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Commit df5bd514 authored by Paul E. McKenney's avatar Paul E. McKenney
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rcu: Reduce expedited GP memory contention via per-CPU variables 



Currently, the piggybacked-work checks carried out by sync_exp_work_done()
atomically increment a small set of variables (the ->expedited_workdone0,
->expedited_workdone1, ->expedited_workdone2, ->expedited_workdone3
fields in the rcu_state structure), which will form a memory-contention
bottleneck given a sufficiently large number of CPUs concurrently invoking
either synchronize_rcu_expedited() or synchronize_sched_expedited().

This commit therefore moves these for fields to the per-CPU rcu_data
structure, eliminating the memory contention.  The show_rcuexp() function
also changes to sum up each field in the rcu_data structures.

Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
parent 1307f214

kernel/rcu/tree.c

+5 −6
Original line number Diff line number Diff line
@@ -3585,7 +3585,7 @@ static bool sync_exp_work_done(struct rcu_state *rsp, struct rcu_node *rnp, 
 */

static struct rcu_node *exp_funnel_lock(struct rcu_state *rsp, unsigned long s)

{

	struct rcu_data *rdp;

	struct rcu_data *rdp = per_cpu_ptr(rsp->rda, raw_smp_processor_id());

	struct rcu_node *rnp0;

	struct rcu_node *rnp1 = NULL;
@@ -3599,7 +3599,7 @@ static struct rcu_node *exp_funnel_lock(struct rcu_state *rsp, unsigned long s) 
	if (!mutex_is_locked(&rnp0->exp_funnel_mutex)) {

		if (mutex_trylock(&rnp0->exp_funnel_mutex)) {

			if (sync_exp_work_done(rsp, rnp0, NULL,

					       &rsp->expedited_workdone0, s))

					       &rdp->expedited_workdone0, s))

				return NULL;

			return rnp0;

		}
@@ -3613,14 +3613,13 @@ static struct rcu_node *exp_funnel_lock(struct rcu_state *rsp, unsigned long s) 
	 * can be inexact, as it is just promoting locality and is not

	 * strictly needed for correctness.

	 */

	rdp = per_cpu_ptr(rsp->rda, raw_smp_processor_id());

	if (sync_exp_work_done(rsp, NULL, NULL, &rsp->expedited_workdone1, s))

	if (sync_exp_work_done(rsp, NULL, NULL, &rdp->expedited_workdone1, s))

		return NULL;

	mutex_lock(&rdp->exp_funnel_mutex);

	rnp0 = rdp->mynode;

	for (; rnp0 != NULL; rnp0 = rnp0->parent) {

		if (sync_exp_work_done(rsp, rnp1, rdp,

				       &rsp->expedited_workdone2, s))

				       &rdp->expedited_workdone2, s))

			return NULL;

		mutex_lock(&rnp0->exp_funnel_mutex);

		if (rnp1)
@@ -3630,7 +3629,7 @@ static struct rcu_node *exp_funnel_lock(struct rcu_state *rsp, unsigned long s) 
		rnp1 = rnp0;

	}

	if (sync_exp_work_done(rsp, rnp1, rdp,

			       &rsp->expedited_workdone3, s))

			       &rdp->expedited_workdone3, s))

		return NULL;

	return rnp1;

}

kernel/rcu/tree.h

+4 −4
Original line number Diff line number Diff line
@@ -386,6 +386,10 @@ struct rcu_data { 
	struct rcu_head oom_head;

#endif /* #ifdef CONFIG_RCU_FAST_NO_HZ */

	struct mutex exp_funnel_mutex;

	atomic_long_t expedited_workdone0;	/* # done by others #0. */

	atomic_long_t expedited_workdone1;	/* # done by others #1. */

	atomic_long_t expedited_workdone2;	/* # done by others #2. */

	atomic_long_t expedited_workdone3;	/* # done by others #3. */



	/* 7) Callback offloading. */

#ifdef CONFIG_RCU_NOCB_CPU
@@ -500,10 +504,6 @@ struct rcu_state { 
	/* End of fields guarded by barrier_mutex. */



	unsigned long expedited_sequence;	/* Take a ticket. */

	atomic_long_t expedited_workdone0;	/* # done by others #0. */

	atomic_long_t expedited_workdone1;	/* # done by others #1. */

	atomic_long_t expedited_workdone2;	/* # done by others #2. */

	atomic_long_t expedited_workdone3;	/* # done by others #3. */

	atomic_long_t expedited_normal;		/* # fallbacks to normal. */

	atomic_t expedited_need_qs;		/* # CPUs left to check in. */

	wait_queue_head_t expedited_wq;		/* Wait for check-ins. */

kernel/rcu/tree_trace.c

+12 −6
Original line number Diff line number Diff line
@@ -183,14 +183,20 @@ static const struct file_operations rcudata_fops = { 


static int show_rcuexp(struct seq_file *m, void *v)

{

	int cpu;

	struct rcu_state *rsp = (struct rcu_state *)m->private;

	struct rcu_data *rdp;

	unsigned long s0 = 0, s1 = 0, s2 = 0, s3 = 0;



	for_each_possible_cpu(cpu) {

		rdp = per_cpu_ptr(rsp->rda, cpu);

		s0 += atomic_long_read(&rdp->expedited_workdone0);

		s1 += atomic_long_read(&rdp->expedited_workdone1);

		s2 += atomic_long_read(&rdp->expedited_workdone2);

		s3 += atomic_long_read(&rdp->expedited_workdone3);

	}

	seq_printf(m, "s=%lu wd0=%lu wd1=%lu wd2=%lu wd3=%lu n=%lu enq=%d sc=%lu\n",

		   rsp->expedited_sequence,

		   atomic_long_read(&rsp->expedited_workdone0),

		   atomic_long_read(&rsp->expedited_workdone1),

		   atomic_long_read(&rsp->expedited_workdone2),

		   atomic_long_read(&rsp->expedited_workdone3),

		   rsp->expedited_sequence, s0, s1, s2, s3,

		   atomic_long_read(&rsp->expedited_normal),

		   atomic_read(&rsp->expedited_need_qs),

		   rsp->expedited_sequence / 2);