Messages in this thread | | | Date | Fri, 9 Nov 2012 16:55:16 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH RESEND v2 1/1] percpu_rw_semaphore: reimplement to not block the readers unnecessarily |
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On Fri, Nov 09, 2012 at 07:10:48PM +0100, Oleg Nesterov wrote: > On 11/09, Paul E. McKenney wrote: > > > > On Fri, Nov 09, 2012 at 04:46:56PM +0100, Oleg Nesterov wrote: > > > Contrary, I am going to try to add some complications later, so that > > > it can have more users. In particular, I think it can replace > > > get_online_cpus/cpu_hotplug_begin, just we need > > > percpu_down_write_but_dont_deadlock_with_recursive_readers(). > > > > I must confess that I am a bit concerned about possible scalability > > bottlenecks in the current get_online_cpus(), so +1 from me on this one. > > OK, thanks... > > And btw percpu_down_write_but_dont_deadlock_with_recursive_readers() is > trivial, just it needs down_write(rw_sem) "inside" wait_event(), not > before. But I'm afraid I will never manage to write the comments ;) > > static bool xxx(brw) > { > down_write(&brw->rw_sem);
down_write_trylock()
As you noted in your later email. Presumably you return false if the attempt to acquire it fails.
> if (!atomic_read(&brw->slow_read_ctr)) > return true; > > up_write(&brw->rw_sem); > return false; > } > > static void __percpu_down_write(struct percpu_rw_semaphore *brw, bool recursive_readers) > { > mutex_lock(&brw->writer_mutex); > > synchronize_sched(); > > atomic_add(clear_fast_ctr(brw), &brw->slow_read_ctr); > > if (recursive_readers) { > wait_event(brw->write_waitq, xxx(brw));
I see what you mean about acquiring brw->rw_sem inside of wait_event().
Cute trick!
The "recursive_readers" is a global initialization-time thing, right?
> } else { > down_write(&brw->rw_sem); > > wait_event(brw->write_waitq, !atomic_read(&brw->slow_read_ctr)); > } > }
Looks like it should work, and would perform and scale nicely even if we end up having to greatly increase the number of calls to get_online_cpus().
> Of course, cpu.c still needs .active_writer to allow get_online_cpus() > under cpu_hotplug_begin(), but this is simple.
Yep, same check as now.
> But first we should do other changes, I think. IMHO we should not do > synchronize_sched() under mutex_lock() and this will add (a bit) more > complications. We will see.
Indeed, that does put considerable delay on the writers. There is always synchronize_sched_expedited(), I suppose.
Thanx, Paul
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