rcu: Add tracing data to support queueing models

The current tracing data is not sufficient to deduce the average time
that a callback spends waiting for a grace period to end.  Add three
per-CPU counters recording the number of callbacks invoked (ci), the
number of callbacks orphaned (co), and the number of callbacks adopted
(ca).  Given the existing callback queue length (ql), the average wait
time in absence of CPU hotplug operations is ql/ci.  The units of wait
time will be in terms of the duration over which ci was measured.

In the presence of CPU hotplug operations, there is room for argument,
but ql/(ci-co+ca) won't steer you too far wrong.

Also fixes a typo called out by Lucas De Marchi <lucas.de.marchi@gmail.com>.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/Documentation/RCU/trace.txt b/Documentation/RCU/trace.txt
index efd8cc95c06b1470db165a74a0d99fd33ee17dea..a851118775d84c7a1d2356ba6a6c8e6208292887 100644 (file)
--- a/Documentation/RCU/trace.txt
+++ b/Documentation/RCU/trace.txt
@@ -125,6 +125,17 @@ o  "b" is the batch limit for this CPU.  If more than this number
        of RCU callbacks is ready to invoke, then the remainder will
        be deferred.
 
+o      "ci" is the number of RCU callbacks that have been invoked for
+       this CPU.  Note that ci+ql is the number of callbacks that have
+       been registered in absence of CPU-hotplug activity.
+
+o      "co" is the number of RCU callbacks that have been orphaned due to
+       this CPU going offline.
+
+o      "ca" is the number of RCU callbacks that have been adopted due to
+       other CPUs going offline.  Note that ci+co-ca+ql is the number of
+       RCU callbacks registered on this CPU.
+
 There is also an rcu/rcudata.csv file with the same information in
 comma-separated-variable spreadsheet format.
 
@@ -180,7 +191,7 @@ o   "s" is the "signaled" state that drives force_quiescent_state()'s
 
 o      "jfq" is the number of jiffies remaining for this grace period
        before force_quiescent_state() is invoked to help push things
-       along.  Note that CPUs in dyntick-idle mode thoughout the grace
+       along.  Note that CPUs in dyntick-idle mode throughout the grace
        period will not report on their own, but rather must be check by
        some other CPU via force_quiescent_state().
 

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 42140a860bb997614af85de2f8515e2cb330b8f1..e75073504a31b7209b24e4e4f0b883af98bf942a 100644 (file)
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1004,6 +1004,7 @@ static void rcu_send_cbs_to_orphanage(struct rcu_state *rsp)
        for (i = 0; i < RCU_NEXT_SIZE; i++)
                rdp->nxttail[i] = &rdp->nxtlist;
        rsp->orphan_qlen += rdp->qlen;
+       rdp->n_cbs_orphaned += rdp->qlen;
        rdp->qlen = 0;
        raw_spin_unlock(&rsp->onofflock);  /* irqs remain disabled. */
 }
@@ -1025,6 +1026,7 @@ static void rcu_adopt_orphan_cbs(struct rcu_state *rsp)
        *rdp->nxttail[RCU_NEXT_TAIL] = rsp->orphan_cbs_list;
        rdp->nxttail[RCU_NEXT_TAIL] = rsp->orphan_cbs_tail;
        rdp->qlen += rsp->orphan_qlen;
+       rdp->n_cbs_adopted += rsp->orphan_qlen;
        rsp->orphan_cbs_list = NULL;
        rsp->orphan_cbs_tail = &rsp->orphan_cbs_list;
        rsp->orphan_qlen = 0;
@@ -1156,6 +1158,7 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
 
        /* Update count, and requeue any remaining callbacks. */
        rdp->qlen -= count;
+       rdp->n_cbs_invoked += count;
        if (list != NULL) {
                *tail = rdp->nxtlist;
                rdp->nxtlist = list;

diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 7918ba61873f48e5641583852ac7909227589253..91d4170c5c13afd2e8997bd59b28e7cc2a4385e8 100644 (file)
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -202,6 +202,9 @@ struct rcu_data {
        long            qlen;           /* # of queued callbacks */
        long            qlen_last_fqs_check;
                                        /* qlen at last check for QS forcing */
+       unsigned long   n_cbs_invoked;  /* count of RCU cbs invoked. */
+       unsigned long   n_cbs_orphaned; /* RCU cbs sent to orphanage. */
+       unsigned long   n_cbs_adopted;  /* RCU cbs adopted from orphanage. */
        unsigned long   n_force_qs_snap;
                                        /* did other CPU force QS recently? */
        long            blimit;         /* Upper limit on a processed batch */

diff --git a/kernel/rcutree_trace.c b/kernel/rcutree_trace.c
index 458e032a3a30e67237c73a98c6d0d1bc1aa60e7a..d15430b9d122f4d619e76fb6b5069aa1f494a575 100644 (file)
--- a/kernel/rcutree_trace.c
+++ b/kernel/rcutree_trace.c
@@ -64,7 +64,9 @@ static void print_one_rcu_data(struct seq_file *m, struct rcu_data *rdp)
                   rdp->dynticks_fqs);
 #endif /* #ifdef CONFIG_NO_HZ */
        seq_printf(m, " of=%lu ri=%lu", rdp->offline_fqs, rdp->resched_ipi);
-       seq_printf(m, " ql=%ld b=%ld\n", rdp->qlen, rdp->blimit);
+       seq_printf(m, " ql=%ld b=%ld", rdp->qlen, rdp->blimit);
+       seq_printf(m, " ci=%lu co=%lu ca=%lu\n",
+                  rdp->n_cbs_invoked, rdp->n_cbs_orphaned, rdp->n_cbs_adopted);
 }
 
 #define PRINT_RCU_DATA(name, func, m) \
@@ -119,7 +121,9 @@ static void print_one_rcu_data_csv(struct seq_file *m, struct rcu_data *rdp)
                   rdp->dynticks_fqs);
 #endif /* #ifdef CONFIG_NO_HZ */
        seq_printf(m, ",%lu,%lu", rdp->offline_fqs, rdp->resched_ipi);
-       seq_printf(m, ",%ld,%ld\n", rdp->qlen, rdp->blimit);
+       seq_printf(m, ",%ld,%ld", rdp->qlen, rdp->blimit);
+       seq_printf(m, ",%lu,%lu,%lu\n",
+                  rdp->n_cbs_invoked, rdp->n_cbs_orphaned, rdp->n_cbs_adopted);
 }
 
 static int show_rcudata_csv(struct seq_file *m, void *unused)
@@ -128,7 +132,7 @@ static int show_rcudata_csv(struct seq_file *m, void *unused)
 #ifdef CONFIG_NO_HZ
        seq_puts(m, "\"dt\",\"dt nesting\",\"dn\",\"df\",");
 #endif /* #ifdef CONFIG_NO_HZ */
-       seq_puts(m, "\"of\",\"ri\",\"ql\",\"b\"\n");
+       seq_puts(m, "\"of\",\"ri\",\"ql\",\"b\",\"ci\",\"co\",\"ca\"\n");
 #ifdef CONFIG_TREE_PREEMPT_RCU
        seq_puts(m, "\"rcu_preempt:\"\n");
        PRINT_RCU_DATA(rcu_preempt_data, print_one_rcu_data_csv, m);