rcu: document TINY_RCU and TINY_PREEMPT_RCU tracing.
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>
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Paul E. McKenney
Paul E. McKenney
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@ -1,18 +1,22 @@
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CONFIG_RCU_TRACE debugfs Files and Formats
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The rcutree implementation of RCU provides debugfs trace output that
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summarizes counters and state. This information is useful for debugging
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RCU itself, and can sometimes also help to debug abuses of RCU.
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The following sections describe the debugfs files and formats.
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The rcutree and rcutiny implementations of RCU provide debugfs trace
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output that summarizes counters and state. This information is useful for
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debugging RCU itself, and can sometimes also help to debug abuses of RCU.
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The following sections describe the debugfs files and formats, first
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for rcutree and next for rcutiny.
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Hierarchical RCU debugfs Files and Formats
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CONFIG_TREE_RCU and CONFIG_TREE_PREEMPT_RCU debugfs Files and Formats
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This implementation of RCU provides three debugfs files under the
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These implementations of RCU provides five debugfs files under the
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top-level directory RCU: rcu/rcudata (which displays fields in struct
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rcu_data), rcu/rcugp (which displays grace-period counters), and
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rcu/rcuhier (which displays the struct rcu_node hierarchy).
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rcu_data), rcu/rcudata.csv (which is a .csv spreadsheet version of
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rcu/rcudata), rcu/rcugp (which displays grace-period counters),
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rcu/rcuhier (which displays the struct rcu_node hierarchy), and
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rcu/rcu_pending (which displays counts of the reasons that the
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rcu_pending() function decided that there was core RCU work to do).
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The output of "cat rcu/rcudata" looks as follows:
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@ -326,3 +330,115 @@ o "nn" is the number of times that this CPU needed nothing. Alert
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readers will note that the rcu "nn" number for a given CPU very
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closely matches the rcu_bh "np" number for that same CPU. This
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is due to short-circuit evaluation in rcu_pending().
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CONFIG_TINY_RCU and CONFIG_TINY_PREEMPT_RCU debugfs Files and Formats
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These implementations of RCU provides a single debugfs file under the
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top-level directory RCU, namely rcu/rcudata, which displays fields in
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rcu_bh_ctrlblk, rcu_sched_ctrlblk and, for CONFIG_TINY_PREEMPT_RCU,
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rcu_preempt_ctrlblk.
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The output of "cat rcu/rcudata" is as follows:
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rcu_preempt: qlen=24 gp=1097669 g197/p197/c197 tasks=...
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ttb=. btg=no ntb=184 neb=0 nnb=183 j=01f7 bt=0274
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normal balk: nt=1097669 gt=0 bt=371 b=0 ny=25073378 nos=0
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exp balk: bt=0 nos=0
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rcu_sched: qlen: 0
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rcu_bh: qlen: 0
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This is split into rcu_preempt, rcu_sched, and rcu_bh sections, with the
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rcu_preempt section appearing only in CONFIG_TINY_PREEMPT_RCU builds.
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The last three lines of the rcu_preempt section appear only in
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CONFIG_RCU_BOOST kernel builds. The fields are as follows:
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o "qlen" is the number of RCU callbacks currently waiting either
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for an RCU grace period or waiting to be invoked. This is the
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only field present for rcu_sched and rcu_bh, due to the
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short-circuiting of grace period in those two cases.
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o "gp" is the number of grace periods that have completed.
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o "g197/p197/c197" displays the grace-period state, with the
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"g" number being the number of grace periods that have started
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(mod 256), the "p" number being the number of grace periods
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that the CPU has responded to (also mod 256), and the "c"
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number being the number of grace periods that have completed
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(once again mode 256).
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Why have both "gp" and "g"? Because the data flowing into
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"gp" is only present in a CONFIG_RCU_TRACE kernel.
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o "tasks" is a set of bits. The first bit is "T" if there are
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currently tasks that have recently blocked within an RCU
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read-side critical section, the second bit is "N" if any of the
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aforementioned tasks are blocking the current RCU grace period,
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and the third bit is "E" if any of the aforementioned tasks are
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blocking the current expedited grace period. Each bit is "."
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if the corresponding condition does not hold.
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o "ttb" is a single bit. It is "B" if any of the blocked tasks
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need to be priority boosted and "." otherwise.
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o "btg" indicates whether boosting has been carried out during
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the current grace period, with "exp" indicating that boosting
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is in progress for an expedited grace period, "no" indicating
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that boosting has not yet started for a normal grace period,
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"begun" indicating that boosting has bebug for a normal grace
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period, and "done" indicating that boosting has completed for
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a normal grace period.
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o "ntb" is the total number of tasks subjected to RCU priority boosting
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periods since boot.
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o "neb" is the number of expedited grace periods that have had
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to resort to RCU priority boosting since boot.
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o "nnb" is the number of normal grace periods that have had
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to resort to RCU priority boosting since boot.
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o "j" is the low-order 12 bits of the jiffies counter in hexadecimal.
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o "bt" is the low-order 12 bits of the value that the jiffies counter
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will have at the next time that boosting is scheduled to begin.
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o In the line beginning with "normal balk", the fields are as follows:
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o "nt" is the number of times that the system balked from
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boosting because there were no blocked tasks to boost.
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Note that the system will balk from boosting even if the
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grace period is overdue when the currently running task
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is looping within an RCU read-side critical section.
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There is no point in boosting in this case, because
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boosting a running task won't make it run any faster.
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o "gt" is the number of times that the system balked
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from boosting because, although there were blocked tasks,
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none of them were preventing the current grace period
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from completing.
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o "bt" is the number of times that the system balked
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from boosting because boosting was already in progress.
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o "b" is the number of times that the system balked from
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boosting because boosting had already completed for
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the grace period in question.
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o "ny" is the number of times that the system balked from
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boosting because it was not yet time to start boosting
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the grace period in question.
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o "nos" is the number of times that the system balked from
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boosting for inexplicable ("not otherwise specified")
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reasons. This can actually happen due to races involving
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increments of the jiffies counter.
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o In the line beginning with "exp balk", the fields are as follows:
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o "bt" is the number of times that the system balked from
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boosting because there were no blocked tasks to boost.
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o "nos" is the number of times that the system balked from
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boosting for inexplicable ("not otherwise specified")
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reasons.
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