each sector ID.
> > (> "sectors" could be considered the same as the resctrl "classes of
> > service") Then, an example of resource group is created as follows.
> > ・ L1: NUMAX-PEY-L1sector0 (X = 0,1,2,3.Y = 0,1,2 ... 11),
> > ・ L2: NUMAX-L2sector0 (X = 0,1,2,3)
> >
> > In this example, sector with same ID(0) of all PEs is allocated to
> > resource group. The L1D caches are numbered from
> > NUMA0_PE0-L1sector0(0) to NUMA4_PE11-L1sector0(47) and the L2
> caches
> > numbered from
> > NUMA0-L2sector0(0) to NUM4-L2sector0(3).
> > (NUMA number X is from 0-4, PE number Y is from 0-11)
> > (1) The number of ways of NUMAX-PEY-L1sector0 can be set independently
> >      for each PEs (0-47). When run a task on this resource group,
> >      we cannot control on which PE the task is running on and how many
> >      cache ways the task is using.
> 
> resctrl does not control the affinity on which PE/CPU a task is run.
> resctrl is an interface with which to configure how resources are allocated on
> the system. resctrl could thus provide interface with which each sector of each
> cache instance is assigned a number of cache ways.
> resctrl also provides an interface to assign a task with a class of service (sector
> id?). Through this the task obtains access to all resources that is allocated to
> the particular class of service (sector id?). Depending on which CPU the task is
> running it may indeed experience different performance if the sector id it is
> running with does not have the same allocations on all cache instances. The
> affinity of the task needs to be managed separately using for example taskset.
> Please see Documentation/x86/resctrl.rst "Examples for RDT allocation usage"
> 
> > (2) Since L2 can only use 2 sectors at a time, when creating more than
> >      2 resource groups, L2setctor0 will have to be allocated to a
> >      different resource group. If the L2sector0 is shared by different
> >      resource groups, the L2 sector settings on resource group will be
> >      influenced by each other.
> > etc... there are various problems, and no merit to using resctrl.
> >
> >
> > In my idea, in order to allocate the L1 and L2 cache to a resource
> > group, allocate NUMA to the resource group.
> > An example of resource group is as follows.
> > ・ NUMA0-PEY-L1sectorZ (Y = 0,1,2...11. Z = 0,1,2,3)
> > ・ NUMA0-L2sectorZZ (ZZ = 0,1,2,3)
> >
> >    #cat /sys/fs/resctrl/p0/cpus
> >    0-11 *1
> >    #cat /sys/fs/resctrl/p0/schemata
> >    L1:0=0xF,0x3,0x1,x0x0 *2
> >    L2:0=0xFFF,0xF,0,0 *3
> >
> > *1: PEs belong one NUMA. (Of course, multiple NUMAs can also be
> >      specified in one resource group)
> > *2: The number of ways for L1sector0,1,2,3. On this resource group
> >      the number of ways of all sector0 is the same(0xF). If 0 way is
> >      specified for one sector, this sector cannot be used. If 4(0xF)
> >      ways are specified for one sector, this sector can use cache fully.
> >      If 4 ways are specified for each sector, there will be no
> >      restriction for using cache.
> > *3: The number of ways for L2 sector 0,1. If L2sector0,1 is used,
> >      the number of ways of L2sector2,3 must be set to 0.
> >
> > All sectors with the same ID on the same resource group were set to
> > the same number of ways, and when running a task on A64FX, the sector
> > ID used by task is determined by [56:57] bits of virtual address.
> > By specifying the PID to /sys/fs/resctrl/tasks, the task will be bound
> > to the resource group, and then, the cache size used by task will not
> > be changed never.
> 
> This completely ignores how this directory and files are currently used.
> What is missing how this implementation maps to the current resctrl
> architecture.

Best regards,
Tan Shaopeng