Re: [PATCH v6 08/21] cpuset: Honour task_cpu_possible_mask() in guarantee_online_cpus()

[Will Deacon]

On Fri, May 21, 2021 at 05:25:24PM +0100, Qais Yousef wrote:
> On 05/18/21 10:47, Will Deacon wrote:
> > Asymmetric systems may not offer the same level of userspace ISA support
> > across all CPUs, meaning that some applications cannot be executed by
> > some CPUs. As a concrete example, upcoming arm64 big.LITTLE designs do
> > not feature support for 32-bit applications on both clusters.
> > 
> > Modify guarantee_online_cpus() to take task_cpu_possible_mask() into
> > account when trying to find a suitable set of online CPUs for a given
> > task. This will avoid passing an invalid mask to set_cpus_allowed_ptr()
> > during ->attach() and will subsequently allow the cpuset hierarchy to be
> > taken into account when forcefully overriding the affinity mask for a
> > task which requires migration to a compatible CPU.
> > 
> > Cc: Li Zefan <lizefan@xxxxxxxxxx>
> > Cc: Tejun Heo <tj@xxxxxxxxxx>
> > Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> > Signed-off-by: Will Deacon <will@xxxxxxxxxx>
> > ---
> >  include/linux/cpuset.h |  2 +-
> >  kernel/cgroup/cpuset.c | 33 +++++++++++++++++++--------------
> >  2 files changed, 20 insertions(+), 15 deletions(-)
> > 
> > diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
> > index ed6ec677dd6b..414a8e694413 100644
> > --- a/include/linux/cpuset.h
> > +++ b/include/linux/cpuset.h
> > @@ -185,7 +185,7 @@ static inline void cpuset_read_unlock(void) { }
> >  static inline void cpuset_cpus_allowed(struct task_struct *p,
> >  				       struct cpumask *mask)
> >  {
> > -	cpumask_copy(mask, cpu_possible_mask);
> > +	cpumask_copy(mask, task_cpu_possible_mask(p));
> >  }
> >  
> >  static inline void cpuset_cpus_allowed_fallback(struct task_struct *p)
> > diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> > index 8c799260a4a2..b532a5333ff9 100644
> > --- a/kernel/cgroup/cpuset.c
> > +++ b/kernel/cgroup/cpuset.c
> > @@ -372,18 +372,26 @@ static inline bool is_in_v2_mode(void)
> >  }
> >  
> >  /*
> > - * Return in pmask the portion of a cpusets's cpus_allowed that
> > - * are online.  If none are online, walk up the cpuset hierarchy
> > - * until we find one that does have some online cpus.
> > + * Return in pmask the portion of a task's cpusets's cpus_allowed that
> > + * are online and are capable of running the task.  If none are found,
> > + * walk up the cpuset hierarchy until we find one that does have some
> > + * appropriate cpus.
> >   *
> >   * One way or another, we guarantee to return some non-empty subset
> >   * of cpu_online_mask.
> >   *
> >   * Call with callback_lock or cpuset_mutex held.
> >   */
> > -static void guarantee_online_cpus(struct cpuset *cs, struct cpumask *pmask)
> > +static void guarantee_online_cpus(struct task_struct *tsk,
> > +				  struct cpumask *pmask)
> >  {
> > -	while (!cpumask_intersects(cs->effective_cpus, cpu_online_mask)) {
> > +	struct cpuset *cs = task_cs(tsk);
> 
> task_cs() requires rcu_read_lock(), but I can't see how the lock is obtained
> from cpuset_attach() path, did I miss it? Running with lockdep should spill
> suspicious RCU usage warning.
> 
> Maybe it makes more sense to move the rcu_read_lock() inside the function now
> with task_cs()?

Well spotted! I'll add the rcu_read_[un]lock() calls to
guarantee_online_cpus().

Will