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From: 王贇 <yun.wang@linux.alibaba.com>
To: "Ingo Molnar" <mingo@redhat.com>,
	"Peter Zijlstra" <peterz@infradead.org>,
	"Juri Lelli" <juri.lelli@redhat.com>,
	"Vincent Guittot" <vincent.guittot@linaro.org>,
	"Dietmar Eggemann" <dietmar.eggemann@arm.com>,
	"Steven Rostedt" <rostedt@goodmis.org>,
	"Ben Segall" <bsegall@google.com>, "Mel Gorman" <mgorman@suse.de>,
	"Luis Chamberlain" <mcgrof@kernel.org>,
	"Kees Cook" <keescook@chromium.org>,
	"Iurii Zaikin" <yzaikin@google.com>,
	"Michal Koutný" <mkoutny@suse.com>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-doc@vger.kernel.org,
	"Paul E. McKenney" <paulmck@linux.ibm.com>,
	"Jonathan Corbet" <corbet@lwn.net>,
	"Iurii Zaikin" <yzaikin@google.com>
Subject: [PATCH v2 3/3] sched/numa: documentation for per-cgroup numa stat
Date: Fri, 15 Nov 2019 10:29:11 +0800
Message-ID: <263bb462-6f7c-c313-a0cc-04beb308076b@linux.alibaba.com> (raw)
In-Reply-To: <896a7da3-f139-32e7-8a64-b3562df1a091@linux.alibaba.com>

Add the description for 'cg_numa_stat', also a new doc to explain
the details on how to deal with the per-cgroup numa statistics.

Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Michal Koutný <mkoutny@suse.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Iurii Zaikin <yzaikin@google.com>
Signed-off-by: Michael Wang <yun.wang@linux.alibaba.com>
---
Since v1:
  * thanks to Iurii for the better sentences
  * thanks to Jonathan for the better format

 Documentation/admin-guide/cg-numa-stat.rst      | 163 ++++++++++++++++++++++++
 Documentation/admin-guide/index.rst             |   1 +
 Documentation/admin-guide/kernel-parameters.txt |   4 +
 Documentation/admin-guide/sysctl/kernel.rst     |   9 ++
 4 files changed, 177 insertions(+)
 create mode 100644 Documentation/admin-guide/cg-numa-stat.rst

diff --git a/Documentation/admin-guide/cg-numa-stat.rst b/Documentation/admin-guide/cg-numa-stat.rst
new file mode 100644
index 000000000000..1aed3b1d23c4
--- /dev/null
+++ b/Documentation/admin-guide/cg-numa-stat.rst
@@ -0,0 +1,163 @@
+===============================
+Per-cgroup NUMA statistics
+===============================
+
+Background
+----------
+
+On NUMA platforms, remote memory accessing always has a performance penalty,
+although we have NUMA balancing working hard to maximize the access locality,
+there are still situations it can't help.
+
+This could happen in modern production environment. When a large number of
+cgroups are used to classify and control resources, this creates a complex
+configuration for memory policy, CPUs and NUMA nodes. In such cases NUMA
+balancing could end up with the wrong memory policy or exhausted local NUMA
+node, which would lead to low percentage of local page accesses.
+
+We need to detect such cases, figure out which workloads from which cgroup
+has introduced the issues, then we get chance to do adjustment to avoid
+performance degradation.
+
+However, there are no hardware counters for per-task local/remote accessing
+info, we don't know how many remote page accesses have occurred for a
+particular task.
+
+Statistics
+----------
+
+Fortunately, we have NUMA Balancing which scans task's mapping and triggers PF
+periodically, gives us the opportunity to record per-task page accessing info.
+
+By "echo 1 > /proc/sys/kernel/cg_numa_stat" at runtime or adding boot parameter
+'cg_numa_stat', we will enable the accounting of per-cgroup numa statistics,
+the 'cpu.numa_stat' entry of CPU cgroup will show statistics::
+
+  locality -- execution time sectioned by task NUMA locality (in ms)
+  exectime -- execution time sectioned by NUMA node (in ms)
+
+We define 'task NUMA locality' as::
+
+  nr_local_page_access * 100 / (nr_local_page_access + nr_remote_page_access)
+
+this per-task percentage value will be updated on the ticks for current task,
+and the access counter will be updated on task's NUMA balancing PF, so only
+the pages which NUMA Balancing paid attention to will be accounted.
+
+On each tick, we acquire the locality of current task on that CPU, accumulating
+the ticks into the counter of corresponding locality region, tasks from the
+same group sharing the counters, becoming the group locality.
+
+Similarly, we acquire the NUMA node of current CPU where the current task is
+executing on, accumulating the ticks into the counter of corresponding node,
+becoming the per-cgroup node execution time.
+
+Note that the accounting is hierarchical, which means the numa statistics for
+a given group represents not only the workload of this group, but also the
+workloads of all it's descendants.
+
+For example the 'cpu.numa_stat' show::
+
+  locality 39541 60962 36842 72519 118605 721778 946553
+  exectime 1220127 1458684
+
+The locality is sectioned into 7 regions, approximately as::
+
+  0-13% 14-27% 28-42% 43-56% 57-71% 72-85% 86-100%
+
+And exectime is sectioned into 2 nodes, 0 and 1 in this case.
+
+Thus we know the workload of this group and it's descendants have totally
+executed 1220127ms on node_0 and 1458684ms on node_1, tasks with locality
+around 0~13% executed for 39541 ms, and tasks with locality around 87~100%
+executed for 946553 ms, which imply most of the memory access are local.
+
+Monitoring
+----------
+
+By monitoring the increments of these statistics, we can easily know whether
+NUMA balancing is working well for a particular workload.
+
+For example we take a 5 secs sample period, and consider locality under 27%
+is bad, then on each sampling we have::
+
+  region_bad = region_1 + region_2
+  region_all = region_1 + region_2 + ... + region_7
+
+and we have the increments as::
+
+  region_bad_diff = region_bad - last_region_bad
+  region_all_diff = region_all - last_region_all
+
+which finally become::
+
+  region_bad_percent = region_bad_diff * 100 / region_all_diff
+
+we can plot a line for region_bad_percent, when the line close to 0 things
+are good, when getting close to 100% something is wrong, we can pick a proper
+watermark to trigger warning message.
+
+You may want to drop the data if the region_all is too small, which implies
+there are not many available pages for NUMA Balancing, ignoring would be fine
+since most likely the workload is insensitive to NUMA.
+
+Monitoring root group helps you control the overall situation, while you may
+also want to monitor all the leaf groups which contain the workloads, this
+helps to catch the mouse.
+
+The exectime could be useful when NUMA Balancing is disabled, or when locality
+becomes too small, for NUMA node X we have::
+
+  exectime_X_diff = exectime_X - last_exectime_X
+  exectime_all_diff = exectime_all - last_exectime_all
+
+try to put your workload into a memory cgroup which providing per-node memory
+consumption by 'memory.numa_stat' entry, then we could get::
+
+  memory_percent_X = memory_X * 100 / memory_all
+  exectime_percent_X = exectime_X_diff * 100 / exectime_all_diff
+
+These two percentages are usually matched on each node, workload should execute
+mostly on the node contain most of it's memory, but it's not guaranteed.
+
+The workload may only access a small part of it's memory, in such cases although
+the majority of memory are remotely, locality could still be good.
+
+Thus to tell if things are fine or not depends on the understanding of system
+resource deployment, however, if you find node X got 100% memory percent but 0%
+exectime percent, definitely something is wrong.
+
+Troubleshooting
+---------------
+
+After identifying which workload introduced the bad locality, check:
+
+1). Is the workload bound to a particular NUMA node?
+2). Has any NUMA node run out of resources?
+
+There are several ways to bind task's memory with a NUMA node, the strict way
+like the MPOL_BIND memory policy or 'cpuset.mems' will limiting the memory
+node where to allocate pages, in this situation, admin should make sure the
+task is allowed to run on the CPUs of that NUMA node, and make sure there are
+available CPU resource there.
+
+There are also ways to bind task's CPU with a NUMA node, like 'cpuset.cpus' or
+sched_setaffinity() syscall, in this situation, NUMA Balancing help to migrate
+pages into that node, admin should make sure there are available memory there.
+
+Admin could try rebind or unbind the NUMA node to erase the damage, make a
+change then observe the statistics see if things get better until the situation
+is acceptable.
+
+Highlights
+----------
+
+For some tasks, NUMA Balancing may found no necessary to scan pages, and
+locality could always be 0 or small number, don't pay attention to them
+since they most likely insensitive to NUMA.
+
+There are no accounting until the option turned on, so enable it in advance
+if you want to have the whole history.
+
+We have per-task migfailed counter to tell how many page migration has been
+failed for a particular task, you will find it in /proc/PID/sched entry.
diff --git a/Documentation/admin-guide/index.rst b/Documentation/admin-guide/index.rst
index 4405b7485312..c75a3fdfcd94 100644
--- a/Documentation/admin-guide/index.rst
+++ b/Documentation/admin-guide/index.rst
@@ -112,6 +112,7 @@ configure specific aspects of kernel behavior to your liking.
    video-output
    wimax/index
    xfs
+   cg-numa-stat

 .. only::  subproject and html

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 5e27d74e2b74..475b8351be6d 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3191,6 +3191,10 @@
 	numa_balancing=	[KNL,X86] Enable or disable automatic NUMA balancing.
 			Allowed values are enable and disable

+	cg_numa_atat	[KNL] Enable advanced per-cgroup numa statistics.
+			Useful to debug NUMA efficiency problems when there are
+			lots of per-cgroup workloads.
+
 	numa_zonelist_order= [KNL, BOOT] Select zonelist order for NUMA.
 			'node', 'default' can be specified
 			This can be set from sysctl after boot.
diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
index 614179dc79a9..719593e8be20 100644
--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/Documentation/admin-guide/sysctl/kernel.rst
@@ -572,6 +572,15 @@ rate for each task.
 numa_balancing_scan_size_mb is how many megabytes worth of pages are
 scanned for a given scan.

+cg_numa_stat:
+=============
+
+Enables/disables advanced per-cgroup NUMA statistic.
+
+0: disabled (default).
+1: enabled.
+
+Check Documentation/admin-guide/cg-numa-stat.rst for details.

 osrelease, ostype & version:
 ============================
-- 
2.14.4.44.g2045bb6


  parent reply index

Thread overview: 62+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-13  3:43 [PATCH 0/3] sched/numa: introduce advanced numa statistic 王贇
2019-11-13  3:44 ` [PATCH 1/3] sched/numa: advanced per-cgroup " 王贇
2019-11-13  3:45 ` [PATCH 2/3] sched/numa: expose per-task pages-migration-failure 王贇
2019-11-13  3:45 ` [PATCH 3/3] sched/numa: documentation for per-cgroup numa stat 王贇
2019-11-13 15:09   ` Jonathan Corbet
2019-11-14  1:52     ` 王贇
2019-11-13 18:28   ` Iurii Zaikin
2019-11-14  2:22     ` 王贇
2019-11-15  2:29   ` 王贇 [this message]
2019-11-20  9:45 ` [PATCH 0/3] sched/numa: introduce advanced numa statistic 王贇
2019-11-25  1:35 ` 王贇
2019-11-27  1:48 ` [PATCH v2 " 王贇
2019-11-27  1:49   ` [PATCH v2 1/3] sched/numa: advanced per-cgroup " 王贇
2019-11-27 10:19     ` Mel Gorman
2019-11-28  2:09       ` 王贇
2019-11-28 12:39         ` Michal Koutný
2019-11-28 13:41           ` 王贇
2019-11-28 15:58             ` Michal Koutný
2019-11-29  1:52               ` 王贇
2019-11-29  5:19                 ` 王贇
2019-11-29 10:06                   ` Michal Koutný
2019-12-02  2:11                     ` 王贇
2019-11-27  1:50   ` [PATCH v2 2/3] sched/numa: expose per-task pages-migration-failure 王贇
2019-11-27 10:00     ` Mel Gorman
2019-12-02  2:22     ` 王贇
2019-11-27  1:50   ` [PATCH v2 3/3] sched/numa: documentation for per-cgroup numa stat 王贇
2019-11-27  4:58     ` Randy Dunlap
2019-11-27  5:54       ` 王贇
2019-12-03  5:59   ` [PATCH v3 0/2] sched/numa: introduce numa locality 王贇
2019-12-03  6:00     ` [PATCH v3 1/2] sched/numa: introduce per-cgroup NUMA locality info 王贇
2019-12-04  2:33       ` Randy Dunlap
2019-12-04  2:38         ` 王贇
2019-12-03  6:02     ` [PATCH v3 2/2] sched/numa: documentation for per-cgroup numa statistics 王贇
2019-12-03 13:43       ` Jonathan Corbet
2019-12-04  2:27         ` 王贇
2019-12-04  7:58     ` [PATCH v4 0/2] sched/numa: introduce numa locality 王贇
2019-12-04  7:59       ` [PATCH v4 1/2] sched/numa: introduce per-cgroup NUMA locality info 王贇
2019-12-05  3:28         ` Randy Dunlap
2019-12-05  3:29           ` Randy Dunlap
2019-12-05  3:52             ` 王贇
2019-12-04  8:00       ` [PATCH v4 2/2] sched/numa: documentation for per-cgroup numa statistics 王贇
2019-12-05  3:40         ` Randy Dunlap
2019-12-05  6:53       ` [PATCH v5 0/2] sched/numa: introduce numa locality 王贇
2019-12-05  6:53         ` [PATCH v5 1/2] sched/numa: introduce per-cgroup NUMA locality info 王贇
2019-12-05  6:54         ` [PATCH v5 2/2] sched/numa: documentation for per-cgroup numa, statistics 王贇
2019-12-10  2:19         ` [PATCH v5 0/2] sched/numa: introduce numa locality 王贇
2019-12-13  1:43         ` [PATCH v6 " 王贇
2019-12-13  1:47           ` [PATCH v6 1/2] sched/numa: introduce per-cgroup NUMA locality info 王贇
2020-01-03 15:14             ` Michal Koutný
2020-01-04  4:51               ` 王贇
2019-12-13  1:48           ` [PATCH v6 2/2] sched/numa: documentation for per-cgroup numa 王贇
2019-12-27  2:22           ` [PATCH v6 0/2] sched/numa: introduce numa locality 王贇
2020-01-17  2:19           ` 王贇
2020-01-19  6:08           ` [PATCH v7 " 王贇
2020-01-19  6:09             ` [PATCH v7 1/2] sched/numa: introduce per-cgroup NUMA locality info 王贇
2020-01-19  6:09             ` [PATCH v7 2/2] sched/numa: documentation for per-cgroup numa, statistics 王贇
2020-01-21  0:12               ` Randy Dunlap
2020-01-21  1:58                 ` 王贇
2020-01-21  1:56             ` [PATCH v8 0/2] sched/numa: introduce numa locality 王贇
2020-01-21  1:57               ` [PATCH v8 1/2] sched/numa: introduce per-cgroup NUMA locality info 王贇
2020-01-21  1:57               ` [PATCH v8 2/2] sched/numa: documentation for per-cgroup numa, statistics 王贇
2020-01-21  2:08                 ` Randy Dunlap

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