mm/vmstat: switch vmstat shepherd to flush per-CPU counters remotely

With a task that busy loops on a given CPU, the kworker interruption to
execute vmstat_update is undesired and may exceed latency thresholds for
certain applications.

Performance details for the kworker interruption:

oslat   1094.456862: sys_mlock(start: 7f7ed0000b60, len: 1000)
oslat   1094.456971: workqueue_queue_work: ... function=vmstat_update ...
oslat   1094.456974: sched_switch: prev_comm=oslat ... ==> next_comm=kworker/5:1 ...
kworker 1094.456978: sched_switch: prev_comm=kworker/5:1 ==> next_comm=oslat ...

The example above shows an additional 7us for the

        oslat -> kworker -> oslat

switches.  In the case of a virtualized CPU, and the vmstat_update
interruption in the host (of a qemu-kvm vcpu), the latency penalty
observed in the guest is higher than 50us, violating the acceptable
latency threshold for certain applications.

To fix this, now that the counters are modified via cmpxchg both CPU
locally (via the account functions), and remotely (via cpu_vm_stats_fold),
its possible to switch vmstat_shepherd to perform the per-CPU vmstats
folding remotely.

Link: https://lkml.kernel.org/r/20230320180745.807656081@redhat.com
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Aaron Tomlin <atomlin@atomlin.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Huacai Chen <chenhuacai@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: "Russell King (Oracle)" <linux@armlinux.org.uk>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/vmstat.c b/mm/vmstat.c
index c93c87d9d7307cf5316b699247eef1d87bc47ba2..0a858e1c5fc6796d6c060acb5b181223b4c9df6e 100644 (file)
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -2049,6 +2049,23 @@ static void vmstat_shepherd(struct work_struct *w);
 
 static DECLARE_DEFERRABLE_WORK(shepherd, vmstat_shepherd);
 
+#ifdef CONFIG_HAVE_CMPXCHG_LOCAL
+/* Flush counters remotely if CPU uses cmpxchg to update its per-CPU counters */
+static void vmstat_shepherd(struct work_struct *w)
+{
+       int cpu;
+
+       cpus_read_lock();
+       for_each_online_cpu(cpu) {
+               cpu_vm_stats_fold(cpu);
+               cond_resched();
+       }
+       cpus_read_unlock();
+
+       schedule_delayed_work(&shepherd,
+               round_jiffies_relative(sysctl_stat_interval));
+}
+#else
 static void vmstat_shepherd(struct work_struct *w)
 {
        int cpu;
@@ -2068,6 +2085,7 @@ static void vmstat_shepherd(struct work_struct *w)
        schedule_delayed_work(&shepherd,
                round_jiffies_relative(sysctl_stat_interval));
 }
+#endif
 
 static void __init start_shepherd_timer(void)
 {