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Vincent Guittot c8b3ed0053 sched/fair: Fix tg->load when offlining a CPU 
[ Upstream commit f60a631ab9 ]

When a CPU is taken offline, the contribution of its cfs_rqs to task_groups'
load may remain and will negatively impact the calculation of the share of
the online CPUs.

To fix this bug, clear the contribution of an offlining CPU to task groups'
load and skip its contribution while it is inactive.

Here's the reproducer of the anomaly, by Imran Khan:

	"So far I have encountered only one rather lengthy way of reproducing this issue,
	which is as follows:

	1. Take a KVM guest (booted with 4 CPUs and can be scaled up to 124 CPUs) and
	   create 2 custom cgroups: /sys/fs/cgroup/cpu/test_group_1 and /sys/fs/cgroup/
	   cpu/test_group_2

	2. Assign a CPU intensive workload to each of these cgroups and start the
	   workload.

	For my tests I am using following app:

	int main(int argc, char *argv[])
	{
		unsigned long count, i, val;
		if (argc != 2) {
		      printf("usage: ./a.out <number of random nums to generate> \n");
		      return 0;
		}

		count = strtoul(argv[1], NULL, 10);

		printf("Generating %lu random numbers \n", count);
		for (i = 0; i < count; i++) {
			val = rand();
			val = val % 2;
			//usleep(1);
		}
		printf("Generated %lu random numbers \n", count);
		return 0;
	}

	Also since the system is booted with 4 CPUs, in order to completely load the
	system I am also launching 4 instances of same test app under:

	   /sys/fs/cgroup/cpu/

	3. We can see that both of the cgroups get similar CPU time:

        # systemd-cgtop --depth 1
	Path                                 Tasks    %CPU  Memory  Input/s    Output/s
	/                                      659      -     5.5G        -        -
	/system.slice                            -      -     5.7G        -        -
	/test_group_1                            4      -        -        -        -
	/test_group_2                            3      -        -        -        -
	/user.slice                             31      -    56.5M        -        -

	Path                                 Tasks   %CPU   Memory  Input/s    Output/s
	/                                      659  394.6     5.5G        -        -
	/test_group_2                            3   65.7        -        -        -
	/user.slice                             29   55.1    48.0M        -        -
	/test_group_1                            4   47.3        -        -        -
	/system.slice                            -    2.2     5.7G        -        -

	Path                                 Tasks  %CPU    Memory  Input/s    Output/s
	/                                      659  394.8     5.5G        -        -
	/test_group_1                            4   62.9        -        -        -
	/user.slice                             28   44.9    54.2M        -        -
	/test_group_2                            3   44.7        -        -        -
	/system.slice                            -    0.9     5.7G        -        -

	Path                                 Tasks  %CPU    Memory  Input/s     Output/s
	/                                      659  394.4     5.5G        -        -
	/test_group_2                            3   58.8        -        -        -
	/test_group_1                            4   51.9        -        -        -
	/user.slice                              30   39.3    59.6M        -        -
	/system.slice                            -    1.9     5.7G        -        -

	Path                                 Tasks  %CPU     Memory  Input/s    Output/s
	/                                      659  394.7     5.5G        -        -
	/test_group_1                            4   60.9        -        -        -
	/test_group_2                            3   57.9        -        -        -
	/user.slice                             28   43.5    36.9M        -        -
	/system.slice                            -    3.0     5.7G        -        -

	Path                                 Tasks  %CPU     Memory  Input/s     Output/s
	/                                      659  395.0     5.5G        -        -
	/test_group_1                            4   66.8        -        -        -
	/test_group_2                            3   56.3        -        -        -
	/user.slice                             29   43.1    51.8M        -        -
	/system.slice                            -    0.7     5.7G        -        -

	4. Now move systemd-udevd to one of these test groups, say test_group_1, and
	   perform scale up to 124 CPUs followed by scale down back to 4 CPUs from the
	   host side.

	5. Run the same workload i.e 4 instances of CPU hogger under /sys/fs/cgroup/cpu
	   and one instance of  CPU hogger each in /sys/fs/cgroup/cpu/test_group_1 and
	   /sys/fs/cgroup/test_group_2.

	It can be seen that test_group_1 (the one where systemd-udevd was moved) is getting
	much less CPU time than the test_group_2, even though at this point of time both of
	these groups have only CPU hogger running:

        # systemd-cgtop --depth 1
	Path                                   Tasks   %CPU   Memory  Input/s   Output/s
	/                                      1219     -     5.4G        -        -
	/system.slice                           -       -     5.6G        -        -
	/test_group_1                           4       -        -        -        -
	/test_group_2                           3       -        -        -        -
	/user.slice                            26       -    91.3M        -        -

	Path                                   Tasks  %CPU     Memory  Input/s   Output/s
	/                                      1221  394.3     5.4G        -        -
	/test_group_2                             3   82.7        -        -        -
	/test_group_1                             4   14.3        -        -        -
	/system.slice                             -    0.8     5.6G        -        -
	/user.slice                              26    0.4    91.2M        -        -

	Path                                   Tasks  %CPU    Memory  Input/s    Output/s
	/                                      1221  394.6     5.4G        -        -
	/test_group_2                             3   67.4        -        -        -
	/system.slice                             -   24.6     5.6G        -        -
	/test_group_1                             4   12.5        -        -        -
	/user.slice                              26    0.4    91.2M        -        -

	Path                                  Tasks  %CPU    Memory  Input/s    Output/s
	/                                     1221  395.2     5.4G        -        -
	/test_group_2                            3   60.9        -        -        -
	/system.slice                            -   27.9     5.6G        -        -
	/test_group_1                            4   12.2        -        -        -
	/user.slice                             26    0.4    91.2M        -        -

	Path                                  Tasks  %CPU    Memory  Input/s    Output/s
	/                                     1221  395.2     5.4G        -        -
	/test_group_2                            3   69.4        -        -        -
	/test_group_1                            4   13.9        -        -        -
	/user.slice                             28    1.6    92.0M        -        -
	/system.slice                            -    1.0     5.6G        -        -

	Path                                  Tasks  %CPU    Memory  Input/s    Output/s
	/                                      1221  395.6     5.4G        -        -
	/test_group_2                             3   59.3        -        -        -
	/test_group_1                             4   14.1        -        -        -
	/user.slice                              28    1.3    92.2M        -        -
	/system.slice                             -    0.7     5.6G        -        -

	Path                                  Tasks  %CPU    Memory  Input/s    Output/s
	/                                      1221  395.5     5.4G        -        -
	/test_group_2                            3   67.2        -        -        -
	/test_group_1                            4   11.5        -        -        -
	/user.slice                             28    1.3    92.5M        -        -
	/system.slice                            -    0.6     5.6G        -        -

	Path                                  Tasks  %CPU    Memory  Input/s    Output/s
	/                                      1221  395.1     5.4G        -        -
	/test_group_2                             3   76.8        -        -        -
	/test_group_1                             4   12.9        -        -        -
	/user.slice                              28    1.3    92.8M        -        -
	/system.slice                             -    1.2     5.6G        -        -

	From sched_debug data it can be seen that in bad case the load.weight of per-CPU
	sched entities corresponding to test_group_1 has reduced significantly and
	also load_avg of test_group_1 remains much higher than that of test_group_2,
	even though systemd-udevd stopped running long time back and at this point of
	time both cgroups just have the CPU hogger app as running entity."

[ mingo: Added details from the original discussion, plus minor edits to the patch. ]

Reported-by: Imran Khan <imran.f.khan@oracle.com>
Tested-by: Imran Khan <imran.f.khan@oracle.com>
Tested-by: Aaron Lu <aaron.lu@intel.com>
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Imran Khan <imran.f.khan@oracle.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Borislav Petkov <bp@alien8.de>
Link: https://lore.kernel.org/r/20231223111545.62135-1-vincent.guittot@linaro.org
Signed-off-by: Sasha Levin <sashal@kernel.org>
History 2024-02-05 20:16:45 +00:00
arch x86/mce: Mark fatal MCE's page as poison to avoid panic in the kdump kernel 2024-02-05 20:16:45 +00:00
block block: Move checking GENHD_FL_NO_PART to bdev_add_partition() 2024-01-31 16:21:20 -08:00
certs This update includes the following changes: 2023-11-02 16:15:30 -10:00
crypto crypto: api - Disallow identical driver names 2024-01-31 16:20:54 -08:00
Documentation drm: Allow drivers to indicate the damage helpers to ignore damage clips 2024-01-31 16:21:13 -08:00
drivers drivers/perf: pmuv3: don't expose SW_INCR event in sysfs 2024-02-05 20:16:44 +00:00
fs cifs: fix stray unlock in cifs_chan_skip_or_disable 2024-01-31 16:21:20 -08:00
include sched/numa: Fix mm numa_scan_seq based unconditional scan 2024-02-05 20:16:45 +00:00
init rootfs: Fix support for rootfstype= when root= is given 2024-01-25 15:45:15 -08:00
io_uring io_uring: adjust defer tw counting 2024-01-25 15:45:30 -08:00
ipc Many singleton patches against the MM code. The patch series which are 2023-11-02 19:38:47 -10:00
kernel sched/fair: Fix tg->load when offlining a CPU 2024-02-05 20:16:45 +00:00
lib debugobjects: Stop accessing objects after releasing hash bucket lock 2024-02-05 20:16:45 +00:00
LICENSES LICENSES: Add the copyleft-next-0.3.1 license 2022-11-08 15:44:01 +01:00
mm memblock: fix crash when reserved memory is not added to memory 2024-01-31 16:21:18 -08:00
net netfilter: nf_tables: reject QUEUE/DROP verdict parameters 2024-01-31 16:21:12 -08:00
rust rust: Ignore preserve-most functions 2024-01-25 15:45:09 -08:00
samples Landlock updates for v6.7-rc1 2023-11-03 09:28:53 -10:00
scripts scripts/get_abi: fix source path leak 2024-01-31 16:21:00 -08:00
security lsm: new security_file_ioctl_compat() hook 2024-01-31 16:21:00 -08:00
sound soundwire: fix initializing sysfs for same devices on different buses 2024-01-31 16:20:53 -08:00
tools selftests: bonding: do not test arp/ns target with mode balance-alb/tlb 2024-01-31 16:21:10 -08:00
usr arch: Remove Itanium (IA-64) architecture 2023-09-11 08:13:17 +00:00
virt KVM/arm64 fixes for 6.7, part #2 2023-12-22 18:03:54 -05:00
.clang-format iommu: Add for_each_group_device() 2023-05-23 08:15:51 +02:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.get_maintainer.ignore get_maintainer: add Alan to .get_maintainer.ignore 2022-08-20 15:17:44 -07:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore kbuild: rpm-pkg: generate kernel.spec in rpmbuild/SPECS/ 2023-10-03 20:49:09 +09:00
.mailmap 12 hotfixes. 2 are cc:stable and the remainder either address post-6.7 2024-01-05 13:46:18 -08:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS 12 hotfixes. 2 are cc:stable and the remainder either address post-6.7 2024-01-05 13:46:18 -08:00
Kbuild Kbuild updates for v6.1 2022-10-10 12:00:45 -07:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS 12 hotfixes. 2 are cc:stable and the remainder either address post-6.7 2024-01-05 13:46:18 -08:00
Makefile Linux 6.7.3 2024-01-31 16:21:21 -08:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.