Re: [PATCH v3 3/7] sched/fair: Add cgroup_mode: max

[Waiman Long]

On 6/10/26 11:09 AM, Waiman Long wrote:
> On 6/5/26 8:40 AM, Peter Zijlstra wrote:
> > In order to avoid the average CPU fraction avg(F_g_n) becoming tiny 
> > '1/N',
> > assume each cgroup is maximally concurrent and distrubute 'N*weight', 
> > such
> > that:
> >
> >     F_g_n' = N * F_g_n
> >
> > Giving:
> >
> >     avg(F_g_n') = N*avg(F_g_n) ~ N * 1/N = 1
> >
> > And while this sounds like it solves things, remember what that ~ 
> > meant. There
> > is the corner case when a cgroup is minimally loaded, eg a single 
> > runnable
> > task, therefore limit the CPU fraction to that of a nice -20 task to 
> > avoid
> > getting too much load.
> >
> > This last bit is what makes it different from a previous proposal to 
> > allow
> > raising cpu.weight to '100 * N', that would not limit the mininal 
> > concurrency
> > case and results in a very large F_g_n. And just like F_g_n << 1 is
> > problematic, so is F_g_n >> 1 for the exact same reasons (it would 
> > drown the
> > kthreads, but it also risks overflowing the load values).
> >
> > So while this might appear to be a better scheme than the current 
> > default
> > scheme, it doesn't really handle less than maximal concurrency nicely 
> > -- it
> > clips and introduces artificially large weights. So where the 
> > traditional SMP
> > mode works well when nr_tasks << nr_cpus, MAX doesn't work well in 
> > that regime
> > and vice-versa.
> >
> > The meaning of "cpu.weight" would be: weight per allowed CPU.
> >
> > Included for completeness (and infrastructure).
> >
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> > ---
> >   include/linux/cpuset.h |    6 +++++
> >   kernel/cgroup/cpuset.c |   15 ++++++++++++++
> >   kernel/sched/debug.c   |    1
> >   kernel/sched/fair.c    |   52 
> > ++++++++++++++++++++++++++++++++++++++++++++-----
> >   4 files changed, 69 insertions(+), 5 deletions(-)
> >
> > --- a/include/linux/cpuset.h
> > +++ b/include/linux/cpuset.h
> > @@ -80,6 +80,7 @@ extern void lockdep_assert_cpuset_lock_h
> >   extern void cpuset_cpus_allowed_locked(struct task_struct *p, 
> > struct cpumask *mask);
> >   extern void cpuset_cpus_allowed(struct task_struct *p, struct 
> > cpumask *mask);
> >   extern bool cpuset_cpus_allowed_fallback(struct task_struct *p);
> > +extern int cpuset_num_cpus(struct cgroup *cgroup);
> >   extern nodemask_t cpuset_mems_allowed(struct task_struct *p);
> >   #define cpuset_current_mems_allowed (current->mems_allowed)
> >   void cpuset_init_current_mems_allowed(void);
> > @@ -216,6 +217,11 @@ static inline bool cpuset_cpus_allowed_f
> >       return false;
> >   }
> >   +static inline int cpuset_num_cpus(struct cgroup *cgroup)
> > +{
> > +    return num_online_cpus();
> > +}
> > +
> >   static inline nodemask_t cpuset_mems_allowed(struct task_struct *p)
> >   {
> >       return node_possible_map;
> > --- a/kernel/cgroup/cpuset.c
> > +++ b/kernel/cgroup/cpuset.c
> > @@ -4116,6 +4116,21 @@ bool cpuset_cpus_allowed_fallback(struct
> >       return changed;
> >   }
> >   +int cpuset_num_cpus(struct cgroup *cgrp)
> > +{
> > +    int nr = num_online_cpus();
> > +    struct cpuset *cs;
> > +
> > +    if (is_in_v2_mode()) {
> > +        guard(rcu)();
> > +        cs = css_cs(cgroup_e_css(cgrp, &cpuset_cgrp_subsys));
> > +        if (cs)
> > +            nr = cpumask_weight(cs->effective_cpus);
> > +    }
> > +
> > +    return nr;
> > +}
>
> I just have a question about cgroup v1 support. I am assuming that 
> cgroup v1 without the cpuset_v2_mode mount option is not supported. To 
> fully support cgroup v1, you may have to use guarantee_active_cpus() 
> to return the actual set of CPUs that the task can run on. Also there 
> is a caveat about the arm64 specific task_cpu_possible_mask() for 
> certain arm64 CPUs. That is for 32-bit binary running on 64-bit core 
> which are allowed only on a selected subset of cores within the CPU.
>
> This is probably not what you want to focus on right now, but it will 
> be good to have a comment to list items that are not fully supported 
> here. 

FYI, you may have to take the callback_lock to ensure the stability of 
the effective_cpus mask.

Cheers,
Longman