[PATCH 1/3] powerpc/pseries/cpuhp: cache node corrections
Daniel Henrique Barboza
danielhb413 at gmail.com
Tue Sep 21 09:59:14 AEST 2021
On 9/20/21 10:55, Nathan Lynch wrote:
> On pseries, cache nodes in the device tree can be added and removed by the
> CPU DLPAR code as well as the partition migration (mobility) code. PowerVM
> partitions in dedicated processor mode typically have L2 and L3 cache
> nodes.
>
> The CPU DLPAR code has the following shortcomings:
>
> * Cache nodes returned as siblings of a new CPU node by
> ibm,configure-connector are silently discarded; only the CPU node is
> added to the device tree.
>
> * Cache nodes which become unreferenced in the processor removal path are
> not removed from the device tree. This can lead to duplicate nodes when
> the post-migration device tree update code replaces cache nodes.
>
> This is long-standing behavior. Presumably it has gone mostly unnoticed
> because the two bugs have the property of obscuring each other in common
> simple scenarios (e.g. remove a CPU and add it back). Likely you'd notice
> only if you cared to inspect the device tree or the sysfs cacheinfo
> information.
>
> Booted with two processors:
>
> $ pwd
> /sys/firmware/devicetree/base/cpus
> $ ls -1d */
> l2-cache at 2010/
> l2-cache at 2011/
> l3-cache at 3110/
> l3-cache at 3111/
> PowerPC,POWER9 at 0/
> PowerPC,POWER9 at 8/
> $ lsprop */l2-cache
> l2-cache at 2010/l2-cache
> 00003110 (12560)
> l2-cache at 2011/l2-cache
> 00003111 (12561)
> PowerPC,POWER9 at 0/l2-cache
> 00002010 (8208)
> PowerPC,POWER9 at 8/l2-cache
> 00002011 (8209)
> $ ls /sys/devices/system/cpu/cpu0/cache/
> index0 index1 index2 index3
>
> After DLPAR-adding PowerPC,POWER9 at 10, we see that its associated cache
> nodes are absent, its threads' L2+L3 cacheinfo is unpopulated, and it is
> missing a cache level in its sched domain hierarchy:
>
> $ ls -1d */
> l2-cache at 2010/
> l2-cache at 2011/
> l3-cache at 3110/
> l3-cache at 3111/
> PowerPC,POWER9 at 0/
> PowerPC,POWER9 at 10/
> PowerPC,POWER9 at 8/
> $ lsprop PowerPC\,POWER9 at 10/l2-cache
> PowerPC,POWER9 at 10/l2-cache
> 00002012 (8210)
> $ ls /sys/devices/system/cpu/cpu16/cache/
> index0 index1
> $ grep . /sys/kernel/debug/sched/domains/cpu{0,8,16}/domain*/name
> /sys/kernel/debug/sched/domains/cpu0/domain0/name:SMT
> /sys/kernel/debug/sched/domains/cpu0/domain1/name:CACHE
> /sys/kernel/debug/sched/domains/cpu0/domain2/name:DIE
> /sys/kernel/debug/sched/domains/cpu8/domain0/name:SMT
> /sys/kernel/debug/sched/domains/cpu8/domain1/name:CACHE
> /sys/kernel/debug/sched/domains/cpu8/domain2/name:DIE
> /sys/kernel/debug/sched/domains/cpu16/domain0/name:SMT
> /sys/kernel/debug/sched/domains/cpu16/domain1/name:DIE
>
> When removing PowerPC,POWER9 at 8, we see that its cache nodes are left
> behind:
>
> $ ls -1d */
> l2-cache at 2010/
> l2-cache at 2011/
> l3-cache at 3110/
> l3-cache at 3111/
> PowerPC,POWER9 at 0/
>
> When DLPAR is combined with VM migration, we can get duplicate nodes. E.g.
> removing one processor, then migrating, adding a processor, and then
> migrating again can result in warnings from the OF core during
> post-migration device tree updates:
>
> Duplicate name in cpus, renamed to "l2-cache at 2011#1"
> Duplicate name in cpus, renamed to "l3-cache at 3111#1"
>
> and nodes with duplicated phandles in the tree, making lookup behavior
> unpredictable:
>
> $ lsprop l[23]-cache@*/ibm,phandle
> l2-cache at 2010/ibm,phandle
> 00002010 (8208)
> l2-cache at 2011#1/ibm,phandle
> 00002011 (8209)
> l2-cache at 2011/ibm,phandle
> 00002011 (8209)
> l3-cache at 3110/ibm,phandle
> 00003110 (12560)
> l3-cache at 3111#1/ibm,phandle
> 00003111 (12561)
> l3-cache at 3111/ibm,phandle
> 00003111 (12561)
>
> Address these issues by:
>
> * Correctly processing siblings of the node returned from
> dlpar_configure_connector().
> * Removing cache nodes in the CPU remove path when it can be determined
> that they are not associated with other CPUs or caches.
>
> Use the of_changeset API in both cases, which allows us to keep the error
> handling in this code from becoming more complex while ensuring that the
> device tree cannot become inconsistent.
>
> Signed-off-by: Nathan Lynch <nathanl at linux.ibm.com>
> Fixes: ac71380 ("powerpc/pseries: Add CPU dlpar remove functionality")
> Fixes: 90edf18 ("powerpc/pseries: Add CPU dlpar add functionality")
> ---
> arch/powerpc/platforms/pseries/hotplug-cpu.c | 72 +++++++++++++++++++-
> 1 file changed, 70 insertions(+), 2 deletions(-)
Tested with a QEMU pseries guest, multiple CPU add/removals of the same CPU,
and no issues found with these new pseries_cpuhp* functions.
Code LGTM as well.
Reviewed-by: Daniel Henrique Barboza <danielhb413 at gmail.com>
Tested-by: Daniel Henrique Barboza <danielhb413 at gmail.com>
>
> diff --git a/arch/powerpc/platforms/pseries/hotplug-cpu.c b/arch/powerpc/platforms/pseries/hotplug-cpu.c
> index d646c22e94ab..87a0fbe9cf12 100644
> --- a/arch/powerpc/platforms/pseries/hotplug-cpu.c
> +++ b/arch/powerpc/platforms/pseries/hotplug-cpu.c
> @@ -521,6 +521,27 @@ static bool valid_cpu_drc_index(struct device_node *parent, u32 drc_index)
> return found;
> }
>
> +static int pseries_cpuhp_attach_nodes(struct device_node *dn)
> +{
> + struct of_changeset cs;
> + int ret;
> +
> + /*
> + * This device node is unattached but may have siblings; open-code the
> + * traversal.
> + */
> + for (of_changeset_init(&cs); dn != NULL; dn = dn->sibling) {
> + ret = of_changeset_attach_node(&cs, dn);
> + if (ret)
> + goto out;
> + }
> +
> + ret = of_changeset_apply(&cs);
> +out:
> + of_changeset_destroy(&cs);
> + return ret;
> +}
> +
> static ssize_t dlpar_cpu_add(u32 drc_index)
> {
> struct device_node *dn, *parent;
> @@ -563,7 +584,7 @@ static ssize_t dlpar_cpu_add(u32 drc_index)
> return -EINVAL;
> }
>
> - rc = dlpar_attach_node(dn, parent);
> + rc = pseries_cpuhp_attach_nodes(dn);
>
> /* Regardless we are done with parent now */
> of_node_put(parent);
> @@ -600,6 +621,53 @@ static ssize_t dlpar_cpu_add(u32 drc_index)
> return rc;
> }
>
> +static unsigned int pseries_cpuhp_cache_use_count(const struct device_node *cachedn)
> +{
> + unsigned int use_count = 0;
> + struct device_node *dn;
> +
> + WARN_ON(!of_node_is_type(cachedn, "cache"));
> +
> + for_each_of_cpu_node(dn) {
> + if (of_find_next_cache_node(dn) == cachedn)
> + use_count++;
> + }
> +
> + for_each_node_by_type(dn, "cache") {
> + if (of_find_next_cache_node(dn) == cachedn)
> + use_count++;
> + }
> +
> + return use_count;
> +}
> +
> +static int pseries_cpuhp_detach_nodes(struct device_node *cpudn)
> +{
> + struct device_node *dn;
> + struct of_changeset cs;
> + int ret = 0;
> +
> + of_changeset_init(&cs);
> + ret = of_changeset_detach_node(&cs, cpudn);
> + if (ret)
> + goto out;
> +
> + dn = cpudn;
> + while ((dn = of_find_next_cache_node(dn))) {
> + if (pseries_cpuhp_cache_use_count(dn) > 1)
> + break;
> +
> + ret = of_changeset_detach_node(&cs, dn);
> + if (ret)
> + goto out;
> + }
> +
> + ret = of_changeset_apply(&cs);
> +out:
> + of_changeset_destroy(&cs);
> + return ret;
> +}
> +
> static ssize_t dlpar_cpu_remove(struct device_node *dn, u32 drc_index)
> {
> int rc;
> @@ -621,7 +689,7 @@ static ssize_t dlpar_cpu_remove(struct device_node *dn, u32 drc_index)
> return rc;
> }
>
> - rc = dlpar_detach_node(dn);
> + rc = pseries_cpuhp_detach_nodes(dn);
> if (rc) {
> int saved_rc = rc;
>
>
More information about the Linuxppc-dev
mailing list