RE: About add an A64FX cache control function into resctrl

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

Hi,

Ping... any comments&advice about add an A64FX cache control function into resctrl?

Best regards
Tan Shaopeng

> Hello
> 
> 
> I'm Tan Shaopeng from Fujitsu Limited.
> 
> I’m trying to implement Fujitsu A64FX’s cache related features.
> It is a cache partitioning function we called sector cache function that using
> the value of the tag that is upper 8 bits of the 64bit address and the value of the
> sector cache register to control virtual cache capacity of the L1D&L2 cache.
> 
> A few days ago, when I sent a driver that realizes this function to
> ARM64 kernel community, Will Deacon and Arnd Bergmann suggested an idea
> to add the sector cache function of A64FX into resctrl.
> https://lore.kernel.org/linux-arm-kernel/CAK8P3a2pFcNTw9NpRtQfYr7A5Oc
> Z=As2kM0D_sbfFcGQ_J2Q+Q@mail.gmail.com/
> 
> Based on my study, I think the sector cache function of A64FX can be added
> into the allocation features of resctrl after James' resctrl rework has finished.
> But, in order to implement this function, more interfaces for resctrl are need.
> The details are as follow, and could you give me some advice?
> 
> [Sector cache function]
> The sector cache function split cache into multiple sectors and control them
> separately. It is implemented on the L1D cache and
> L2 cache in the A64FX processor and can be controlled individually for L1D
> cache and L2 cache. A64FX has no L3 cache. Each L1D cache and L2 cache
> has 4 sectors. Which L1D sector is used is specified by the value of [57:56] bits
> of address, how many ways of sector are specified by the value of register
> (IMP_SCCR_L1_EL0).
> Which L2 sector is used is specified by the value of [56] bits of address, and
> how many ways of sector are specified by value of register
> (IMP_SCCR_ASSIGN_EL1, IMP_SCCR_SET0_L2_EL1,
> IMP_SCCR_SET1_L2_EL1).
> 
> For more details of sector cache function, see A64FX HPC extension
> specification (1.2. Sector cache) in https://github.com/fujitsu/A64FX
> 
> [Difference between resctrl(CAT) and this sector cache function]
> L2/L3 CAT (Cache Allocation Technology) enables the user to specify some
> physical partition of cache space that an application can fill.
> A64FX's L1D/L2 cache has 4 sectors and 16ways. This sector function enables
> a user to specify number of ways each sector uses.
> Therefore, for CAT it is enough to specify a cache portion for each cache_id
> (socket). On the other hand, sector cache needs to specify cache portion of
> each sector for each cache_id, and following extension to resctrl interface is
> needed to support sector cache.
> 
> [Idear for A64FX sector cache function control interface (schemata file
> details)]
> L1:<cache_id0>=<cwbm>,<cwbm>,<cwbm>,<cwbm>;<cache_id1>=<cw
> bm>,<cwbm>,<cwbm>,<cwbm>;…
> L2:<cache_id0>=>=<cwbm>,<cwbm>,<cwbm>,<cwbm>;<cache_id1>=
> <cwbm>,<cwbm>,<cwbm>,<cwbm>;…
> 
> ・L1: Add a new interface to control the L1D cache.
> ・<cwbm>,<cwbm>,<cwbm>,<cwbm>：Specify the number of ways for each
> sector.
> ・cwbm：Specify the number of ways in each sector as a bitmap (percentage),
>   but the bitmap does not indicate the location of the cache.
> * In the sector cache function, L2 sector cache way setting register is
>   shared among PEs (Processor Element) in shared domain. If two PEs
>   which share L2 cache belongs to different resource groups, one resource
>   group's L2 setting will affect to other resource group's L2 setting.
> * Since A64FX does not support MPAM, it is not necessary to consider
>   how to switch between MPAM and sector cache function now.
> 
> Some questions:
> 1.I'm still studying about RDT, could you tell me whether RDT has
>   the similar mechanism with sector cache function?
> 2.In RDT, L3 cache is shared among cores in socket. If two cores which
>   share L3 cache belongs to different resource groups, one resource
>   group's L3 setting will affect to other resource group's L3 setting?
> 3.Is this approach acceptable? could you give me some advice?
> 
> 
> Best regards
> Tan Shaopeng