Messages in this thread | | | Date | Mon, 30 Sep 2013 18:38:10 +0200 | From | Oleg Nesterov <> | Subject | Re: [RFC] introduce synchronize_sched_{enter,exit}() |
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On 09/30, Peter Zijlstra wrote: > > On Sun, Sep 29, 2013 at 08:36:34PM +0200, Oleg Nesterov wrote: > > Why? Say, percpu_rw_semaphore, or upcoming changes in get_online_cpus(), > > (Peter, I think they should be unified anyway, but lets ignore this for > > now). > > If you think the percpu_rwsem users can benefit sure.. So far its good I > didn't go the percpu_rwsem route for it looks like we got something > better at the end of it ;-)
I think you could simply improve percpu_rwsem instead. Once we add task_struct->cpuhp_ctr percpu_rwsem and get_online_cpus/hotplug_begin becomes absolutely congruent.
OTOH, it would be simpler to change hotplug first, then copy-and-paste the improvents into percpu_rwsem, then see if we can simply convert cpu_hotplug_begin/end into percpu_down/up_write.
> Well, if we make percpu_rwsem the defacto container of the pattern and > use that throughout, we'd have only a single implementation
Not sure. I think it can have other users. But even if not, please look at "struct sb_writers". Yes, I believe it makes sense to use percpu_rwsem here, but note that it is actually array of semaphores. I do not think each element needs its own xxx_struct.
> and don't > need the abstraction.
And even if struct percpu_rw_semaphore will be the only container of xxx_struct, I think the code looks better and more understandable this way, exactly because it adds the new abstraction layer. Performance-wise this should be free.
> > static void cb_rcu_func(struct rcu_head *rcu) > > { > > struct xxx_struct *xxx = container_of(rcu, struct xxx_struct, cb_head); > > long flags; > > > > BUG_ON(xxx->gp_state != GP_PASSED); > > BUG_ON(xxx->cb_state == CB_IDLE); > > > > spin_lock_irqsave(&xxx->xxx_lock, flags); > > if (xxx->gp_count) { > > xxx->cb_state = CB_IDLE; > > This seems to be when a new xxx_begin() has happened after our last > xxx_end() and the sync_sched() from xxx_begin() merges with the > xxx_end() one and we're done.
Yes,
> > } else if (xxx->cb_state == CB_REPLAY) { > > xxx->cb_state = CB_PENDING; > > call_rcu_sched(&xxx->cb_head, cb_rcu_func); > > A later xxx_exit() has happened, and we need to requeue to catch a later > GP.
Exactly.
> So I don't immediately see the point of the concurrent write side; > percpu_rwsem wouldn't allow this and afaict neither would > freeze_super().
Oh I disagree. Even ignoring the fact I believe xxx_struct itself can have more users (I can be wrong of course), I do think that percpu_down_write_nonexclusive() makes sense (except "exclusive" should be the argument of percpu_init_rwsem). And in fact the initial implementation I sent didn't even has the "exclusive" mode.
Please look at uprobes (currently the only user). We do not really need the global write-lock, we can do the per-uprobe locking. However, every caller needs to block the percpu_down_read() callers (dup_mmap).
> Other than that; yes this makes sense if you care about write side > performance and I think its solid.
Great ;)
Oleg.
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