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

[Qais Yousef <qais.yousef@arm.com>]

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.

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().

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

--
Qais Yousef