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    SubjectRe: [PATCH v5 07/15] cpuset: Don't use the cpu_possible_mask as a last resort for cgroup v1
    On 12/08/20 13:28, Will Deacon wrote:
    > If the scheduler cannot find an allowed CPU for a task,
    > cpuset_cpus_allowed_fallback() will widen the affinity to cpu_possible_mask
    > if cgroup v1 is in use.
    >
    > In preparation for allowing architectures to provide their own fallback
    > mask, just return early if we're not using cgroup v2 and allow
    > select_fallback_rq() to figure out the mask by itself.
    >
    > Cc: Li Zefan <lizefan@huawei.com>
    > Cc: Tejun Heo <tj@kernel.org>
    > Cc: Johannes Weiner <hannes@cmpxchg.org>
    > Reviewed-by: Quentin Perret <qperret@google.com>
    > Signed-off-by: Will Deacon <will@kernel.org>
    > ---
    > kernel/cgroup/cpuset.c | 6 ++++--
    > 1 file changed, 4 insertions(+), 2 deletions(-)
    >
    > diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
    > index 57b5b5d0a5fd..e970737c3ed2 100644
    > --- a/kernel/cgroup/cpuset.c
    > +++ b/kernel/cgroup/cpuset.c
    > @@ -3299,9 +3299,11 @@ void cpuset_cpus_allowed(struct task_struct *tsk, struct cpumask *pmask)
    >
    > void cpuset_cpus_allowed_fallback(struct task_struct *tsk)
    > {
    > + if (!is_in_v2_mode())
    > + return; /* select_fallback_rq will try harder */
    > +
    > rcu_read_lock();
    > - do_set_cpus_allowed(tsk, is_in_v2_mode() ?
    > - task_cs(tsk)->cpus_allowed : cpu_possible_mask);
    > + do_set_cpus_allowed(tsk, task_cs(tsk)->cpus_allowed);

    Why is it safe to return that for cpuset v2? task_cs(tsk)->cpus_allowed is the
    original user configured settings of the cpuset.cpus; which could have empty
    intersection with task_cpu_possible_mask(), no?

    do_set_cpus_allowed() will call set_cpus_allowed_common() which will end up
    copying the mask as-is.

    So unless I missed something there's a risk a 32bit task ends up having a 64bit
    only cpu_mask when using cpuset v2.

    Thanks

    --
    Qais Yousef

    > rcu_read_unlock();
    >
    > /*
    > --
    > 2.29.2.576.ga3fc446d84-goog
    >

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