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rcu: document TINY_RCU and TINY_PREEMPT_RCU tracing.

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Add the required verbiage to Documentation/RCU/trace.txt.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Paul E. McKenney 2010-11-04 14:31:19 -07:00 committed by Paul E. McKenney
parent 9e571a82f0
commit 8e79e1f961
1 changed files with 124 additions and 8 deletions
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132
Documentation/RCU/trace.txt
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@ -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.