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@xxxxxxxxxxxxx> Cc: Michal Koutný <mkoutny@xxxxxxxx> Cc: Mel Gorman <mgorman@xxxxxxx> Signed-off-by: Michael Wang <yun.wang@xxxxxxxxxxxxxxxxx> --- Documentation/admin-guide/cg-numa-stat.rst | 161 ++++++++++++++++++++++++ Documentation/admin-guide/kernel-parameters.txt | 4 + Documentation/admin-guide/sysctl/kernel.rst | 9 ++ 3 files changed, 174 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..87b716c51e16 --- /dev/null +++ b/Documentation/admin-guide/cg-numa-stat.rst @@ -0,0 +1,161 @@ +=============================== +Per-cgroup NUMA statistics +=============================== + +Background +---------- + +On NUMA platforms, remote memory accessing always has a performance penalty, +although we have NUMA balancing working hard to maximum the local accessing +proportion, there are still situations it can't helps. + +This could happen in modern production environment, using bunch of cgroups +to classify and control resources which introduced complex configuration on +memory policy, CPUs and NUMA node, NUMA balancing could facing the wrong +memory policy or exhausted local NUMA node, lead into the low local page +accessing proportion. + +We need to perceive such cases, figure out which workloads from which cgroup +has introduced the issues, then we got chance to do adjustment to avoid +performance damages. + +However, there are no hardware counter for per-task local/remote accessing +info, we don't know how many remote page accessing has been done for a +particular task. + +Statistics +---------- + +Fortunately, we have NUMA Balancing which scan task's mapping and trigger PF +periodically, give us the opportunity to record per-task page accessing info. + +By "echo 1 > /proc/sys/kernel/cg_numa_stat" on runtime or add 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. + +To be noticed, the accounting is in a hierarchy way, which means the numa +statistics representing 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, closely 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 draw 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 imply +there are not much available pages for NUMA Balancing, just ignore would be +fine since most likely the workload is insensitive to NUMA. + +Monitoring root group help you control the overall situation, while you may +also want to monitoring all the leaf groups which contain the workloads, this +help to catch the mouse. + +The exectime could be useful when NUMA Balancing is disabled, or when locality +become 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 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 percentage are usually matched on each node, workload should execute +mostly on the node contain most of it's memory, but it's not guaranteed. + +Depends on which part of the memory accessed mostly by the workload, locality +could still be good with just a little piece of memory locally. + +Thus to tell if things are find 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 locate which workloads introduced the bad locality, check: + +1). Is the workloads bind into a particular NUMA node? +2). Is there 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/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index 5e27d74e2b74..220df1f0beb8 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 + lot's 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
