Ping!
Does anyone have any comments or concerns about this patch?
Best,
Joel Savitz
Best,
Joel Savitz
On Thu, Mar 7, 2019 at 9:42 AM Joel Savitz <jsav...@redhat.com> wrote:
>
> On Wed, Mar 6, 2019 at 7:55 PM Joel Savitz <jsav...@redhat.com> wrote:
> >
> > 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 simply resetting the mask
> > to cpu_possible_mask.
> >
> > Signed-off-by: Joel Savitz <jsav...@redhat.com>
> > ---
> > kernel/cgroup/cpuset.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> > index 479743db6c37..5f65a2167bdf 100644
> > --- a/kernel/cgroup/cpuset.c
> > +++ b/kernel/cgroup/cpuset.c
> > @@ -3243,7 +3243,7 @@ void cpuset_cpus_allowed(struct task_struct *tsk,
> > struct cpumask *pmask)
> > 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, cpu_possible_mask);
> > rcu_read_unlock();
> >
> > /*
> > --
> > 2.20.1
> >
