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From: Marcelo Tosatti <mtosatti@redhat.com>
To: Christoph Lameter <cl@linux.com>
Cc: Aaron Tomlin <atomlin@atomlin.com>,
	Frederic Weisbecker <frederic@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	Russell King <linux@armlinux.org.uk>,
	Huacai Chen <chenhuacai@kernel.org>,
	Heiko Carstens <hca@linux.ibm.com>,
	x86@kernel.org, Vlastimil Babka <vbabka@suse.cz>,
	Michal Hocko <mhocko@suse.com>,
	Marcelo Tosatti <mtosatti@redhat.com>
Subject: [PATCH v8 11/13] mm/vmstat: switch vmstat shepherd to flush per-CPU counters remotely
Date: Mon, 15 May 2023 15:00:26 -0300	[thread overview]
Message-ID: <20230515180138.692835888@redhat.com> (raw)
In-Reply-To: 20230515180015.016409657@redhat.com

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.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

---

Index: linux-vmstat-remote/mm/vmstat.c
===================================================================
--- linux-vmstat-remote.orig/mm/vmstat.c
+++ linux-vmstat-remote/mm/vmstat.c
@@ -2049,6 +2049,23 @@ static void vmstat_shepherd(struct work_
 
 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_
 	schedule_delayed_work(&shepherd,
 		round_jiffies_relative(sysctl_stat_interval));
 }
+#endif
 
 static void __init start_shepherd_timer(void)
 {



  parent reply	other threads:[~2023-05-15 18:07 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-05-15 18:00 [PATCH v8 00/13] fold per-CPU vmstats remotely Marcelo Tosatti
2023-05-15 18:00 ` [PATCH v8 01/13] vmstat: allow_direct_reclaim should use zone_page_state_snapshot Marcelo Tosatti
2023-05-24 12:34   ` Michal Hocko
2023-05-25  7:05   ` Aaron Tomlin
2023-05-15 18:00 ` [PATCH v8 02/13] this_cpu_cmpxchg: ARM64: switch this_cpu_cmpxchg to locked, add _local function Marcelo Tosatti
2023-05-15 18:00 ` [PATCH v8 03/13] this_cpu_cmpxchg: loongarch: " Marcelo Tosatti
2023-05-15 18:00 ` [PATCH v8 04/13] this_cpu_cmpxchg: S390: " Marcelo Tosatti
2023-05-15 18:00 ` [PATCH v8 05/13] this_cpu_cmpxchg: x86: " Marcelo Tosatti
2023-05-15 18:00 ` [PATCH v8 06/13] add this_cpu_cmpxchg_local and asm-generic definitions Marcelo Tosatti
2023-05-15 18:00 ` [PATCH v8 07/13] convert this_cpu_cmpxchg users to this_cpu_cmpxchg_local Marcelo Tosatti
2023-05-15 18:00 ` [PATCH v8 08/13] mm/vmstat: switch counter modification to cmpxchg Marcelo Tosatti
2023-05-15 18:00 ` [PATCH v8 09/13] vmstat: switch per-cpu vmstat counters to 32-bits Marcelo Tosatti
2023-05-15 18:00 ` [PATCH v8 10/13] mm/vmstat: use xchg in cpu_vm_stats_fold Marcelo Tosatti
2023-05-15 18:00 ` Marcelo Tosatti [this message]
2023-05-15 18:00 ` [PATCH v8 12/13] mm/vmstat: refresh stats remotely instead of via work item Marcelo Tosatti
2023-05-15 18:00 ` [PATCH v8 13/13] vmstat: add pcp remote node draining via cpu_vm_stats_fold Marcelo Tosatti
2023-05-16  8:09 ` [PATCH v8 00/13] fold per-CPU vmstats remotely Christoph Lameter
2023-05-16 18:02   ` Marcelo Tosatti
2023-05-24 12:51 ` Michal Hocko
2023-05-24 13:53   ` Marcelo Tosatti
2023-05-25  6:47     ` Michal Hocko

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