On 03/05/2024 11:17, Barry Song wrote: > On Fri, May 3, 2024 at 5:16 PM Ryan Roberts <ryan.roberts@xxxxxxx> wrote: >> >> On 03/05/2024 03:09, Barry Song wrote: >>> From: Barry Song <v-songbaohua@xxxxxxxx> >>> >>> When unplugging a CPU, the current code merges its vm_events >>> with an online CPU. Because, during summation, it only considers >>> online CPUs, which is a crude workaround. By transitioning to >>> summing up all possible CPUs, we can eliminate the need for >>> vm_events_fold_cpu. >>> >>> Suggested-by: Ryan Roberts <ryan.roberts@xxxxxxx> >>> Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx> >>> --- >>> originally suggested by Ryan while he reviewed mTHP counters >>> patchset[1]; I am also applying this suggestion to vm_events >>> >>> -v2: >>> also drop cpus_read_lock() as we don't care about cpu hotplug any more; >>> -v1: >>> https://lore.kernel.org/linux-mm/20240412123039.442743-1-21cnbao@xxxxxxxxx/ >>> >>> [1] https://lore.kernel.org/linux-mm/ca73cbf1-8304-4790-a721-3c3a42f9d293@xxxxxxx/ >>> >>> include/linux/vmstat.h | 5 ----- >>> mm/page_alloc.c | 8 -------- >>> mm/vmstat.c | 21 +-------------------- >>> 3 files changed, 1 insertion(+), 33 deletions(-) >>> >>> diff --git a/include/linux/vmstat.h b/include/linux/vmstat.h >>> index 735eae6e272c..f7eaeb8bfa47 100644 >>> --- a/include/linux/vmstat.h >>> +++ b/include/linux/vmstat.h >>> @@ -83,8 +83,6 @@ static inline void count_vm_events(enum vm_event_item item, long delta) >>> >>> extern void all_vm_events(unsigned long *); >>> >>> -extern void vm_events_fold_cpu(int cpu); >>> - >>> #else >>> >>> /* Disable counters */ >>> @@ -103,9 +101,6 @@ static inline void __count_vm_events(enum vm_event_item item, long delta) >>> static inline void all_vm_events(unsigned long *ret) >>> { >>> } >>> -static inline void vm_events_fold_cpu(int cpu) >>> -{ >>> -} >>> >>> #endif /* CONFIG_VM_EVENT_COUNTERS */ >>> >>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c >>> index cd584aace6bf..8b56d785d587 100644 >>> --- a/mm/page_alloc.c >>> +++ b/mm/page_alloc.c >>> @@ -5826,14 +5826,6 @@ static int page_alloc_cpu_dead(unsigned int cpu) >>> mlock_drain_remote(cpu); >>> drain_pages(cpu); >>> >>> - /* >>> - * Spill the event counters of the dead processor >>> - * into the current processors event counters. >>> - * This artificially elevates the count of the current >>> - * processor. >>> - */ >>> - vm_events_fold_cpu(cpu); >>> - >>> /* >>> * Zero the differential counters of the dead processor >>> * so that the vm statistics are consistent. >>> diff --git a/mm/vmstat.c b/mm/vmstat.c >>> index db79935e4a54..aaa32418652e 100644 >>> --- a/mm/vmstat.c >>> +++ b/mm/vmstat.c >>> @@ -114,7 +114,7 @@ static void sum_vm_events(unsigned long *ret) >>> >>> memset(ret, 0, NR_VM_EVENT_ITEMS * sizeof(unsigned long)); >>> >>> - for_each_online_cpu(cpu) { >>> + for_each_possible_cpu(cpu) { >> >> One thought comes to mind (due to my lack of understanding exactly what >> "possible" means): Linux is compiled with a max number of cpus - NR_CPUS - 512 >> for arm64's defconfig. Does all possible cpus include all 512? On an 8 CPU >> system that would be increasing the number of loops by 64 times. >> >> Or perhaps possible just means CPUs that have ever been online? > > Hi Ryan, > > On arm64, we get possible CPUs either from device tree or ACPI. they are both > much less than NR_CPUS. Ahh great. In that case, this patch seems good to me, although I'm not too familiar with the code. > > /* > * Enumerate the possible CPU set from the device tree or ACPI and build the > * cpu logical map array containing MPIDR values related to logical > * cpus. Assumes that cpu_logical_map(0) has already been initialized. > */ > void __init smp_init_cpus(void) > > for device tree case, it is, > > /* > * Enumerate the possible CPU set from the device tree and build the > * cpu logical map array containing MPIDR values related to logical > * cpus. Assumes that cpu_logical_map(0) has already been initialized. > */ > static void __init of_parse_and_init_cpus(void) > { > struct device_node *dn; > > for_each_of_cpu_node(dn) { > u64 hwid = of_get_cpu_hwid(dn, 0); > > if (hwid & ~MPIDR_HWID_BITMASK) > goto next; > > if (is_mpidr_duplicate(cpu_count, hwid)) { > pr_err("%pOF: duplicate cpu reg properties in the DT\n", > dn); > goto next; > } > > /* > * The numbering scheme requires that the boot CPU > * must be assigned logical id 0. Record it so that > * the logical map built from DT is validated and can > * be used. > */ > if (hwid == cpu_logical_map(0)) { > if (bootcpu_valid) { > pr_err("%pOF: duplicate boot cpu reg > property in DT\n", > dn); > goto next; > } > > bootcpu_valid = true; > early_map_cpu_to_node(0, of_node_to_nid(dn)); > > /* > * cpu_logical_map has already been > * initialized and the boot cpu doesn't need > * the enable-method so continue without > * incrementing cpu. > */ > continue; > } > > if (cpu_count >= NR_CPUS) > goto next; > > pr_debug("cpu logical map 0x%llx\n", hwid); > set_cpu_logical_map(cpu_count, hwid); > > early_map_cpu_to_node(cpu_count, of_node_to_nid(dn)); > next: > cpu_count++; > } > } > > even for ARM32, we get that sometimes from scu_get_core_count(), > eg. > static void __init omap4_smp_init_cpus(void) > { > unsigned int i = 0, ncores = 1, cpu_id; > > /* Use ARM cpuid check here, as SoC detection will not work so early */ > cpu_id = read_cpuid_id() & CPU_MASK; > if (cpu_id == CPU_CORTEX_A9) { > /* > * Currently we can't call ioremap here because > * SoC detection won't work until after init_early. > */ > cfg.scu_base = OMAP2_L4_IO_ADDRESS(scu_a9_get_base()); > BUG_ON(!cfg.scu_base); > ncores = scu_get_core_count(cfg.scu_base); > } else if (cpu_id == CPU_CORTEX_A15) { > ncores = OMAP5_CORE_COUNT; > } > > /* sanity check */ > if (ncores > nr_cpu_ids) { > pr_warn("SMP: %u cores greater than maximum (%u), clipping\n", > ncores, nr_cpu_ids); > ncores = nr_cpu_ids; > } > > for (i = 0; i < ncores; i++) > set_cpu_possible(i, true); > } > > Other architectures do exactly the same jobs. > > > >> >> Either way, I guess it's not considered a performance bottleneck because, from >> memory, the scheduler and many other places are iterating over all possible cpus. >> >>> struct vm_event_state *this = &per_cpu(vm_event_states, cpu); >>> >>> for (i = 0; i < NR_VM_EVENT_ITEMS; i++) >>> @@ -129,29 +129,10 @@ static void sum_vm_events(unsigned long *ret) >>> */ >>> void all_vm_events(unsigned long *ret) >>> { >>> - cpus_read_lock(); >>> sum_vm_events(ret); >>> - cpus_read_unlock(); >>> } >>> EXPORT_SYMBOL_GPL(all_vm_events); >>> >>> -/* >>> - * Fold the foreign cpu events into our own. >>> - * >>> - * This is adding to the events on one processor >>> - * but keeps the global counts constant. >>> - */ >>> -void vm_events_fold_cpu(int cpu) >>> -{ >>> - struct vm_event_state *fold_state = &per_cpu(vm_event_states, cpu); >>> - int i; >>> - >>> - for (i = 0; i < NR_VM_EVENT_ITEMS; i++) { >>> - count_vm_events(i, fold_state->event[i]); >>> - fold_state->event[i] = 0; >>> - } >>> -} >>> - >>> #endif /* CONFIG_VM_EVENT_COUNTERS */ >>> >>> /* >> > > Thanks > Barry
