On 07/07/20 13:36, Qais Yousef wrote: > On 07/07/20 12:30, Valentin Schneider wrote: >> >> 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@xxxxxxxxxxxxxxx >> >> > --- >> >> > >> >> > 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()? > > Correct. > >> >> __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 > > AFAIU rcu_read_lock() is light weight. So having the protection applied is more > robust against future changes. So I think the one thing you win by having this dance with mb's and the suggested handling of the task list is that you do not need any rcu_synchronize() anymore. Both approaches have merit, it's just that the way I understood the suggestion to add sched_post_fork() was to simplify the ordering of the update with the aforementioned scheme. > >> >> sched_post_fork() being preempted out is a bit more annoying, but what >> prevents us from making that bit preempt-disabled? > > preempt_disable() is not friendly to RT and heavy handed approach IMO. > True, but this is both an infrequent and slow sysctl path, so I don't think RT would care much. >> >> I have to point out I'm assuming here updaters are serialized, which does >> seem to be see the case (cf. uclamp_mutex). > > Correct. > > Thanks
