Re: [PATCH] sched: Provide a pointer to the valid CPU mask

Ingo Molnar
2019-05-02T15:11:02Z

* Sebastian Andrzej Siewior <bigeasy@linutronix.de> wrote:

> In commit 4b53a3412d66 ("sched/core: Remove the tsk_nr_cpus_allowed()
> wrapper") the tsk_nr_cpus_allowed() wrapper was removed. There was not
> much difference in !RT but in RT we used this to implement
> migrate_disable(). Within a migrate_disable() section the CPU mask is
> restricted to single CPU while the "normal" CPU mask remains untouched.
>
> As an alternative implementation Ingo suggested to use
> struct task_struct {
> const cpumask_t *cpus_ptr;
> cpumask_t cpus_mask;
> };
> with
> t->cpus_allowed_ptr = &t->cpus_allowed;
>
> In -RT we then can switch the cpus_ptr to
> t->cpus_allowed_ptr = &cpumask_of(task_cpu(p));
>
> in a migration disabled region. The rules are simple:
> - Code that 'uses' ->cpus_allowed would use the pointer.
> - Code that 'modifies' ->cpus_allowed would use the direct mask.
>
> I proposed this patch as a series earlier and it was shutdown due to the
> migrate_disable() bits. It has been said that migrate_disable() should
> only be used with RT and thus not introduced without it.
> I hereby propose only the mask CPU-bits.
>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@kernel.org>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
> arch/ia64/kernel/mca.c | 2 +-
> arch/mips/include/asm/switch_to.h | 4 +--
> arch/mips/kernel/mips-mt-fpaff.c | 2 +-
> arch/mips/kernel/traps.c | 6 ++--
> arch/powerpc/platforms/cell/spufs/sched.c | 2 +-
> arch/x86/kernel/cpu/resctrl/pseudo_lock.c | 2 +-
> drivers/infiniband/hw/hfi1/affinity.c | 6 ++--
> drivers/infiniband/hw/hfi1/sdma.c | 3 +-
> drivers/infiniband/hw/qib/qib_file_ops.c | 7 ++--
> fs/proc/array.c | 4 +--
> include/linux/sched.h | 5 +--
> init/init_task.c | 3 +-
> kernel/cgroup/cpuset.c | 2 +-
> kernel/fork.c | 2 ++
> kernel/sched/core.c | 40 +++++++++++-----------
> kernel/sched/cpudeadline.c | 4 +--
> kernel/sched/cpupri.c | 4 +--
> kernel/sched/deadline.c | 6 ++--
> kernel/sched/fair.c | 34 +++++++++---------
> kernel/sched/rt.c | 4 +--
> kernel/trace/trace_hwlat.c | 2 +-
> lib/smp_processor_id.c | 2 +-
> samples/trace_events/trace-events-sample.c | 2 +-
> 23 files changed, 75 insertions(+), 73 deletions(-)

Looks good to me in principle - Peter, Thomas, any fundamental
objections?

Thanks,

Ingo