Re: [PATCH 3/3] locking/percpu-rwsem: Avoid unnecessary writer wakeups

[Oleg Nesterov]

On 11/21, Davidlohr Bueso wrote:
>
> On Mon, 21 Nov 2016, Oleg Nesterov wrote:
>
>> On 11/21, Oleg Nesterov wrote:
>>>
>>> No, no, I meant that afaics both readers can see per_cpu_sum() != 0 and
>>> thus the writer won't be woken up. Till the next down_read/up_read.
>>>
>>> Suppose that we have 2 CPU's, both counters == 1, both readers decrement.
>>> its counter at the same time.
>>>
>>> 	READER_ON_CPU_0			READER_ON_CPU_1
>>>
>>> 	--ctr_0;			--ctr_1;
>>>
>>> 	if (ctr_0 + ctr_1)		if (ctr_0 + ctr_1)
>>> 		wakeup();			wakeup();
>>>
>>> Why we can't miss a wakeup?
>
> But the patch is really: if (!(ctr_0 + ctr_1)).

Of course, I meant if (ctr_0 + ctr_1 == 0).

>> And in fact I am not sure this optimization makes sense... But it would be
>> nice to avoid wake_up() when the writer sleeps in rcu_sync_enter(). Or this
>> is the "slow mode" sem (cgroup_threadgroup_rwsem).
>
> Why do you think using per_cpu_sum() does not make sense? As mentioned in the
> changelog it optimizes for incoming readers while the writer is doing sync_enter
> and getting the regular rwsem. What am I missing?

And this does make sense, but see below,

>> I need to re-check, but what do you think about the change below?
>
> While optimizing for multiple writers (rcu_sync_enter) is certainly valid
> (at least considering the cgroups rwsem you mention),

No, it is not for multiple writers. rcu_sync_enter() is slow, the new
readers can come and acquire/release this lock. And if it is a "slow mode"
sem then every up() does wakeup which we want to eliminate.

But after sem->readers_block is already true, I am not sure the additional
per_cpu_sum() is a win (even if it was correct), the new readers can't come.

Except __percpu_down_read()->__percpu_up_read() which we want to optimize
too, but in this case we do not need per_cpu_sum() too.

I'll try to make a patch this week... I had this optimization in mind from
the very beginning, I event mentioned it during the last discussion, but
never had time. Basically we should not inc if readers_block == T.

Oleg.