On Wed, Jul 29, 2020 at 09:57:56AM -0300, Daniel Henrique Barboza wrote: > This patch adds a new documentation file, ppc-spapr-numa.rst, > informing what developers and user can expect of the NUMA distance > support for the pseries machine, up to QEMU 5.1. > > In the (hopefully soon) future, when we rework the NUMA mechanics > of the pseries machine to at least attempt to contemplate user > choice, this doc will be extended to inform about the new > support. > > Signed-off-by: Daniel Henrique Barboza <danielhb...@gmail.com>
Applied to ppc-for-5.2, thanks. > --- > docs/specs/ppc-spapr-numa.rst | 191 ++++++++++++++++++++++++++++++++++ > 1 file changed, 191 insertions(+) > create mode 100644 docs/specs/ppc-spapr-numa.rst > > diff --git a/docs/specs/ppc-spapr-numa.rst b/docs/specs/ppc-spapr-numa.rst > new file mode 100644 > index 0000000000..e762038022 > --- /dev/null > +++ b/docs/specs/ppc-spapr-numa.rst > @@ -0,0 +1,191 @@ > + > +NUMA mechanics for sPAPR (pseries machines) > +============================================ > + > +NUMA in sPAPR works different than the System Locality Distance > +Information Table (SLIT) in ACPI. The logic is explained in the LOPAPR > +1.1 chapter 15, "Non Uniform Memory Access (NUMA) Option". This > +document aims to complement this specification, providing details > +of the elements that impacts how QEMU views NUMA in pseries. > + > +Associativity and ibm,associativity property > +-------------------------------------------- > + > +Associativity is defined as a group of platform resources that has > +similar mean performance (or in our context here, distance) relative to > +everyone else outside of the group. > + > +The format of the ibm,associativity property varies with the value of > +bit 0 of byte 5 of the ibm,architecture-vec-5 property. The format with > +bit 0 equal to zero is deprecated. The current format, with the bit 0 > +with the value of one, makes ibm,associativity property represent the > +physical hierarchy of the platform, as one or more lists that starts > +with the highest level grouping up to the smallest. Considering the > +following topology: > + > +:: > + > + Mem M1 ---- Proc P1 | > + ----------------- | Socket S1 ---| > + chip C1 | | > + | HW module 1 (MOD1) > + Mem M2 ---- Proc P2 | | > + ----------------- | Socket S2 ---| > + chip C2 | > + > +The ibm,associativity property for the processors would be: > + > +* P1: {MOD1, S1, C1, P1} > +* P2: {MOD1, S2, C2, P2} > + > +Each allocable resource has an ibm,associativity property. The LOPAPR > +specification allows multiple lists to be present in this property, > +considering that the same resource can have multiple connections to the > +platform. > + > +Relative Performance Distance and ibm,associativity-reference-points > +-------------------------------------------------------------------- > + > +The ibm,associativity-reference-points property is an array that is used > +to define the relevant performance/distance related boundaries, defining > +the NUMA levels for the platform. > + > +The definition of its elements also varies with the value of bit 0 of byte 5 > +of the ibm,architecture-vec-5 property. The format with bit 0 equal to zero > +is also deprecated. With the current format, each integer of the > +ibm,associativity-reference-points represents an 1 based ordinal index (i.e. > +the first element is 1) of the ibm,associativity array. The first > +boundary is the most significant to application performance, followed by > +less significant boundaries. Allocated resources that belongs to the > +same performance boundaries are expected to have relative NUMA distance > +that matches the relevancy of the boundary itself. Resources that belongs > +to the same first boundary will have the shortest distance from each > +other. Subsequent boundaries represents greater distances and degraded > +performance. > + > +Using the previous example, the following setting reference points defines > +three NUMA levels: > + > +* ibm,associativity-reference-points = {0x3, 0x2, 0x1} > + > +The first NUMA level (0x3) is interpreted as the third element of each > +ibm,associativity array, the second level is the second element and > +the third level is the first element. Let's also consider that elements > +belonging to the first NUMA level have distance equal to 10 from each > +other, and each NUMA level doubles the distance from the previous. This > +means that the second would be 20 and the third level 40. For the P1 and > +P2 processors, we would have the following NUMA levels: > + > +:: > + > + * ibm,associativity-reference-points = {0x3, 0x2, 0x1} > + > + * P1: associativity{MOD1, S1, C1, P1} > + > + First NUMA level (0x3) => associativity[2] = C1 > + Second NUMA level (0x2) => associativity[1] = S1 > + Third NUMA level (0x1) => associativity[0] = MOD1 > + > + * P2: associativity{MOD1, S2, C2, P2} > + > + First NUMA level (0x3) => associativity[2] = C2 > + Second NUMA level (0x2) => associativity[1] = S2 > + Third NUMA level (0x1) => associativity[0] = MOD1 > + > + P1 and P2 have the same third NUMA level, MOD1: Distance between them = 40 > + > +Changing the ibm,associativity-reference-points array changes the performance > +distance attributes for the same associativity arrays, as the following > +example illustrates: > + > +:: > + > + * ibm,associativity-reference-points = {0x2} > + > + * P1: associativity{MOD1, S1, C1, P1} > + > + First NUMA level (0x2) => associativity[1] = S1 > + > + * P2: associativity{MOD1, S2, C2, P2} > + > + First NUMA level (0x2) => associativity[1] = S2 > + > + P1 and P2 does not have a common performance boundary. Since this is a one > level > + NUMA configuration, distance between them is one boundary above the first > + level, 20. > + > + > +In a hypothetical platform where all resources inside the same hardware > module > +is considered to be on the same performance boundary: > + > +:: > + > + * ibm,associativity-reference-points = {0x1} > + > + * P1: associativity{MOD1, S1, C1, P1} > + > + First NUMA level (0x1) => associativity[0] = MOD0 > + > + * P2: associativity{MOD1, S2, C2, P2} > + > + First NUMA level (0x1) => associativity[0] = MOD0 > + > + P1 and P2 belongs to the same first order boundary. The distance between > then > + is 10. > + > + > +How the pseries Linux guest calculates NUMA distances > +===================================================== > + > +Another key difference between ACPI SLIT and the LOPAPR regarding NUMA is > +how the distances are expressed. The SLIT table provides the NUMA distance > +value between the relevant resources. LOPAPR does not provide a standard > +way to calculate it. We have the ibm,associativity for each resource, which > +provides a common-performance hierarchy, and the > ibm,associativity-reference-points > +array that tells which level of associativity is considered to be relevant > +or not. > + > +The result is that each OS is free to implement and to interpret the distance > +as it sees fit. For the pseries Linux guest, each level of NUMA duplicates > +the distance of the previous level, and the maximum amount of levels is > +limited to MAX_DISTANCE_REF_POINTS = 4 (from arch/powerpc/mm/numa.c in the > +kernel tree). This results in the following distances: > + > +* both resources in the first NUMA level: 10 > +* resources one NUMA level apart: 20 > +* resources two NUMA levels apart: 40 > +* resources three NUMA levels apart: 80 > +* resources four NUMA levels apart: 160 > + > + > +Consequences for QEMU NUMA tuning > +--------------------------------- > + > +The way the pseries Linux guest calculates NUMA distances has a direct effect > +on what QEMU users can expect when doing NUMA tuning. As of QEMU 5.1, this is > +the default ibm,associativity-reference-points being used in the pseries > +machine: > + > +ibm,associativity-reference-points = {0x4, 0x4, 0x2} > + > +The first and second level are equal, 0x4, and a third one was added in > +commit a6030d7e0b35 exclusively for NVLink GPUs support. This means that > +regardless of how the ibm,associativity properties are being created in > +the device tree, the pseries Linux guest will only recognize three scenarios > +as far as NUMA distance goes: > + > +* if the resources belongs to the same first NUMA level = 10 > +* second level is skipped since it's equal to the first > +* all resources that aren't a NVLink GPU, it is guaranteed that they will > belong > + to the same third NUMA level, having distance = 40 > +* for NVLink GPUs, distance = 80 from everything else > + > +In short, we can summarize the NUMA distances seem in pseries Linux guests, > using > +QEMU up to 5.1, as follows: > + > +* local distance, i.e. the distance of the resource to its own NUMA node: 10 > +* if it's a NVLink GPU device, distance: 80 > +* every other resource, distance: 40 > + > +This also means that user input in QEMU command line does not change the > +NUMA distancing inside the guest for the pseries machine. -- David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ | _way_ _around_! http://www.ozlabs.org/~dgibson
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