Messages in this thread Patch in this message | | | From | Joel Savitz <> | Subject | [RESEND PATCH v2] cpuset: restore sanity to cpuset_cpus_allowed_fallback() | Date | Tue, 7 May 2019 17:12:45 -0400 |
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If a process is limited by taskset (i.e. cpuset) to only be allowed to run on cpu N, and then cpu N is offlined via hotplug, the process will be assigned the current value of its cpuset cgroup's effective_cpus field in a call to do_set_cpus_allowed() in cpuset_cpus_allowed_fallback(). This argument's value does not makes sense for this case, because task_cs(tsk)->effective_cpus is modified by cpuset_hotplug_workfn() to reflect the new value of cpu_active_mask after cpu N is removed from the mask. While this may make sense for the cgroup affinity mask, it does not make sense on a per-task basis, as a task that was previously limited to only be run on cpu N will be limited to every cpu _except_ for cpu N after it is offlined/onlined via hotplug.
Pre-patch behavior:
$ grep Cpus /proc/$$/status Cpus_allowed: ff Cpus_allowed_list: 0-7
$ taskset -p 4 $$ pid 19202's current affinity mask: f pid 19202's new affinity mask: 4
$ grep Cpus /proc/self/status Cpus_allowed: 04 Cpus_allowed_list: 2
# echo off > /sys/devices/system/cpu/cpu2/online $ grep Cpus /proc/$$/status Cpus_allowed: 0b Cpus_allowed_list: 0-1,3
# echo on > /sys/devices/system/cpu/cpu2/online $ grep Cpus /proc/$$/status Cpus_allowed: 0b Cpus_allowed_list: 0-1,3
On a patched system, the final grep produces the following output instead:
$ grep Cpus /proc/$$/status Cpus_allowed: ff Cpus_allowed_list: 0-7
This patch changes the above behavior by instead resetting the mask to task_cs(tsk)->cpus_allowed by default, and cpu_possible mask in legacy mode.
This fallback mechanism is only triggered if _every_ other valid avenue has been traveled, and it is the last resort before calling BUG().
Signed-off-by: Joel Savitz <jsavitz@redhat.com> --- Makefile | 2 +- kernel/cgroup/cpuset.c | 15 ++++++++++++++- 2 files changed, 15 insertions(+), 2 deletions(-)
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c index 6a1942ed781c..515525ff1cfd 100644 --- a/kernel/cgroup/cpuset.c +++ b/kernel/cgroup/cpuset.c @@ -3254,10 +3254,23 @@ void cpuset_cpus_allowed(struct task_struct *tsk, struct cpumask *pmask) spin_unlock_irqrestore(&callback_lock, flags); } +/** + * cpuset_cpus_allowed_fallback - final fallback before complete catastrophe. + * @tsk: pointer to task_struct with which the scheduler is struggling + * + * Description: In the case that the scheduler cannot find an allowed cpu in + * tsk->cpus_allowed, we fall back to task_cs(tsk)->cpus_allowed. In legacy + * mode however, this value is the same as task_cs(tsk)->effective_cpus, + * which will not contain a sane cpumask during cases such as cpu hotplugging. + * This is the absolute last resort for the scheduler and it is only used if + * _every_ other avenue has been traveled. + **/ + void cpuset_cpus_allowed_fallback(struct task_struct *tsk) { rcu_read_lock(); - do_set_cpus_allowed(tsk, task_cs(tsk)->effective_cpus); + do_set_cpus_allowed(tsk, is_in_v2_mode() ? + task_cs(tsk)->cpus_allowed : cpu_possible_mask); rcu_read_unlock(); /* -- 2.18.1
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