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Date: Wed, 2 Nov 2022 19:31:40 +0100
From: Uladzislau Rezki <urezki@...il.com>
To: Joel Fernandes <joel@...lfernandes.org>,
"Paul E. McKenney" <paulmck@...nel.org>
Cc: Uladzislau Rezki <urezki@...il.com>,
"Paul E. McKenney" <paulmck@...nel.org>, rcu@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] rcu/kfree: Do not request RCU when not needed
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@...il.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@...il.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@...lfernandes.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@...il.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@...il.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
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