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Subject[PATCH v2] rcu/kfree: Do not request RCU when not needed
Date
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@joelfernandes.org>
---
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);
}

@@ -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);
}
}

--
2.38.1.431.g37b22c650d-goog

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