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rcu: Make rcu_gp_cleanup() more accurately predict need for new GP

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Currently, rcu_gp_cleanup() scans the rcu_node tree in order to reset
state to reflect the end of the grace period.  It also checks to see
whether a new grace period is needed, but in a number of cases, rather
than directly cause the new grace period to be immediately started, it
instead leaves the grace-period-needed state where various fail-safes
can find it.  This works fine, but results in higher contention on the
root rcu_node structure's ->lock, which is undesirable, and contention
on that lock has recently become noticeable.

This commit therefore makes rcu_gp_cleanup() immediately start a new
grace period if there is any need for one.

It is quite possible that it will later be necessary to throttle the
grace-period rate, but that can be dealt with when and if.

Reported-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
This commit is contained in:
Paul E. McKenney 2018-04-12 07:20:30 -07:00
parent 5fe0a56298
commit fb31340f8a
3 changed files with 10 additions and 24 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

16
kernel/rcu/tree.c
View file

@ -1763,14 +1763,14 @@ rcu_start_future_gp(struct rcu_node *rnp, struct rcu_data *rdp,
* Clean up any old requests for the just-ended grace period. Also return
* whether any additional grace periods have been requested.
*/
static int rcu_future_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp)
static bool rcu_future_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp)
{
int c = rnp->completed;
int needmore;
bool needmore;
struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
need_future_gp_element(rnp, c) = 0;
needmore = need_future_gp_element(rnp, c + 1);
needmore = need_any_future_gp(rnp);
trace_rcu_future_gp(rnp, rdp, c,
needmore ? TPS("CleanupMore") : TPS("Cleanup"));
return needmore;
@ -2113,7 +2113,6 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
{
unsigned long gp_duration;
bool needgp = false;
int nocb = 0;
struct rcu_data *rdp;
struct rcu_node *rnp = rcu_get_root(rsp);
struct swait_queue_head *sq;
@ -2152,7 +2151,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
if (rnp == rdp->mynode)
needgp = __note_gp_changes(rsp, rnp, rdp) || needgp;
/* smp_mb() provided by prior unlock-lock pair. */
nocb += rcu_future_gp_cleanup(rsp, rnp);
needgp = rcu_future_gp_cleanup(rsp, rnp) || needgp;
sq = rcu_nocb_gp_get(rnp);
raw_spin_unlock_irq_rcu_node(rnp);
rcu_nocb_gp_cleanup(sq);
@ -2162,13 +2161,18 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
}
rnp = rcu_get_root(rsp);
raw_spin_lock_irq_rcu_node(rnp); /* Order GP before ->completed update. */
rcu_nocb_gp_set(rnp, nocb);
/* Declare grace period done. */
WRITE_ONCE(rsp->completed, rsp->gpnum);
trace_rcu_grace_period(rsp->name, rsp->completed, TPS("end"));
rsp->gp_state = RCU_GP_IDLE;
/* Check for GP requests since above loop. */
rdp = this_cpu_ptr(rsp->rda);
if (need_any_future_gp(rnp)) {
trace_rcu_future_gp(rnp, rdp, rsp->completed - 1,
TPS("CleanupMore"));
needgp = true;
}
/* Advance CBs to reduce false positives below. */
needgp = rcu_advance_cbs(rsp, rnp, rdp) || needgp;
if (needgp || cpu_needs_another_gp(rsp, rdp)) {

1
kernel/rcu/tree.h
View file

@ -469,7 +469,6 @@ static void print_cpu_stall_info_end(void);
static void zero_cpu_stall_ticks(struct rcu_data *rdp);
static void increment_cpu_stall_ticks(void);
static bool rcu_nocb_cpu_needs_barrier(struct rcu_state *rsp, int cpu);
static void rcu_nocb_gp_set(struct rcu_node *rnp, int nrq);
static struct swait_queue_head *rcu_nocb_gp_get(struct rcu_node *rnp);
static void rcu_nocb_gp_cleanup(struct swait_queue_head *sq);
static void rcu_init_one_nocb(struct rcu_node *rnp);

17
kernel/rcu/tree_plugin.h
View file

@ -1780,19 +1780,6 @@ static void rcu_nocb_gp_cleanup(struct swait_queue_head *sq)
swake_up_all(sq);
}
/*
* Set the root rcu_node structure's ->need_future_gp field
* based on the sum of those of all rcu_node structures. This does
* double-count the root rcu_node structure's requests, but this
* is necessary to handle the possibility of a rcu_nocb_kthread()
* having awakened during the time that the rcu_node structures
* were being updated for the end of the previous grace period.
*/
static void rcu_nocb_gp_set(struct rcu_node *rnp, int nrq)
{
need_future_gp_element(rnp, rnp->completed + 1) += nrq;
}
static struct swait_queue_head *rcu_nocb_gp_get(struct rcu_node *rnp)
{
return &rnp->nocb_gp_wq[rnp->completed & 0x1];
@ -2495,10 +2482,6 @@ static void rcu_nocb_gp_cleanup(struct swait_queue_head *sq)
{
}
static void rcu_nocb_gp_set(struct rcu_node *rnp, int nrq)
{
}
static struct swait_queue_head *rcu_nocb_gp_get(struct rcu_node *rnp)
{
return NULL;