Re: [PATCH 1/2] cpuset: Fix cpuset_cpus_allowed() to not filter offline CPUs

[Waiman Long <longman@redhat.com>]

On 2/1/23 04:14, Peter Zijlstra wrote:
> On Tue, Jan 31, 2023 at 11:14:27PM -0500, Waiman Long wrote:
>> On 1/31/23 17:17, Will Deacon wrote:
>>> From: Peter Zijlstra <peterz@infradead.org>
>>>
>>> There is a difference in behaviour between CPUSET={y,n} that is now
>>> wrecking havoc with {relax,force}_compatible_cpus_allowed_ptr().
>>>
>>> Specifically, since commit 8f9ea86fdf99 ("sched: Always preserve the
>>> user requested cpumask")  relax_compatible_cpus_allowed_ptr() is
>>> calling __sched_setaffinity() unconditionally.
>>>
>>> But the underlying problem goes back a lot further, possibly to
>>> commit: ae1c802382f7 ("cpuset: apply cs->effective_{cpus,mems}") which
>>> switched cpuset_cpus_allowed() from cs->cpus_allowed to
>>> cs->effective_cpus.
>>>
>>> The problem is that for CPUSET=y cpuset_cpus_allowed() will filter out
>>> all offline CPUs. For tasks that are part of a (!root) cpuset this is
>>> then later fixed up by the cpuset hotplug notifiers that re-evaluate
>>> and re-apply cs->effective_cpus, but for (normal) tasks in the root
>>> cpuset this does not happen and they will forever after be excluded
>>> from CPUs onlined later.
>>>
>>> As such, rewrite cpuset_cpus_allowed() to return a wider mask,
>>> including the offline CPUs.
>>>
>>> Fixes: 8f9ea86fdf99 ("sched: Always preserve the user requested cpumask")
>>> Reported-by: Will Deacon <will@kernel.org>
>>> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>>> Link: https://lkml.kernel.org/r/20230117160825.GA17756@willie-the-truck
>>> Signed-off-by: Will Deacon <will@kernel.org>
>> Before cgroup v2, cpuset had only one cpumask - cpus_allowed. It only
>> tracked online cpus and ignored the offline ones. It behaves more like
>> effective_cpus in cpuset v2. With v2, we have 2 cpumasks - cpus_allowed and
>> effective_cpus. When cpuset v1 is mounted, cpus_allowed and effective_cpus
>> are effectively the same and track online cpus. With cpuset v2, cpus_allowed
>> contains what the user has written into and it won't be changed until
>> another write happen. However, what the user written may not be what the
>> system can give it and effective_cpus is what the system decides a cpuset
>> can use.
>>
>> Cpuset v2 is able to handle hotplug correctly and update the task's cpumask
>> accordingly. So missing previously offline cpus won't happen with v2.
>>
>> Since v1 keeps the old behavior, previously offlined cpus are lost in the
>> cpuset's cpus_allowed. However tasks in the root cpuset will still be fine
>> with cpu hotplug as its cpus_allowed should track cpu_online_mask. IOW, only
>> tasks in a non-root cpuset suffer this problem.
>>
>> It was a known issue in v1 and I believe is one of the major reasons of the
>> cpuset v2 redesign.
>>
>> A major concern I have is the overhead of creating a poor man version of v2
>> cpus_allowed. This issue can be worked around even for cpuset v1 if it is
>> mounted with the cpuset_v2_mode option to behave more like v2 in its cpumask
>> handling. Alternatively we may be able to provide a config option to make
>> this the default for v1 without the special mount option, if necessary.
> You're still not getting it -- even cpuset (be it v1 or v2) *MUST* *NOT*
> mask offline cpus for root cgroup tasks, ever. (And the only reason it
> gets away with masking offline for !root is that it re-applies the mask
> every time it changes.)
>
> Yes it did that for a fair while -- but it is wrong and broken and a
> very big behavioural difference between CONFIG_CPUSET={y,n}. This must
> not be.
>
> Arguably cpuset-v2 is still wrong for masking offline cpus in it's
> effective_cpus mask, but I really didn't want to go rewrite cpuset.c for
> something that needs to go into /urgent *now*.
>
> Hence this minimal patch that at least lets sched_setaffinity() work as
> intended.

I don't object to the general idea of keeping offline cpus in a task's 
cpu affinity. In the case of cpu offline event, we can skip removing 
that offline cpu from the task's cpu affinity. That will partially solve 
the problem here and is also simpler.

I believe a main reason why effective_cpus holds only online cpus is 
because of the need to detect when there is no online cpus available in 
a given cpuset. In this case, it will fall back to the nearest ancestors 
with online cpus.

This offline cpu problem with cpuset v1 is a known problem for a long 
time. It is not a recent regression.

Cheers,
Longman