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Message-Id: <1289095532-5398-5-git-send-email-paulmck@linux.vnet.ibm.com>
Date: Sat, 6 Nov 2010 19:05:25 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: linux-kernel@...r.kernel.org
Cc: mingo@...e.hu, laijs@...fujitsu.com, dipankar@...ibm.com,
akpm@...ux-foundation.org, mathieu.desnoyers@...ymtl.ca,
josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH RFC tip/core/rcu 05/12] rcu: document TINY_RCU and TINY_PREEMPT_RCU tracing.
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
Documentation/RCU/trace.txt | 132 ++++++++++++++++++++++++++++++++++++++++---
1 files changed, 124 insertions(+), 8 deletions(-)
diff --git a/Documentation/RCU/trace.txt b/Documentation/RCU/trace.txt
index a851118..ff6d3f1 100644
--- a/Documentation/RCU/trace.txt
+++ b/Documentation/RCU/trace.txt
@@ -1,18 +1,22 @@
CONFIG_RCU_TRACE debugfs Files and Formats
-The rcutree implementation of RCU provides debugfs trace output that
-summarizes counters and state. This information is useful for debugging
-RCU itself, and can sometimes also help to debug abuses of RCU.
-The following sections describe the debugfs files and formats.
+The rcutree and rcutiny implementations of RCU provide debugfs trace
+output that summarizes counters and state. This information is useful for
+debugging RCU itself, and can sometimes also help to debug abuses of RCU.
+The following sections describe the debugfs files and formats, first
+for rcutree and next for rcutiny.
-Hierarchical RCU debugfs Files and Formats
+CONFIG_TREE_RCU and CONFIG_TREE_PREEMPT_RCU debugfs Files and Formats
-This implementation of RCU provides three debugfs files under the
+These implementations of RCU provides five debugfs files under the
top-level directory RCU: rcu/rcudata (which displays fields in struct
-rcu_data), rcu/rcugp (which displays grace-period counters), and
-rcu/rcuhier (which displays the struct rcu_node hierarchy).
+rcu_data), rcu/rcudata.csv (which is a .csv spreadsheet version of
+rcu/rcudata), rcu/rcugp (which displays grace-period counters),
+rcu/rcuhier (which displays the struct rcu_node hierarchy), and
+rcu/rcu_pending (which displays counts of the reasons that the
+rcu_pending() function decided that there was core RCU work to do).
The output of "cat rcu/rcudata" looks as follows:
@@ -326,3 +330,115 @@ o "nn" is the number of times that this CPU needed nothing. Alert
readers will note that the rcu "nn" number for a given CPU very
closely matches the rcu_bh "np" number for that same CPU. This
is due to short-circuit evaluation in rcu_pending().
+
+
+CONFIG_TINY_RCU and CONFIG_TINY_PREEMPT_RCU debugfs Files and Formats
+
+These implementations of RCU provides a single debugfs file under the
+top-level directory RCU, namely rcu/rcudata, which displays fields in
+rcu_bh_ctrlblk, rcu_sched_ctrlblk and, for CONFIG_TINY_PREEMPT_RCU,
+rcu_preempt_ctrlblk.
+
+The output of "cat rcu/rcudata" is as follows:
+
+rcu_preempt: qlen=24 gp=1097669 g197/p197/c197 tasks=...
+ ttb=. btg=no ntb=184 neb=0 nnb=183 j=01f7 bt=0274
+ normal balk: nt=1097669 gt=0 bt=371 b=0 ny=25073378 nos=0
+ exp balk: bt=0 nos=0
+rcu_sched: qlen: 0
+rcu_bh: qlen: 0
+
+This is split into rcu_preempt, rcu_sched, and rcu_bh sections, with the
+rcu_preempt section appearing only in CONFIG_TINY_PREEMPT_RCU builds.
+The last three lines of the rcu_preempt section appear only in
+CONFIG_RCU_BOOST kernel builds. The fields are as follows:
+
+o "qlen" is the number of RCU callbacks currently waiting either
+ for an RCU grace period or waiting to be invoked. This is the
+ only field present for rcu_sched and rcu_bh, due to the
+ short-circuiting of grace period in those two cases.
+
+o "gp" is the number of grace periods that have completed.
+
+o "g197/p197/c197" displays the grace-period state, with the
+ "g" number being the number of grace periods that have started
+ (mod 256), the "p" number being the number of grace periods
+ that the CPU has responded to (also mod 256), and the "c"
+ number being the number of grace periods that have completed
+ (once again mode 256).
+
+ Why have both "gp" and "g"? Because the data flowing into
+ "gp" is only present in a CONFIG_RCU_TRACE kernel.
+
+o "tasks" is a set of bits. The first bit is "T" if there are
+ currently tasks that have recently blocked within an RCU
+ read-side critical section, the second bit is "N" if any of the
+ aforementioned tasks are blocking the current RCU grace period,
+ and the third bit is "E" if any of the aforementioned tasks are
+ blocking the current expedited grace period. Each bit is "."
+ if the corresponding condition does not hold.
+
+o "ttb" is a single bit. It is "B" if any of the blocked tasks
+ need to be priority boosted and "." otherwise.
+
+o "btg" indicates whether boosting has been carried out during
+ the current grace period, with "exp" indicating that boosting
+ is in progress for an expedited grace period, "no" indicating
+ that boosting has not yet started for a normal grace period,
+ "begun" indicating that boosting has bebug for a normal grace
+ period, and "done" indicating that boosting has completed for
+ a normal grace period.
+
+o "ntb" is the total number of tasks subjected to RCU priority boosting
+ periods since boot.
+
+o "neb" is the number of expedited grace periods that have had
+ to resort to RCU priority boosting since boot.
+
+o "nnb" is the number of normal grace periods that have had
+ to resort to RCU priority boosting since boot.
+
+o "j" is the low-order 12 bits of the jiffies counter in hexadecimal.
+
+o "bt" is the low-order 12 bits of the value that the jiffies counter
+ will have at the next time that boosting is scheduled to begin.
+
+o In the line beginning with "normal balk", the fields are as follows:
+
+ o "nt" is the number of times that the system balked from
+ boosting because there were no blocked tasks to boost.
+ Note that the system will balk from boosting even if the
+ grace period is overdue when the currently running task
+ is looping within an RCU read-side critical section.
+ There is no point in boosting in this case, because
+ boosting a running task won't make it run any faster.
+
+ o "gt" is the number of times that the system balked
+ from boosting because, although there were blocked tasks,
+ none of them were preventing the current grace period
+ from completing.
+
+ o "bt" is the number of times that the system balked
+ from boosting because boosting was already in progress.
+
+ o "b" is the number of times that the system balked from
+ boosting because boosting had already completed for
+ the grace period in question.
+
+ o "ny" is the number of times that the system balked from
+ boosting because it was not yet time to start boosting
+ the grace period in question.
+
+ o "nos" is the number of times that the system balked from
+ boosting for inexplicable ("not otherwise specified")
+ reasons. This can actually happen due to races involving
+ increments of the jiffies counter.
+
+o In the line beginning with "exp balk", the fields are as follows:
+
+ o "bt" is the number of times that the system balked from
+ boosting because there were no blocked tasks to boost.
+
+ o "nos" is the number of times that the system balked from
+ boosting for inexplicable ("not otherwise specified")
+ reasons.
--
1.7.0.6
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