linux-kernel - Re: [PATCH RFC] rcu/kfree: Do not request RCU when not needed

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Date:   Thu, 3 Nov 2022 13:41:43 +0100
From:   Uladzislau Rezki <urezki@...il.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        Uladzislau Rezki <urezki@...il.com>, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] rcu/kfree: Do not request RCU when not needed

> > >> /**
> > >> @@ -3066,10 +3068,12 @@ static void kfree_rcu_work(struct work_struct *work)
> > >>    struct kfree_rcu_cpu_work *krwp;
> > >>    int i, j;
> > >> 
> > >> -    krwp = container_of(to_rcu_work(work),
> > >> +    krwp = container_of(work,
> > >>                struct kfree_rcu_cpu_work, rcu_work);
> > >>    krcp = krwp->krcp;
> > >> 
> > >> +    cond_synchronize_rcu(krwp->gp_snap);
> > > 
> > > Might this provoke OOMs in case of callback flooding?
> > > 
> > > An alternative might be something like this:
> > > 
> > >    if (!poll_state_synchronize_rcu(krwp->gp_snap)) {
> > >        queue_rcu_work(system_wq, &krwp->rcu_work);
> > >        return;
> > >    }
> > > 
> > > Either way gets you a non-lazy callback in the case where a grace
> > > period has not yet elapsed.
> > > Or am I missing something that prevents OOMs here?
> > 
> > The memory consumptions appears to be much less in his testing with the onslaught of kfree, which makes OOM probably less likely.
> > 
> > Though, was your reasoning that in case of a grace period not elapsing, we need a non lazy callback queued, so as to make the reclaim happen sooner?
> > 
> > If so, the cond_synchronize_rcu() should already be conditionally queueing non-lazy CB since we don’t make synchronous users wait for seconds. Or did I miss something?
> 
> My concern is that the synchronize_rcu() will block a kworker kthread
> for some time, and that in callback-flood situations this might slow
> things down due to exhausting the supply of kworkers.
> 
This concern works in both cases. I mean in default configuration and
with a posted patch. The reclaim work, which name is kfree_rcu_work() only
does a progress when a gp is passed so the rcu_work_rcufn() can queue
our reclaim kworker.

As it is now:

1. Collect pointers, then we decide to drop them we queue the
   monitro_work() worker to the system_wq.

2. The monitor work, kfree_rcu_work(), tries to attach or saying
it by another words bypass a "backlog" to "free" channels.

3. It invokes the queue_rcu_work() that does call_rcu_flush() and
in its turn it queues our worker from the handler. So the worker
is run after GP is passed.

With a patch: 

[1] and [2] steps are the same. But on third step we do:

1. Record the GP status for last in channel;
2. Directly queue the drain work without any call_rcu() helpers;
3. On the reclaim worker entry we check if GP is passed;
4. If not it invokes synchronize_rcu().

The patch eliminates extra steps by not going via RCU-core route
instead it directly invokes the reclaim worker where it either
proceed or wait a GP if needed.

--
Uladzislau Rezki