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af98d8a36a
CPU controller limits are not properly enforced during CPU hotplug operations, particularly during CPU offline. When a CPU goes offline, throttled processes are unintentionally being unthrottled across all CPUs in the system, allowing them to exceed their assigned quota limits. Consider below for an example, Assigning 6.25% bandwidth limit to a cgroup in a 8 CPU system, where, workload is running 8 threads for 20 seconds at 100% CPU utilization, expected (user+sys) time = 10 seconds. $ cat /sys/fs/cgroup/test/cpu.max 50000 100000 $ ./ebizzy -t 8 -S 20 // non-hotplug case real 20.00 s user 10.81 s // intended behaviour sys 0.00 s $ ./ebizzy -t 8 -S 20 // hotplug case real 20.00 s user 14.43 s // Workload is able to run for 14 secs sys 0.00 s // when it should have only run for 10 secs During CPU hotplug, scheduler domains are rebuilt and cpu_attach_domain is called for every active CPU to update the root domain. That ends up calling rq_offline_fair which un-throttles any throttled hierarchies. Unthrottling should only occur for the CPU being hotplugged to allow its throttled processes to become runnable and get migrated to other CPUs. With current patch applied, $ ./ebizzy -t 8 -S 20 // hotplug case real 21.00 s user 10.16 s // intended behaviour sys 0.00 s This also has another symptom, when a CPU goes offline, and if the cfs_rq is not in throttled state and the runtime_remaining still had plenty remaining, it gets reset to 1 here, causing the runtime_remaining of cfs_rq to be quickly depleted. Note: hotplug operation (online, offline) was performed in while(1) loop v3: https://lore.kernel.org/all/20241210102346.228663-2-vishalc@linux.ibm.com v2: https://lore.kernel.org/all/20241207052730.1746380-2-vishalc@linux.ibm.com v1: https://lore.kernel.org/all/20241126064812.809903-2-vishalc@linux.ibm.com Suggested-by: Zhang Qiao <zhangqiao22@huawei.com> Signed-off-by: Vishal Chourasia <vishalc@linux.ibm.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Acked-by: Vincent Guittot <vincent.guittot@linaro.org> Tested-by: Madadi Vineeth Reddy <vineethr@linux.ibm.com> Tested-by: Samir Mulani <samir@linux.ibm.com> Link: https://lore.kernel.org/r/20241212043102.584863-2-vishalc@linux.ibm.com |
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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 reStructuredText 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.