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

From: Davidlohr Bueso
Date: Mon Nov 21 2016 - 22:59:48 EST

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.


--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)). wrt to stale values is this
like due to the data dependency we only see the real value of this_cpu ctr,
and no guarantee for the other cpus? If so I had not considered that scenario,
and yes we'd need stronger guarantees.

I'd have to wonder if other users of per_cpu_sum() would fall into a similar
trap. Hmm and each user seems to implement its own copy of the same thing.

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?

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), I think that my
heuristic covers the otherwise more common case. Could both optimizations
not work together?

Of course, the window of where readers_block == 1 is quite large, so there
can be a lot of false positives.