> Hi,
>
> On Thu, Nov 10, 2022 at 8:05 AM Uladzislau Rezki <urezki@xxxxxxxxx> wrote:
>
> > > On ChromeOS, using this with the increased timeout, we see that we
> > almost always
> > > never need to initiate a new grace period. Testing also shows this frees
> > large
> > > amounts of unreclaimed memory, under intense kfree_rcu() pressure.
> > >
> > > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> > > ---
> > > v1->v2: Same logic but use polled grace periods instead of sampling
> > gp_seq.
> > >
> > > kernel/rcu/tree.c | 8 +++++++-
> > > 1 file changed, 7 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > index 591187b6352e..ed41243f7a49 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
> > > + * @gp_snap: The GP snapshot recorded at the last scheduling of monitor
> > work.
> > > * @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 +2965,7 @@ struct kfree_rcu_cpu {
> > > struct kfree_rcu_cpu_work krw_arr[KFREE_N_BATCHES];
> > > raw_spinlock_t lock;
> > > struct delayed_work monitor_work;
> > > + unsigned long gp_snap;
> > > bool initialized;
> > > int count;
> > >
> > > @@ -3167,6 +3169,7 @@ schedule_delayed_monitor_work(struct kfree_rcu_cpu
> > *krcp)
> > > mod_delayed_work(system_wq, &krcp->monitor_work,
> > delay);
> > > return;
> > > }
> > > + krcp->gp_snap = get_state_synchronize_rcu();
> > > queue_delayed_work(system_wq, &krcp->monitor_work, delay);
> > > }
> > >
> > How do you guarantee a full grace period for objects which proceed
> > to be placed into an input stream that is not yet detached?
>
>
> Just replying from phone as I’m OOO today.
>
> Hmm, so you’re saying that objects can be queued after the delayed work has
> been queued, but processed when the delayed work is run? I’m looking at
> this code after few years so I may have missed something.
>
> That’s a good point and I think I missed that. I think your version did too
> but I’ll have to double check.
>
> The fix then is to sample the clock for the latest object queued, not for
> when the delayed work is queued.
>
The patch i sent gurantee it. Just in case see v2:
>From 7ff4038d5dac8d2044b240adeee630ad7944ab8d Mon Sep 17 00:00:00 2001
From: "Uladzislau Rezki (Sony)" <urezki@xxxxxxxxx>
Date: Wed, 2 Nov 2022 19:26:27 +0100
Subject: [PATCH] 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@xxxxxxxxx>
---
kernel/rcu/tree.c | 32 ++++++++++++++++++++++++++------
1 file changed, 26 insertions(+), 6 deletions(-)
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 76973d716921..2be4f80867f2 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;
};
/**
@@ -3064,10 +3066,11 @@ static void kfree_rcu_work(struct work_struct *work)
struct rcu_head *head, *next;
struct kfree_rcu_cpu *krcp;
struct kfree_rcu_cpu_work *krwp;
+ unsigned long this_krwp_gp_snap;
int i, j;
- krwp = container_of(to_rcu_work(work),
- struct kfree_rcu_cpu_work, rcu_work);
+ krwp = container_of(work,
+ struct kfree_rcu_cpu_work, rcu_work);
krcp = krwp->krcp;
raw_spin_lock_irqsave(&krcp->lock, flags);
@@ -3080,8 +3083,15 @@ static void kfree_rcu_work(struct work_struct *work)
// Channel 3.
head = krwp->head_free;
krwp->head_free = NULL;
+
+ // Get the latest saved state of grace period
+ // associated with this "krwp" and objects there.
+ this_krwp_gp_snap = krwp->gp_snap;
raw_spin_unlock_irqrestore(&krcp->lock, flags);
+ // Check the state.
+ cond_synchronize_rcu(this_krwp_gp_snap);
+
// Handle the first two channels.
for (i = 0; i < FREE_N_CHANNELS; i++) {
for (; bkvhead[i]; bkvhead[i] = bnext) {
@@ -3212,12 +3222,22 @@ static void kfree_rcu_monitor(struct work_struct *work)
WRITE_ONCE(krcp->count, 0);
+ // 1. Take a snapshot for this krwp. Please note no
+ // more any objects can be added to channels which
+ // have already been attached.
+ //
+ // 2. Since a "krwp" has several free channels a GP
+ // status of latest attached one is recorded, i.e.
+ // it allows us to maintain a GP status for last in
+ // objects among all channels.
+ krwp->gp_snap = get_state_synchronize_rcu();
+
// One work is per one batch, so there are three
// "free channels", the batch can handle. It can
// 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 +4828,7 @@ 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;
}
> > > @@ -3217,7 +3220,10 @@ 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);
> > > + if (poll_state_synchronize_rcu(krcp->gp_snap))
> > > + queue_work(system_wq, &krwp->rcu_work.work
> > );
> > > + else
> > > + queue_rcu_work(system_wq, &krwp->rcu_work);
> > > }
> > >
> > Why do you want to queue a work over RCU-core?
> >
> > 1.
> > call_rcu()
> > -> queue_work();
> > -> do reclaim
> >
> > if it can be improved and simplified as:
> >
> > 2.
> > queue_work();
> > -> cond_synchronize_rcu(), do reclaim
>
>
> The second one blocks, the first doesn’t. So I’m not sure how that’s an
> improvement? I think we don’t want to block in kworker due to possible
> scheduling issues and hurting other kwork during critical operations, if
> possible.
>
Does the first wait for a full grace period so our kworker is not even run?
--
Uladzislau Rezki
