Hi Valentin: > -----Original Message----- > From: Valentin Schneider [mailto:valentin.schnei...@arm.com] > Sent: Saturday, August 29, 2020 9:00 PM > To: linux-kernel@vger.kernel.org; linux-arm-ker...@lists.infradead.org > Cc: Catalin Marinas; Will Deacon; Sudeep Holla; Robin Murphy; Jeremy > Linton; Dietmar Eggemann; Morten Rasmussen; Zengtao (B) > Subject: [PATCH] arm64: topology: Stop using MPIDR for topology > information > > In the absence of ACPI or DT topology data, we fallback to haphazardly > decoding *something* out of MPIDR. Sadly, the contents of that register > are > mostly unusable due to the implementation leniancy and things like Aff0 > having to be capped to 15 (despite being encoded on 8 bits). > > Consider a simple system with a single package of 32 cores, all under the > same LLC. We ought to be shoving them in the same core_sibling mask, > but > MPIDR is going to look like: > > | CPU | 0 | ... | 15 | 16 | ... | 31 | > |------+---+-----+----+----+-----+----+ > | Aff0 | 0 | ... | 15 | 0 | ... | 15 | > | Aff1 | 0 | ... | 0 | 1 | ... | 1 | > | Aff2 | 0 | ... | 0 | 0 | ... | 0 | > > Which will eventually yield > > core_sibling(0-15) == 0-15 > core_sibling(16-31) == 16-31 > > NUMA woes > ========= > > If we try to play games with this and set up NUMA boundaries within those > groups of 16 cores via e.g. QEMU: > > # Node0: 0-9; Node1: 10-19 > $ qemu-system-aarch64 <blah> \ > -smp 20 -numa node,cpus=0-9,nodeid=0 -numa > node,cpus=10-19,nodeid=1 > > The scheduler's MC domain (all CPUs with same LLC) is going to be built via > > arch_topology.c::cpu_coregroup_mask() > > In there we try to figure out a sensible mask out of the topology > information we have. In short, here we'll pick the smallest of NUMA or > core sibling mask. > > node_mask(CPU9) == 0-9 > core_sibling(CPU9) == 0-15 > > MC mask for CPU9 will thus be 0-9, not a problem. > > node_mask(CPU10) == 10-19 > core_sibling(CPU10) == 0-15 > > MC mask for CPU10 will thus be 10-19, not a problem. > > node_mask(CPU16) == 10-19 > core_sibling(CPU16) == 16-19 > > MC mask for CPU16 will thus be 16-19... Uh oh. CPUs 16-19 are in two > different unique MC spans, and the scheduler has no idea what to make of > that. That triggers the WARN_ON() added by commit > > ccf74128d66c ("sched/topology: Assert non-NUMA topology masks > don't (partially) overlap") > > Fixing MPIDR-derived topology > ============================= > > We could try to come up with some cleverer scheme to figure out which of > the available masks to pick, but really if one of those masks resulted from > MPIDR then it should be discarded because it's bound to be bogus. > > I was hoping to give MPIDR a chance for SMT, to figure out which threads > are > in the same core using Aff1-3 as core ID, but Sudeep and Robin pointed out > to me that there are systems out there where *all* cores have non-zero > values in their higher affinity fields (e.g. RK3288 has "5" in all of its > cores' MPIDR.Aff1), which would expose a bogus core ID to userspace. > > Stop using MPIDR for topology information. When no other source of > topology > information is available, mark each CPU as its own core and its NUMA > node > as its LLC domain.
I agree with your idea to remove the topology functionality of MPIDR , but I think we need also consider ARM32 and GIC. > > Signed-off-by: Valentin Schneider <valentin.schnei...@arm.com> > --- > arch/arm64/kernel/topology.c | 32 +++++++++++++++++--------------- > 1 file changed, 17 insertions(+), 15 deletions(-) > > diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c > index 0801a0f3c156..ff1dd1dbfe64 100644 > --- a/arch/arm64/kernel/topology.c > +++ b/arch/arm64/kernel/topology.c > @@ -36,21 +36,23 @@ void store_cpu_topology(unsigned int cpuid) > if (mpidr & MPIDR_UP_BITMASK) > return; > > - /* Create cpu topology mapping based on MPIDR. */ > - if (mpidr & MPIDR_MT_BITMASK) { > - /* Multiprocessor system : Multi-threads per core */ > - cpuid_topo->thread_id = MPIDR_AFFINITY_LEVEL(mpidr, 0); > - cpuid_topo->core_id = MPIDR_AFFINITY_LEVEL(mpidr, 1); > - cpuid_topo->package_id = MPIDR_AFFINITY_LEVEL(mpidr, 2) | > - MPIDR_AFFINITY_LEVEL(mpidr, 3) << 8; > - } else { > - /* Multiprocessor system : Single-thread per core */ > - cpuid_topo->thread_id = -1; > - cpuid_topo->core_id = MPIDR_AFFINITY_LEVEL(mpidr, 0); > - cpuid_topo->package_id = MPIDR_AFFINITY_LEVEL(mpidr, 1) | > - MPIDR_AFFINITY_LEVEL(mpidr, 2) << 8 | > - MPIDR_AFFINITY_LEVEL(mpidr, 3) << 16; > - } > + /* > + * This would be the place to create cpu topology based on MPIDR. > + * > + * However, it cannot be trusted to depict the actual topology; some > + * pieces of the architecture enforce an artificial cap on Aff0 values > + * (e.g. GICv3's ICC_SGI1R_EL1 limits it to 15), leading to an > + * artificial cycling of Aff1, Aff2 and Aff3 values. IOW, these end up > + * having absolutely no relationship to the actual underlying system > + * topology, and cannot be reasonably used as core / package ID. > + * > + * If the MT bit is set, Aff0 *could* be used to define a thread ID, but > + * we still wouldn't be able to obtain a sane core ID. This means we > + * need to entirely ignore MPIDR for any topology deduction. > + */ > + cpuid_topo->thread_id = -1; > + cpuid_topo->core_id = cpuid; > + cpuid_topo->package_id = cpu_to_node(cpuid); > > pr_debug("CPU%u: cluster %d core %d thread %d mpidr %#016llx\n", > cpuid, cpuid_topo->package_id, cpuid_topo->core_id, > -- > 2.27.0