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From: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>
Sent: Friday, April 8, 2022 5:55 PM To: Oscar Salvador <osalvador@xxxxxxx> Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>; linuxppc-dev <linuxppc-dev@xxxxxxxxxxxxxxxx>; linux-mm@xxxxxxxxx <linux-mm@xxxxxxxxx>; Michal Hocko <mhocko@xxxxxxxxxx>; Geetika Moolchandani1 <Geetika.Moolchandani1@xxxxxxx> Subject: Re: [PATCH] powerpc/numa: Handle partially initialized numa nodes * Oscar Salvador <osalvador@xxxxxxx> [2022-04-06 18:19:00]:
> On Wed, Mar 30, 2022 at 07:21:23PM +0530, Srikar Dronamraju wrote: > > arch/powerpc/mm/numa.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c > > index b9b7fefbb64b..13022d734951 100644 > > --- a/arch/powerpc/mm/numa.c > > +++ b/arch/powerpc/mm/numa.c > > @@ -1436,7 +1436,7 @@ int find_and_online_cpu_nid(int cpu) > > if (new_nid < 0 || !node_possible(new_nid)) > > new_nid = first_online_node; > > > > - if (NODE_DATA(new_nid) == NULL) { > > + if (!node_online(new_nid)) { > > #ifdef CONFIG_MEMORY_HOTPLUG > > /* > > * Need to ensure that NODE_DATA is initialized for a node from > > Because of this fix, I wanted to check whether we might have any more fallouts due > to ("mm: handle uninitialized numa nodes gracefully"), and it made me look closer > as to why powerpc is the only platform that special cases try_online_node(), > while all others rely on cpu_up()->try_online_node() to do the right thing. > > So, I had a look. > Let us rewind a bit: > > The commit that sets find_and_online_cpu_nid() in dlpar_online_cpu was > e67e02a544e9 ("powerpc/pseries: Fix cpu hotplug crash with memoryless nodes"). > > In there, it says that we have the following picture: > > partition_sched_domains > arch_update_cpu_topology > numa_update_cpu_topology > find_and_online_cpu_nid > > and by the time find_and_online_cpu_nid() gets called to online the node, it might be > too late as we might have referenced some NODE_DATA() fields. > Note that this happens at a much later stage in cpuhp. > > Also note that at a much earlier stage, we do already have a try_online_node() in cpu_up(), > which should allocate-and-online the node and prevent accessing garbage. > But the problem is that, on powerpc, all possible cpus have the same node set at boot stage > (see arch/powerpc/mm/numa.c:mem_topology_setup), > so cpu_to_node() returns the same thing until it the mapping gets update (which happens in > start_secondary()->set_numa_node()), and so, the try_online_node() from cpu_up() does not > apply on the right node, because it still holds the not-up-to-date mapping node <-> cpu. > > (e.g: in my test case, when onlining a CPU belongin to node1, cpu_up()->try_online_node() > tries to online node0, or whatever old mapping numa<->cpu is there). > > So, we have something like: > > dlpar_online_cpu > device_online > dev->bus->online > cpu_subsys_online > cpu_device_up > cpu_up > try_online_node (old mapping nid <-> cpu ) > ... > ... > cphp_callbacks > sched_cpu_activate > cpuset_update_active_cpus > schedule_work(&cpuset_hotplug_work) > cpuset_hotplug_work > partition_sched_domains > arch_update_cpu_topology > numa_update_cpu_topology > find_and_online_cpu_nid (online new_nid) > > > So, yeah, the real onlining in numa_update_cpu_topology()->find_and_online_cpu_nid() > happens too late, that is why adding find_and_online_cpu_nid() back in dlpar_online_cpu() > fixed the issue, but we should not need this special casing at all. > > We do already know the numa<->cpu associativity in > dlpar_online_cpu()->find_and_online_cpu_nid(), so we should just be able to > update the numa<->cpu mapping, and let the try_online_node() do the right thing. > > With this in mind, I came up with the following patch, where I carried out a battery > of tests for CPU hotplug stuff and it worked as expected. > But I am not familiar with all powerpc pitfalls, e.g: dedicated vs shared cpus etc, so > I would like to hear from more familiar people. > > The patch is: > > From: Oscar Salvador <osalvador@xxxxxxx> > Date: Wed, 6 Apr 2022 14:39:15 +0200 > Subject: [PATCH] powerpc/numa: Associate numa node to its cpu earlier > > powerpc is the only platform that do not rely on > cpu_up()->try_online_node() to bring up a numa node, > and special cases it, instead, deep in its own machinery: > > dlpar_online_cpu > find_and_online_cpu_nid > try_online_node > > This should not be needed, but the thing is that the try_online_node() > from cpu_up() will not apply on the right node, because cpu_to_node() > will return the old mapping numa<->cpu that gets set on boot stage > for all possible cpus. > > That can be seen easily if we try to print out the numa node passed > to try_online_node() in cpu_up(). > > The thing is that the numa<->cpu mapping does not get updated till a much > later stage in start_secondary: > > start_secondary: > set_numa_node(numa_cpu_lookup_table[cpu]) > > But we do not really care, as we already now the > CPU <-> NUMA associativity back in find_and_online_cpu_nid(), > so let us make use of that and set the proper numa<->cpu mapping, > so cpu_to_node() in cpu_up() returns the right node and > try_online_node() can do its work. > > Signed-off-by: Oscar Salvador <osalvador@xxxxxxx> Looks good to me. Reviewed-by: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx> > --- > arch/powerpc/include/asm/topology.h | 8 ++----- > arch/powerpc/mm/numa.c | 31 +++++++--------------------- > arch/powerpc/platforms/pseries/hotplug-cpu.c | 2 +- > 3 files changed, 11 insertions(+), 30 deletions(-) > > diff --git a/arch/powerpc/include/asm/topology.h b/arch/powerpc/include/asm/topology.h > index 36fcafb1fd6d..6ae1b2dce83e 100644 > --- a/arch/powerpc/include/asm/topology.h > +++ b/arch/powerpc/include/asm/topology.h > @@ -111,14 +111,10 @@ static inline void unmap_cpu_from_node(unsigned long cpu) {} > #endif /* CONFIG_NUMA */ > > #if defined(CONFIG_NUMA) && defined(CONFIG_PPC_SPLPAR) > -extern int find_and_online_cpu_nid(int cpu); > +extern void find_and_update_cpu_nid(int cpu); > extern int cpu_to_coregroup_id(int cpu); > #else > -static inline int find_and_online_cpu_nid(int cpu) > -{ > - return 0; > -} > - > +static inline void find_and_update_cpu_nid(int cpu) {} > static inline int cpu_to_coregroup_id(int cpu) > { > #ifdef CONFIG_SMP > diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c > index b9b7fefbb64b..b5bc8b1a833d 100644 > --- a/arch/powerpc/mm/numa.c > +++ b/arch/powerpc/mm/numa.c > @@ -1423,43 +1423,28 @@ static long vphn_get_associativity(unsigned long cpu, > return rc; > } > > -int find_and_online_cpu_nid(int cpu) > +void find_and_update_cpu_nid(int cpu) > { > __be32 associativity[VPHN_ASSOC_BUFSIZE] = {0}; > int new_nid; > > /* Use associativity from first thread for all siblings */ > if (vphn_get_associativity(cpu, associativity)) > - return cpu_to_node(cpu); > + return; > > + /* Do not have previous associativity, so find it now. */ > new_nid = associativity_to_nid(associativity); > + > if (new_nid < 0 || !node_possible(new_nid)) > new_nid = first_online_node; > - > - if (NODE_DATA(new_nid) == NULL) { > -#ifdef CONFIG_MEMORY_HOTPLUG > - /* > - * Need to ensure that NODE_DATA is initialized for a node from > - * available memory (see memblock_alloc_try_nid). If unable to > - * init the node, then default to nearest node that has memory > - * installed. Skip onlining a node if the subsystems are not > - * yet initialized. > - */ > - if (!topology_inited || try_online_node(new_nid)) > - new_nid = first_online_node; > -#else > - /* > - * Default to using the nearest node that has memory installed. > - * Otherwise, it would be necessary to patch the kernel MM code > - * to deal with more memoryless-node error conditions. > + else > + /* Associate node <-> cpu, so cpu_up() calls > + * try_online_node() on the right node. > */ > - new_nid = first_online_node; > -#endif > - } > + set_cpu_numa_node(cpu, new_nid); > > pr_debug("%s:%d cpu %d nid %d\n", __FUNCTION__, __LINE__, > cpu, new_nid); > - return new_nid; > } > > int cpu_to_coregroup_id(int cpu) > diff --git a/arch/powerpc/platforms/pseries/hotplug-cpu.c b/arch/powerpc/platforms/pseries/hotplug-cpu.c > index b81fc846d99c..0f8cd8b06432 100644 > --- a/arch/powerpc/platforms/pseries/hotplug-cpu.c > +++ b/arch/powerpc/platforms/pseries/hotplug-cpu.c > @@ -398,7 +398,7 @@ static int dlpar_online_cpu(struct device_node *dn) > if (get_hard_smp_processor_id(cpu) != thread) > continue; > cpu_maps_update_done(); > - find_and_online_cpu_nid(cpu); > + find_and_update_cpu_nid(cpu); > rc = device_online(get_cpu_device(cpu)); > if (rc) { > dlpar_offline_cpu(dn); > -- > 2.16.4 > > -- > Oscar Salvador > SUSE Labs Tested-by: Geetika Moolchandani <Geetika.Moolchandani1@xxxxxxx>
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