Re: [PATCH v2 5/6] spapr_numa: consider user input when defining associativity
On 9/25/20 12:39 AM, David Gibson wrote: On Thu, Sep 24, 2020 at 04:50:57PM -0300, Daniel Henrique Barboza wrote: This patch puts all the pieces together to finally allow user input when defining the NUMA topology of the spapr guest. We have one more kernel restriction to handle in this patch: the associativity array of node 0 must be filled with zeroes [1]. The strategy below ensures that this will happen. Can you clarify exactly what "node 0" means? Qemu and the guest kernel each have node numberings, which I don't think are necessarily the same. With PAPR's scheme, it's not totally clear what "node 0" even means. spapr_numa_define_associativity_domains() will read the distance (already PAPRified) between the nodes from numa_state and determine the appropriate NUMA level. The NUMA domains, processed in ascending order, are going to be matched via NUMA levels, and the lowest associativity domain value is assigned to that specific level for both. This will create an heuristic where the associativities of the first nodes have higher priority and are re-used in new matches, instead of overwriting them with a new associativity match. This is necessary because neither QEMU, nor the pSeries kernel, supports multiple associativity domains for each resource, meaning that we have to decide which associativity relation is relevant. Hmm. I find that a bit hard to follow. So IIUC, what's going on is you start out by treating every node as as distant as possible from every other: they have a unique value at every level of the associativity array. Then you collapse together nodes that are supposed to be closer by making some of their associativity entries match. Is that right? Ultimately, all of this results in a best effort approximation for the actual NUMA distances the user input in the command line. Given the nature of how PAPR itself interprets NUMA distances versus the expectations risen by how ACPI SLIT works, there might be better algorithms but, in the end, it'll also result in another way to approximate what the user really wanted. To keep this commit message no longer than it already is, the next patch will update the existing documentation in ppc-spapr-numa.rst with more in depth details and design considerations/drawbacks. [1] https://lore.kernel.org/linuxppc-dev/5e8fbea3-8faf-0951-172a-b41a2138f...@gmail.com/ Signed-off-by: Daniel Henrique Barboza --- hw/ppc/spapr_numa.c | 101 +++- 1 file changed, 100 insertions(+), 1 deletion(-) diff --git a/hw/ppc/spapr_numa.c b/hw/ppc/spapr_numa.c index ea33439a15..21cae3f799 100644 --- a/hw/ppc/spapr_numa.c +++ b/hw/ppc/spapr_numa.c @@ -80,12 +80,99 @@ static void spapr_numa_PAPRify_distances(MachineState *ms) } } +static uint8_t spapr_numa_get_numa_level(uint8_t distance) +{ +uint8_t numa_level; This function reinforces my feeling that pre-PAPRizing the distances might not be the best idea. It could go directly from the user distance to level just as easily. Yeah, the logic from patch 3 will ended up being folded into this function. + +switch (distance) { +case 10: +numa_level = 0x4; +break; +case 20: +numa_level = 0x3; +break; +case 40: +numa_level = 0x2; +break; +case 80: +numa_level = 0x1; +break; +default: +numa_level = 0; +} + +return numa_level; +} + +static void spapr_numa_define_associativity_domains(SpaprMachineState *spapr) +{ +MachineState *ms = MACHINE(spapr); +NodeInfo *numa_info = ms->numa_state->nodes; +int nb_numa_nodes = ms->numa_state->num_nodes; +int src, dst; + +for (src = 0; src < nb_numa_nodes; src++) { +for (dst = src; dst < nb_numa_nodes; dst++) { +/* + * This is how the associativity domain between A and B + * is calculated: + * + * - get the distance between them + * - get the correspondent NUMA level for this distance + * - the arrays were initialized with their own numa_ids, + * and we're calculating the distance in node_id ascending order, + * starting from node 0. This will have a cascade effect in the + * algorithm because the associativity domains that node 0 defines + * will be carried over to the other nodes, and node 1 + * associativities will be carried over unless there's already a + * node 0 associativity assigned, and so on. This happens because + * we'll assign the lowest value of assoc_src and assoc_dst to be + * the associativity domain of both, for the given NUMA level. Ok, I follow this description better than the one in the commit message. + * The PPC kernel expects the associativity domains of node 0 to + * be always 0, and this algorithm will grant that by default. + */ +
Re: [PATCH v2 5/6] spapr_numa: consider user input when defining associativity
On Thu, Sep 24, 2020 at 04:50:57PM -0300, Daniel Henrique Barboza wrote: > This patch puts all the pieces together to finally allow user > input when defining the NUMA topology of the spapr guest. > > We have one more kernel restriction to handle in this patch: > the associativity array of node 0 must be filled with zeroes > [1]. The strategy below ensures that this will happen. Can you clarify exactly what "node 0" means? Qemu and the guest kernel each have node numberings, which I don't think are necessarily the same. With PAPR's scheme, it's not totally clear what "node 0" even means. > spapr_numa_define_associativity_domains() will read the distance > (already PAPRified) between the nodes from numa_state and determine > the appropriate NUMA level. The NUMA domains, processed in ascending > order, are going to be matched via NUMA levels, and the lowest > associativity domain value is assigned to that specific level for > both. > > This will create an heuristic where the associativities of the first > nodes have higher priority and are re-used in new matches, instead of > overwriting them with a new associativity match. This is necessary > because neither QEMU, nor the pSeries kernel, supports multiple > associativity domains for each resource, meaning that we have to > decide which associativity relation is relevant. Hmm. I find that a bit hard to follow. So IIUC, what's going on is you start out by treating every node as as distant as possible from every other: they have a unique value at every level of the associativity array. Then you collapse together nodes that are supposed to be closer by making some of their associativity entries match. Is that right? > Ultimately, all of this results in a best effort approximation for > the actual NUMA distances the user input in the command line. Given > the nature of how PAPR itself interprets NUMA distances versus the > expectations risen by how ACPI SLIT works, there might be better > algorithms but, in the end, it'll also result in another way to > approximate what the user really wanted. > > To keep this commit message no longer than it already is, the next > patch will update the existing documentation in ppc-spapr-numa.rst > with more in depth details and design considerations/drawbacks. > > [1] > https://lore.kernel.org/linuxppc-dev/5e8fbea3-8faf-0951-172a-b41a2138f...@gmail.com/ > > Signed-off-by: Daniel Henrique Barboza > --- > hw/ppc/spapr_numa.c | 101 +++- > 1 file changed, 100 insertions(+), 1 deletion(-) > > diff --git a/hw/ppc/spapr_numa.c b/hw/ppc/spapr_numa.c > index ea33439a15..21cae3f799 100644 > --- a/hw/ppc/spapr_numa.c > +++ b/hw/ppc/spapr_numa.c > @@ -80,12 +80,99 @@ static void spapr_numa_PAPRify_distances(MachineState *ms) > } > } > > +static uint8_t spapr_numa_get_numa_level(uint8_t distance) > +{ > +uint8_t numa_level; This function reinforces my feeling that pre-PAPRizing the distances might not be the best idea. It could go directly from the user distance to level just as easily. > + > +switch (distance) { > +case 10: > +numa_level = 0x4; > +break; > +case 20: > +numa_level = 0x3; > +break; > +case 40: > +numa_level = 0x2; > +break; > +case 80: > +numa_level = 0x1; > +break; > +default: > +numa_level = 0; > +} > + > +return numa_level; > +} > + > +static void spapr_numa_define_associativity_domains(SpaprMachineState *spapr) > +{ > +MachineState *ms = MACHINE(spapr); > +NodeInfo *numa_info = ms->numa_state->nodes; > +int nb_numa_nodes = ms->numa_state->num_nodes; > +int src, dst; > + > +for (src = 0; src < nb_numa_nodes; src++) { > +for (dst = src; dst < nb_numa_nodes; dst++) { > +/* > + * This is how the associativity domain between A and B > + * is calculated: > + * > + * - get the distance between them > + * - get the correspondent NUMA level for this distance > + * - the arrays were initialized with their own numa_ids, > + * and we're calculating the distance in node_id ascending order, > + * starting from node 0. This will have a cascade effect in the > + * algorithm because the associativity domains that node 0 > defines > + * will be carried over to the other nodes, and node 1 > + * associativities will be carried over unless there's already a > + * node 0 associativity assigned, and so on. This happens because > + * we'll assign the lowest value of assoc_src and assoc_dst to be > + * the associativity domain of both, for the given NUMA level. Ok, I follow this description better than the one in the commit message. > + * The PPC kernel expects the associativity domains of node 0 to > + * be always 0, and this algorithm w