@@ -2122,10 +2122,20 @@ static void __init start_shepherd_timer(void)
{
int cpu;
- for_each_possible_cpu(cpu)
+ for_each_possible_cpu(cpu) {
INIT_DEFERRABLE_WORK(per_cpu_ptr(&vmstat_work, cpu),
vmstat_update);
+ /*
+ * For secondary CPUs during CPU hotplug scenarios,
+ * vmstat_cpu_online() will enable the work.
+ * mm/vmstat:online enables and disables vmstat_work
+ * symmetrically during CPU hotplug events.
+ */
+ if (!cpu_online(cpu))
+ disable_delayed_work_sync(&per_cpu(vmstat_work, cpu));
+ }
+
schedule_delayed_work(&shepherd,
round_jiffies_relative(sysctl_stat_interval));
}
@@ -2148,13 +2158,14 @@ static int vmstat_cpu_online(unsigned int cpu)
if (!node_state(cpu_to_node(cpu), N_CPU)) {
node_set_state(cpu_to_node(cpu), N_CPU);
}
+ enable_delayed_work(&per_cpu(vmstat_work, cpu));
return 0;
}
static int vmstat_cpu_down_prep(unsigned int cpu)
{
- cancel_delayed_work_sync(&per_cpu(vmstat_work, cpu));
+ disable_delayed_work_sync(&per_cpu(vmstat_work, cpu));
return 0;
}
The upstream commit adcfb264c3ed ("vmstat: disable vmstat_work on vmstat_cpu_down_prep()") introduced another warning during the boot phase so was soon reverted on upstream by commit cd6313beaeae ("Revert "vmstat: disable vmstat_work on vmstat_cpu_down_prep()""). This commit resolves it and reattempts the original fix. Even after mm/vmstat:online teardown, shepherd may still queue work for the dying cpu until the cpu is removed from online mask. While it's quite rare, this means that after unbind_workers() unbinds a per-cpu kworker, it potentially runs vmstat_update for the dying CPU on an irrelevant cpu before entering atomic AP states. When CONFIG_DEBUG_PREEMPT=y, it results in the following error with the backtrace. BUG: using smp_processor_id() in preemptible [00000000] code: \ kworker/7:3/1702 caller is refresh_cpu_vm_stats+0x235/0x5f0 CPU: 0 UID: 0 PID: 1702 Comm: kworker/7:3 Tainted: G Tainted: [N]=TEST Workqueue: mm_percpu_wq vmstat_update Call Trace: <TASK> dump_stack_lvl+0x8d/0xb0 check_preemption_disabled+0xce/0xe0 refresh_cpu_vm_stats+0x235/0x5f0 vmstat_update+0x17/0xa0 process_one_work+0x869/0x1aa0 worker_thread+0x5e5/0x1100 kthread+0x29e/0x380 ret_from_fork+0x2d/0x70 ret_from_fork_asm+0x1a/0x30 </TASK> So, for mm/vmstat:online, disable vmstat_work reliably on teardown and symmetrically enable it on startup. For secondary CPUs during CPU hotplug scenarios, ensure the delayed work is disabled immediately after the initialization. These CPUs are not yet online when start_shepherd_timer() runs on boot CPU. vmstat_cpu_online() will enable the work for them. Suggested-by: Huacai Chen <chenhuacai@kernel.org> Signed-off-by: Huacai Chen <chenhuacai@kernel.org> Signed-off-by: Koichiro Den <koichiro.den@canonical.com> --- v2: https://lore.kernel.org/all/20241221033321.4154409-1-koichiro.den@canonical.com/ v1: https://lore.kernel.org/all/20241220134234.3809621-1-koichiro.den@canonical.com/ --- mm/vmstat.c | 15 +++++++++++++-- 1 file changed, 13 insertions(+), 2 deletions(-)