Re: [PATCH v2 2/2] cgroup/cpuset: align DL bandwidth reservation with attach target mask

[Guopeng Zhang]

在 2026/5/7 23:52, Waiman Long 写道:
> On 5/7/26 6:33 AM, Guopeng Zhang wrote:
>> cpuset_can_attach() preallocates destination SCHED_DEADLINE bandwidth
>> before the attach commit point, while set_cpus_allowed_dl() later
>> subtracts bandwidth from the source root domain when the task affinity is
>> actually updated.
>>
>> Those two decisions must be made with the same CPU mask.
>> cpuset_can_attach() used the destination cpuset effective mask directly,
>> but cpuset_attach_task() first builds a per-task target mask which is
>> constrained by task_cpu_possible_mask() and, if needed, by walking up the
>> cpuset hierarchy. On asymmetric systems, the actual target mask can
>> therefore be a strict subset of cs->effective_cpus.
> 
> The task_cpu_possible_mask() is there for a special class of arm64 CPUs where only some of the cores are able to run legacy 32-bit applications on 64-bit arm CPUs. We can argue how likely that a DL task can be a legacy 32 bit application that is inherently slower than the same application compiled into native 64-bit code. Perhaps we can just disallow such a legacy 32-bit application from moving to a DL scheduling class in the first place.
> 
> I am not in favor of the idea of making the cpuset code more complex to support such a corner case which may never be utilized. Could you strip out the task_possible_cpu_mask() part from this patch? We can revisit this with another patch if such a special use case can be useful to support in the future.
> 
Thanks for the review.

I agree. The task_cpu_possible_mask() case makes the fix broader and
adds more cpuset-side complexity than needed for this series.

I will drop the cpuset_attach_task() target-mask mirroring from v3 and
keep cpuset_can_attach() using cs->effective_cpus. The updated patch will
only share the root-domain bandwidth-move test with set_cpus_allowed_dl()
and only add a migrating DL task to sum_migrate_dl_bw when that task
actually needs a root-domain bandwidth move.

The task_cpu_possible_mask() corner case can be revisited separately if
there is a real need to support that scenario.

Thanks,
Guopeng
> Cheers,
> Longman
> 
>>
>> If the source root domain intersects cs->effective_cpus only on CPUs
>> outside the task's possible mask, can_attach() can skip the destination
>> reservation even though set_cpus_allowed_dl() later sees a real
>> root-domain move and subtracts from the source domain.
>>
>> Extract the root-domain bandwidth-move test used by
>> set_cpus_allowed_dl() into dl_task_needs_bw_move(), and make
>> cpuset_can_attach() compute the same per-task target mask that
>> cpuset_attach_task() applies.
>>
>> Keep nr_migrate_dl_tasks counting all migrating deadline tasks for
>> cpuset DL task accounting. Restrict sum_migrate_dl_bw to the subset of
>> tasks that need destination root-domain bandwidth reservation, because a
>> deadline task can move between cpusets without moving bandwidth between
>> root domains.
>>
>> This keeps the existing per-attach aggregate reservation model; it only
>> changes the per-task mask used to decide which tasks contribute to that
>> aggregate. The broader can_attach()/attach() transaction window is left
>> unchanged.
>>
>> Fixes: 431c69fac05b ("cpuset: Honour task_cpu_possible_mask() in guarantee_online_cpus()")
>> Fixes: 2ef269ef1ac0 ("cgroup/cpuset: Free DL BW in case can_attach() fails")
>> Signed-off-by: Guopeng Zhang <zhangguopeng@xxxxxxxxxx>
>> ---
>>   include/linux/sched/deadline.h  |  9 +++
>>   kernel/cgroup/cpuset-internal.h |  1 +
>>   kernel/cgroup/cpuset.c          | 97 ++++++++++++++++++++++-----------
>>   kernel/sched/deadline.c         | 13 ++++-
>>   4 files changed, 86 insertions(+), 34 deletions(-)
>>