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

[Uladzislau Rezki <urezki@gmail.com>]

On Wed, Nov 02, 2022 at 01:29:17PM -0400, Joel Fernandes wrote:
> On Wed, Nov 2, 2022 at 1:24 PM Uladzislau Rezki <urezki@gmail.com> wrote:
> >
> > On Wed, Nov 02, 2022 at 09:35:44AM -0700, Paul E. McKenney wrote:
> > > On Wed, Nov 02, 2022 at 12:13:17PM -0400, Joel Fernandes wrote:
> > > > On Wed, Nov 2, 2022 at 8:37 AM Uladzislau Rezki <urezki@gmail.com> wrote:
> > > > >
> > > > > On Sat, Oct 29, 2022 at 01:28:56PM +0000, Joel Fernandes (Google) wrote:
> > > > > > On ChromeOS, I am (almost) always seeing the optimization trigger.
> > > > > > Tested boot up and trace_printk'ing how often it triggers.
> > > > > >
> > > > > > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > > > > > ---
> > > > > >  kernel/rcu/tree.c | 18 +++++++++++++++++-
> > > > > >  1 file changed, 17 insertions(+), 1 deletion(-)
> > > > > >
> > > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > > > > index 591187b6352e..3e4c50b9fd33 100644
> > > > > > --- a/kernel/rcu/tree.c
> > > > > > +++ b/kernel/rcu/tree.c
> > > > > > @@ -2935,6 +2935,7 @@ struct kfree_rcu_cpu_work {
> > > > > >
> > > > > >  /**
> > > > > >   * struct kfree_rcu_cpu - batch up kfree_rcu() requests for RCU grace period
> > > > > > + * @rdp: The rdp of the CPU that this kfree_rcu corresponds to.
> > > > > >   * @head: List of kfree_rcu() objects not yet waiting for a grace period
> > > > > >   * @bkvhead: Bulk-List of kvfree_rcu() objects not yet waiting for a grace period
> > > > > >   * @krw_arr: Array of batches of kfree_rcu() objects waiting for a grace period
> > > > > > @@ -2964,6 +2965,8 @@ struct kfree_rcu_cpu {
> > > > > >       struct kfree_rcu_cpu_work krw_arr[KFREE_N_BATCHES];
> > > > > >       raw_spinlock_t lock;
> > > > > >       struct delayed_work monitor_work;
> > > > > > +     struct rcu_data *rdp;
> > > > > > +     unsigned long last_gp_seq;
> > > > > >       bool initialized;
> > > > > >       int count;
> > > > > >
> > > > > > @@ -3167,6 +3170,7 @@ schedule_delayed_monitor_work(struct kfree_rcu_cpu *krcp)
> > > > > >                       mod_delayed_work(system_wq, &krcp->monitor_work, delay);
> > > > > >               return;
> > > > > >       }
> > > > > > +     krcp->last_gp_seq = krcp->rdp->gp_seq;
> > > > > >       queue_delayed_work(system_wq, &krcp->monitor_work, delay);
> > > > > >  }
> > > > > >
> > > > > > @@ -3217,7 +3221,17 @@ static void kfree_rcu_monitor(struct work_struct *work)
> > > > > >                       // be that the work is in the pending state when
> > > > > >                       // channels have been detached following by each
> > > > > >                       // other.
> > > > > > -                     queue_rcu_work(system_wq, &krwp->rcu_work);
> > > > > > +                     //
> > > > > > +                     // NOTE about gp_seq wrap: In case of gp_seq overflow,
> > > > > > +                     // it is possible for rdp->gp_seq to be less than
> > > > > > +                     // krcp->last_gp_seq even though a GP might be over. In
> > > > > > +                     // this rare case, we would just have one extra GP.
> > > > > > +                     if (krcp->last_gp_seq &&
> > > > > >
> > > > > This check can be eliminated i think. A kfree_rcu_cpu is defined as
> > > > > static so by default the last_gp_set is set to zero.
> > > >
> > > > Ack.
> > > >
> > > > > > @@ -4802,6 +4816,8 @@ static void __init kfree_rcu_batch_init(void)
> > > > > >       for_each_possible_cpu(cpu) {
> > > > > >               struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
> > > > > >
> > > > > > +             krcp->rdp = per_cpu_ptr(&rcu_data, cpu);
> > > > > > +             krcp->last_gp_seq = 0;
> > > > > >
> > > > > Yep. This one can be just dropped.
> > > > >
> > > > > But all the rest looks good :) I will give it a try from test point of
> > > > > view. It is interested from the memory footprint point of view.
> > > >
> > > > Ack. Thanks. Even though we should not sample rdp->gp_seq, I think it
> > > > is still worth a test.
> > >
> > > Just for completeness, the main purpose of rdp->gp_seq is to reject
> > > quiescent states that were seen during already-completed grace periods.
> > >
> > So it means that instead of gp_seq reading we should take a snaphshot
> > of the current state:
> >
> > snp = get_state_synchronize_rcu();
> >
> > and later on do a:
> >
> > cond_synchronize_rcu(snp);
> >
> > to wait for a GP.
> 
> This can't be called from the timer IRQ handler though (monitor)
> 
> > Or if the poll_state_synchronize_rcu(oldstate)) != 0
> > queue_rcu_work().
> 
> But something like this should be possible (maybe)
> 
> > Sorry for a description using the RCU API functions name :)
> 
> I believe you will have to call rcu_poll_gp_seq_start() as well if you
> are using polled API. I am planning to look at this properly more,
> soon. Right now I am going to write up the rcutop doc and share with
> you guys.
> 
> (Maybe RCU polling is the right thing to do as we reuse all the infra
> and any corner case it is handling)
> 
OK. This is in my todo list also. Since we have discussed it let's move
it forward.

Below what i have came up with to switch for polling APIs:

From 799ce1653d159ef3d35f34a284f738c2c267c75f Mon Sep 17 00:00:00 2001
From: "Uladzislau Rezki (Sony)" <urezki@gmail.com>
Date: Wed, 2 Nov 2022 19:26:27 +0100
Subject: [PATCH 1/1] rcu: kvfree_rcu: Reduce a memory footptint by using
 polling APIs

Total time taken by all kfree'ers: 6564718459 ns, loops: 10000, batches: 1110, memory footprint: 5057MB
Total time taken by all kfree'ers: 8431051895 ns, loops: 10000, batches: 1109, memory footprint: 2749MB
Total time taken by all kfree'ers: 9477830789 ns, loops: 10000, batches: 1158, memory footprint: 2934MB
Total time taken by all kfree'ers: 9950211144 ns, loops: 10000, batches: 981, memory footprint: 2704MB

with a patch:

Total time taken by all kfree'ers: 7712110118 ns, loops: 10000, batches: 1660, memory footprint: 91MB
Total time taken by all kfree'ers: 7002403664 ns, loops: 10000, batches: 1482, memory footprint: 86MB
Total time taken by all kfree'ers: 7842282319 ns, loops: 10000, batches: 1738, memory footprint: 86MB
Total time taken by all kfree'ers: 7230161977 ns, loops: 10000, batches: 1542, memory footprint: 72MB

Tested with NOCB option, all offloading CPUs:

kvm.sh --memory 10G --torture rcuscale --allcpus --duration 1 \
  --kconfig CONFIG_NR_CPUS=64 \
  --kconfig CONFIG_RCU_NOCB_CPU=y \
  --kconfig CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y \
  --bootargs "rcuscale.kfree_rcu_test=1 rcuscale.kfree_nthreads=16 \
  rcuscale.holdoff=20 rcuscale.kfree_loops=10000 torture.disable_onoff_at_boot" --trust-make

According to data there is a big gain in memory footprint with a patch.
It is because of call_rcu() and call_rcu_flush() take more effort and
time to queue a callback and then wait for a gp.

With polling API:
  a) we do not need to queue any callback;
  b) we might not even need wait for a GP completion.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
---
 kernel/rcu/tree.c | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 76973d716921..17c3d6f2c55b 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2919,18 +2919,20 @@ struct kvfree_rcu_bulk_data {
 	((PAGE_SIZE - sizeof(struct kvfree_rcu_bulk_data)) / sizeof(void *))
 
 /**
+ * @rcu_work: A work to reclaim a memory after a grace period
  * struct kfree_rcu_cpu_work - single batch of kfree_rcu() requests
- * @rcu_work: Let queue_rcu_work() invoke workqueue handler after grace period
  * @head_free: List of kfree_rcu() objects waiting for a grace period
  * @bkvhead_free: Bulk-List of kvfree_rcu() objects waiting for a grace period
  * @krcp: Pointer to @kfree_rcu_cpu structure
+ * @gp_snap: A snapshot of current grace period
  */
 
 struct kfree_rcu_cpu_work {
-	struct rcu_work rcu_work;
+	struct work_struct rcu_work;
 	struct rcu_head *head_free;
 	struct kvfree_rcu_bulk_data *bkvhead_free[FREE_N_CHANNELS];
 	struct kfree_rcu_cpu *krcp;
+	unsigned long gp_snap;
 };
 
 /**
@@ -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);
+
 	raw_spin_lock_irqsave(&krcp->lock, flags);
 	// Channels 1 and 2.
 	for (i = 0; i < FREE_N_CHANNELS; i++) {
@@ -3194,6 +3198,13 @@ static void kfree_rcu_monitor(struct work_struct *work)
 		if ((krcp->bkvhead[0] && !krwp->bkvhead_free[0]) ||
 			(krcp->bkvhead[1] && !krwp->bkvhead_free[1]) ||
 				(krcp->head && !krwp->head_free)) {
+			/*
+			 * Take a snapshot for this krwp. Please note no
+			 * more any objects can be added to this krwp free
+			 * channels.
+			 */
+			krwp->gp_snap = get_state_synchronize_rcu();
+
 			// Channel 1 corresponds to the SLAB-pointer bulk path.
 			// Channel 2 corresponds to vmalloc-pointer bulk path.
 			for (j = 0; j < FREE_N_CHANNELS; j++) {
@@ -3217,7 +3228,7 @@ static void kfree_rcu_monitor(struct work_struct *work)
 			// be that the work is in the pending state when
 			// channels have been detached following by each
 			// other.
-			queue_rcu_work(system_wq, &krwp->rcu_work);
+			queue_work(system_wq, &krwp->rcu_work);
 		}
 	}
 
@@ -4808,7 +4819,7 @@ static void __init kfree_rcu_batch_init(void)
 		struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
 
 		for (i = 0; i < KFREE_N_BATCHES; i++) {
-			INIT_RCU_WORK(&krcp->krw_arr[i].rcu_work, kfree_rcu_work);
+			INIT_WORK(&krcp->krw_arr[i].rcu_work, kfree_rcu_work);
 			krcp->krw_arr[i].krcp = krcp;
 		}
 
-- 
2.30.2

--
Uladzislau Rezki