Re: [PATCH v6 1/2] sched/uclamp: Add a new sysctl to control RT default boost value

[Valentin Schneider <valentin.schneider@arm.com>]

On 07/07/20 10:34, Qais Yousef wrote:
> On 07/06/20 16:49, Valentin Schneider wrote:
>>
>> On 06/07/20 15:28, Qais Yousef wrote:
>> > CC: linux-fsdevel@vger.kernel.org
>> > ---
>> >
>> > Peter
>> >
>> > I didn't do the
>> >
>> >       read_lock(&taslist_lock);
>> >       smp_mb__after_spinlock();
>> >       read_unlock(&tasklist_lock);
>> >
>> > dance you suggested on IRC as it didn't seem necessary. But maybe I missed
>> > something.
>> >
>>
>> So the annoying bit with just uclamp_fork() is that it happens *before* the
>> task is appended to the tasklist. This means without too much care we
>> would have (if we'd do a sync at uclamp_fork()):
>>
>>   CPU0 (sysctl write)                                CPU1 (concurrent forker)
>>
>>                                                        copy_process()
>>                                                          uclamp_fork()
>>                                                            p.uclamp_min = state
>>     state = foo
>>
>>     for_each_process_thread(p, t)
>>       update_state(t);
>>                                                          list_add(p)
>>
>> i.e. that newly forked process would entirely sidestep the update. Now,
>> with Peter's suggested approach we can be in a much better situation. If we
>> have this in the sysctl update:
>>
>>   state = foo;
>>
>>   read_lock(&taslist_lock);
>>   smp_mb__after_spinlock();
>>   read_unlock(&tasklist_lock);
>>
>>   for_each_process_thread(p, t)
>>     update_state(t);
>>
>> While having this in the fork:
>>
>>   write_lock(&tasklist_lock);
>>   list_add(p);
>>   write_unlock(&tasklist_lock);
>>
>>   sched_post_fork(p); // state re-read here; probably wants an mb first
>>
>> Then we can no longer miss an update. If the forked p doesn't see the new
>> value, it *must* have been added to the tasklist before the updater loops
>> over it, so the loop will catch it. If it sees the new value, we're done.
>
> uclamp_fork() has nothing to do with the race. If copy_process() duplicates the
> task_struct of an RT task, it'll copy the old value.
>

Quite so; my point was if we were to use uclamp_fork() as to re-read the value.

> I'd expect the newly introduced sched_post_fork() (also in copy_process() after
> the list update) to prevent this race altogether.
>
> Now we could end up with a problem if for_each_process_thread() doesn't see the
> newly forked task _after_ sched_post_fork(). Hence my question to Peter.
>


>>
>> AIUI, the above strategy doesn't require any use of RCU. The update_state()
>> and sched_post_fork() can race, but as per the above they should both be
>> writing the same value.
>
> for_each_process_thread() must be protected by either tasklist_lock or
> rcu_read_lock().
>

Right

> The other RCU logic I added is not to protect against the race above. I
> describe the other race condition in a comment.

I take it that's the one in uclamp_sync_util_min_rt_default()?

__setscheduler_uclamp() can't be preempted as we hold task_rq_lock(). It
can indeed race with the sync though, but again with the above suggested
setup it would either:
- see the old value, but be guaranteed to be iterated over later by the
  updater
- see the new value

sched_post_fork() being preempted out is a bit more annoying, but what
prevents us from making that bit preempt-disabled?

I have to point out I'm assuming here updaters are serialized, which does
seem to be see the case (cf. uclamp_mutex).


> Basically another updater on a
> different cpu via fork() and sched_setattr() might read an old value and get
> preempted. The rcu synchronization will ensure concurrent updaters have
> finished before iterating the list.
>

> Thanks