Donate to e Foundation | Murena handsets with /e/OS | Own a part of Murena! Learn more

Commit bd58b430 authored by Paul E. McKenney's avatar Paul E. McKenney Committed by Ingo Molnar
Browse files

rcu: Update trace.txt documentation to reflect recent changes 



o	Remove the CONFIG_PREEMPT_RCU documentation since this
	config option has now been removed.

o	Change the now-incorrect references to "rcu" labels to
	instead be "rcu_sched".

o	Add notes stating that CONFIG_TREE_PREEMPT_RCU kernels will
	have additional "rcu_preempt" output.

o	Note the new "oqlen" field in the rcuhier output (for
	RCU callbacks orphaned by an offlined CPU).

Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: mathieu.desnoyers@polymtl.ca
Cc: josh@joshtriplett.org
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
Cc: rostedt@goodmis.org
Cc: Valdis.Kletnieks@vt.edu
Cc: dhowells@redhat.com
Cc: npiggin@suse.de
Cc: jens.axboe@oracle.com
LKML-Reference: <1255540559799-git-send-email->
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent 3397e040

Documentation/RCU/trace.txt

+33 −198
Original line number Original line Diff line number Diff line 
CONFIG_RCU_TRACE debugfs Files and Formats

CONFIG_RCU_TRACE debugfs Files and Formats









The rcupreempt and rcutree implementations of RCU provide debugfs trace

The rcutree implementation of RCU provides debugfs trace output that

output that summarizes counters and state.  This information is useful for

summarizes counters and state.  This information is useful for debugging

debugging RCU itself, and can sometimes also help to debug abuses of RCU.

RCU itself, and can sometimes also help to debug abuses of RCU.

Note that the rcuclassic implementation of RCU does not provide debugfs

The following sections describe the debugfs files and formats.

trace output.



The following sections describe the debugfs files and formats for

preemptable RCU (rcupreempt) and hierarchical RCU (rcutree).





Preemptable RCU debugfs Files and Formats



This implementation of RCU provides three debugfs files under the

top-level directory RCU: rcu/rcuctrs (which displays the per-CPU

counters used by preemptable RCU) rcu/rcugp (which displays grace-period

counters), and rcu/rcustats (which internal counters for debugging RCU).



The output of "cat rcu/rcuctrs" looks as follows:



CPU last cur F M

  0    5  -5 0 0

  1   -1   0 0 0

  2    0   1 0 0

  3    0   1 0 0

  4    0   1 0 0

  5    0   1 0 0

  6    0   2 0 0

  7    0  -1 0 0

  8    0   1 0 0

ggp = 26226, state = waitzero



The per-CPU fields are as follows:



o	"CPU" gives the CPU number.  Offline CPUs are not displayed.



o	"last" gives the value of the counter that is being decremented

	for the current grace period phase.  In the example above,

	the counters sum to 4, indicating that there are still four

	RCU read-side critical sections still running that started

	before the last counter flip.



o	"cur" gives the value of the counter that is currently being

	both incremented (by rcu_read_lock()) and decremented (by

	rcu_read_unlock()).  In the example above, the counters sum to

	1, indicating that there is only one RCU read-side critical section

	still running that started after the last counter flip.



o	"F" indicates whether RCU is waiting for this CPU to acknowledge

	a counter flip.  In the above example, RCU is not waiting on any,

	which is consistent with the state being "waitzero" rather than

	"waitack".



o	"M" indicates whether RCU is waiting for this CPU to execute a

	memory barrier.  In the above example, RCU is not waiting on any,

	which is consistent with the state being "waitzero" rather than

	"waitmb".



o	"ggp" is the global grace-period counter.



o	"state" is the RCU state, which can be one of the following:



	o	"idle": there is no grace period in progress.



	o	"waitack": RCU just incremented the global grace-period

		counter, which has the effect of reversing the roles of

		the "last" and "cur" counters above, and is waiting for

		all the CPUs to acknowledge the flip.  Once the flip has

		been acknowledged, CPUs will no longer be incrementing

		what are now the "last" counters, so that their sum will

		decrease monotonically down to zero.



	o	"waitzero": RCU is waiting for the sum of the "last" counters

		to decrease to zero.



	o	"waitmb": RCU is waiting for each CPU to execute a memory

		barrier, which ensures that instructions from a given CPU's

		last RCU read-side critical section cannot be reordered

		with instructions following the memory-barrier instruction.



The output of "cat rcu/rcugp" looks as follows:



oldggp=48870  newggp=48873



Note that reading from this file provokes a synchronize_rcu().  The

"oldggp" value is that of "ggp" from rcu/rcuctrs above, taken before

executing the synchronize_rcu(), and the "newggp" value is also the

"ggp" value, but taken after the synchronize_rcu() command returns.





The output of "cat rcu/rcugp" looks as follows:



na=1337955 nl=40 wa=1337915 wl=44 da=1337871 dl=0 dr=1337871 di=1337871

1=50989 e1=6138 i1=49722 ie1=82 g1=49640 a1=315203 ae1=265563 a2=49640

z1=1401244 ze1=1351605 z2=49639 m1=5661253 me1=5611614 m2=49639



These are counters tracking internal preemptable-RCU events, however,

some of them may be useful for debugging algorithms using RCU.  In

particular, the "nl", "wl", and "dl" values track the number of RCU

callbacks in various states.  The fields are as follows:



o	"na" is the total number of RCU callbacks that have been enqueued

	since boot.



o	"nl" is the number of RCU callbacks waiting for the previous

	grace period to end so that they can start waiting on the next

	grace period.



o	"wa" is the total number of RCU callbacks that have started waiting

	for a grace period since boot.  "na" should be roughly equal to

	"nl" plus "wa".



o	"wl" is the number of RCU callbacks currently waiting for their

	grace period to end.



o	"da" is the total number of RCU callbacks whose grace periods

	have completed since boot.  "wa" should be roughly equal to

	"wl" plus "da".



o	"dr" is the total number of RCU callbacks that have been removed

	from the list of callbacks ready to invoke.  "dr" should be roughly

	equal to "da".



o	"di" is the total number of RCU callbacks that have been invoked

	since boot.  "di" should be roughly equal to "da", though some

	early versions of preemptable RCU had a bug so that only the

	last CPU's count of invocations was displayed, rather than the

	sum of all CPU's counts.



o	"1" is the number of calls to rcu_try_flip().  This should be

	roughly equal to the sum of "e1", "i1", "a1", "z1", and "m1"

	described below.  In other words, the number of times that

	the state machine is visited should be equal to the sum of the

	number of times that each state is visited plus the number of

	times that the state-machine lock acquisition failed.



o	"e1" is the number of times that rcu_try_flip() was unable to

	acquire the fliplock.



o	"i1" is the number of calls to rcu_try_flip_idle().



o	"ie1" is the number of times rcu_try_flip_idle() exited early

	due to the calling CPU having no work for RCU.



o	"g1" is the number of times that rcu_try_flip_idle() decided

	to start a new grace period.  "i1" should be roughly equal to

	"ie1" plus "g1".



o	"a1" is the number of calls to rcu_try_flip_waitack().



o	"ae1" is the number of times that rcu_try_flip_waitack() found

	that at least one CPU had not yet acknowledge the new grace period

	(AKA "counter flip").



o	"a2" is the number of time rcu_try_flip_waitack() found that

	all CPUs had acknowledged.  "a1" should be roughly equal to

	"ae1" plus "a2".  (This particular output was collected on

	a 128-CPU machine, hence the smaller-than-usual fraction of

	calls to rcu_try_flip_waitack() finding all CPUs having already

	acknowledged.)



o	"z1" is the number of calls to rcu_try_flip_waitzero().



o	"ze1" is the number of times that rcu_try_flip_waitzero() found

	that not all of the old RCU read-side critical sections had

	completed.



o	"z2" is the number of times that rcu_try_flip_waitzero() finds

	the sum of the counters equal to zero, in other words, that

	all of the old RCU read-side critical sections had completed.

	The value of "z1" should be roughly equal to "ze1" plus

	"z2".



o	"m1" is the number of calls to rcu_try_flip_waitmb().



o	"me1" is the number of times that rcu_try_flip_waitmb() finds

	that at least one CPU has not yet executed a memory barrier.



o	"m2" is the number of times that rcu_try_flip_waitmb() finds that

	all CPUs have executed a memory barrier.









Hierarchical RCU debugfs Files and Formats

Hierarchical RCU debugfs Files and Formats
@@ -210,9 +35,10 @@ rcu_bh: 
  6 c=-275 g=-275 pq=1 pqc=-275 qp=0 dt=859/1 dn=0 df=15 of=0 ri=0 ql=0 b=10

  6 c=-275 g=-275 pq=1 pqc=-275 qp=0 dt=859/1 dn=0 df=15 of=0 ri=0 ql=0 b=10

  7 c=-275 g=-275 pq=1 pqc=-275 qp=0 dt=3761/1 dn=0 df=15 of=0 ri=0 ql=0 b=10

  7 c=-275 g=-275 pq=1 pqc=-275 qp=0 dt=3761/1 dn=0 df=15 of=0 ri=0 ql=0 b=10





The first section lists the rcu_data structures for rcu, the second for

The first section lists the rcu_data structures for rcu_sched, the second

rcu_bh.  Each section has one line per CPU, or eight for this 8-CPU system.

for rcu_bh.  Note that CONFIG_TREE_PREEMPT_RCU kernels will have an

The fields are as follows:

additional section for rcu_preempt.  Each section has one line per CPU,

or eight for this 8-CPU system.  The fields are as follows:





o	The number at the beginning of each line is the CPU number.

o	The number at the beginning of each line is the CPU number.

	CPUs numbers followed by an exclamation mark are offline,

	CPUs numbers followed by an exclamation mark are offline,
@@ -223,9 +49,9 @@ o The number at the beginning of each line is the CPU number. 




o	"c" is the count of grace periods that this CPU believes have

o	"c" is the count of grace periods that this CPU believes have

	completed.  CPUs in dynticks idle mode may lag quite a ways

	completed.  CPUs in dynticks idle mode may lag quite a ways

	behind, for example, CPU 4 under "rcu" above, which has slept

	behind, for example, CPU 4 under "rcu_sched" above, which has

	through the past 25 RCU grace periods.	It is not unusual to

	slept through the past 25 RCU grace periods.  It is not unusual

	see CPUs lagging by thousands of grace periods.

	to see CPUs lagging by thousands of grace periods.





o	"g" is the count of grace periods that this CPU believes have

o	"g" is the count of grace periods that this CPU believes have

	started.  Again, CPUs in dynticks idle mode may lag behind.

	started.  Again, CPUs in dynticks idle mode may lag behind.
@@ -308,8 +134,10 @@ The output of "cat rcu/rcugp" looks as follows: 
rcu_sched: completed=33062  gpnum=33063

rcu_sched: completed=33062  gpnum=33063

rcu_bh: completed=464  gpnum=464

rcu_bh: completed=464  gpnum=464





Again, this output is for both "rcu" and "rcu_bh".  The fields are

Again, this output is for both "rcu_sched" and "rcu_bh".  Note that

taken from the rcu_state structure, and are as follows:

kernels built with CONFIG_TREE_PREEMPT_RCU will have an additional

"rcu_preempt" line.  The fields are taken from the rcu_state structure,

and are as follows:





o	"completed" is the number of grace periods that have completed.

o	"completed" is the number of grace periods that have completed.

	It is comparable to the "c" field from rcu/rcudata in that a

	It is comparable to the "c" field from rcu/rcudata in that a
@@ -324,23 +152,24 @@ o "gpnum" is the number of grace periods that have started. It is 
	If these two fields are equal (as they are for "rcu_bh" above),

	If these two fields are equal (as they are for "rcu_bh" above),

	then there is no grace period in progress, in other words, RCU

	then there is no grace period in progress, in other words, RCU

	is idle.  On the other hand, if the two fields differ (as they

	is idle.  On the other hand, if the two fields differ (as they

	do for "rcu" above), then an RCU grace period is in progress.

	do for "rcu_sched" above), then an RCU grace period is in progress.









The output of "cat rcu/rcuhier" looks as follows, with very long lines:

The output of "cat rcu/rcuhier" looks as follows, with very long lines:





c=6902 g=6903 s=2 jfq=3 j=72c7 nfqs=13142/nfqsng=0(13142) fqlh=6

c=6902 g=6903 s=2 jfq=3 j=72c7 nfqs=13142/nfqsng=0(13142) fqlh=6 oqlen=0

1/1 0:127 ^0    

1/1 0:127 ^0    

3/3 0:35 ^0    0/0 36:71 ^1    0/0 72:107 ^2    0/0 108:127 ^3    

3/3 0:35 ^0    0/0 36:71 ^1    0/0 72:107 ^2    0/0 108:127 ^3    

3/3f 0:5 ^0    2/3 6:11 ^1    0/0 12:17 ^2    0/0 18:23 ^3    0/0 24:29 ^4    0/0 30:35 ^5    0/0 36:41 ^0    0/0 42:47 ^1    0/0 48:53 ^2    0/0 54:59 ^3    0/0 60:65 ^4    0/0 66:71 ^5    0/0 72:77 ^0    0/0 78:83 ^1    0/0 84:89 ^2    0/0 90:95 ^3    0/0 96:101 ^4    0/0 102:107 ^5    0/0 108:113 ^0    0/0 114:119 ^1    0/0 120:125 ^2    0/0 126:127 ^3    

3/3f 0:5 ^0    2/3 6:11 ^1    0/0 12:17 ^2    0/0 18:23 ^3    0/0 24:29 ^4    0/0 30:35 ^5    0/0 36:41 ^0    0/0 42:47 ^1    0/0 48:53 ^2    0/0 54:59 ^3    0/0 60:65 ^4    0/0 66:71 ^5    0/0 72:77 ^0    0/0 78:83 ^1    0/0 84:89 ^2    0/0 90:95 ^3    0/0 96:101 ^4    0/0 102:107 ^5    0/0 108:113 ^0    0/0 114:119 ^1    0/0 120:125 ^2    0/0 126:127 ^3    

rcu_bh:

rcu_bh:

c=-226 g=-226 s=1 jfq=-5701 j=72c7 nfqs=88/nfqsng=0(88) fqlh=0

c=-226 g=-226 s=1 jfq=-5701 j=72c7 nfqs=88/nfqsng=0(88) fqlh=0 oqlen=0

0/1 0:127 ^0    

0/1 0:127 ^0    

0/3 0:35 ^0    0/0 36:71 ^1    0/0 72:107 ^2    0/0 108:127 ^3    

0/3 0:35 ^0    0/0 36:71 ^1    0/0 72:107 ^2    0/0 108:127 ^3    

0/3f 0:5 ^0    0/3 6:11 ^1    0/0 12:17 ^2    0/0 18:23 ^3    0/0 24:29 ^4    0/0 30:35 ^5    0/0 36:41 ^0    0/0 42:47 ^1    0/0 48:53 ^2    0/0 54:59 ^3    0/0 60:65 ^4    0/0 66:71 ^5    0/0 72:77 ^0    0/0 78:83 ^1    0/0 84:89 ^2    0/0 90:95 ^3    0/0 96:101 ^4    0/0 102:107 ^5    0/0 108:113 ^0    0/0 114:119 ^1    0/0 120:125 ^2    0/0 126:127 ^3

0/3f 0:5 ^0    0/3 6:11 ^1    0/0 12:17 ^2    0/0 18:23 ^3    0/0 24:29 ^4    0/0 30:35 ^5    0/0 36:41 ^0    0/0 42:47 ^1    0/0 48:53 ^2    0/0 54:59 ^3    0/0 60:65 ^4    0/0 66:71 ^5    0/0 72:77 ^0    0/0 78:83 ^1    0/0 84:89 ^2    0/0 90:95 ^3    0/0 96:101 ^4    0/0 102:107 ^5    0/0 108:113 ^0    0/0 114:119 ^1    0/0 120:125 ^2    0/0 126:127 ^3





This is once again split into "rcu" and "rcu_bh" portions.  The fields are

This is once again split into "rcu_sched" and "rcu_bh" portions,

as follows:

and CONFIG_TREE_PREEMPT_RCU kernels will again have an additional

"rcu_preempt" section.  The fields are as follows:





o	"c" is exactly the same as "completed" under rcu/rcugp.

o	"c" is exactly the same as "completed" under rcu/rcugp.
@@ -372,6 +201,11 @@ o "fqlh" is the number of calls to force_quiescent_state() that 
	exited immediately (without even being counted in nfqs above)

	exited immediately (without even being counted in nfqs above)

	due to contention on ->fqslock.

	due to contention on ->fqslock.





o	"oqlen" is the number of callbacks on the "orphan" callback

	list.  RCU callbacks are placed on this list by CPUs going

	offline, and are "adopted" either by the CPU helping the outgoing

	CPU or by the next rcu_barrier*() call, whichever comes first.



o	Each element of the form "1/1 0:127 ^0" represents one struct

o	Each element of the form "1/1 0:127 ^0" represents one struct

	rcu_node.  Each line represents one level of the hierarchy, from

	rcu_node.  Each line represents one level of the hierarchy, from

	root to leaves.  It is best to think of the rcu_data structures

	root to leaves.  It is best to think of the rcu_data structures
@@ -389,10 +223,10 @@ o Each element of the form "1/1 0:127 ^0" represents one struct 
		The value of qsmaskinit is assigned to that of qsmask

		The value of qsmaskinit is assigned to that of qsmask

		at the beginning of each grace period.

		at the beginning of each grace period.





		For example, for "rcu", the qsmask of the first entry

		For example, for "rcu_sched", the qsmask of the first

		of the lowest level is 0x14, meaning that we are still

		entry of the lowest level is 0x14, meaning that we

		waiting for CPUs 2 and 4 to check in for the current

		are still waiting for CPUs 2 and 4 to check in for the

		grace period.

		current grace period.





	o	The numbers separated by the ":" are the range of CPUs

	o	The numbers separated by the ":" are the range of CPUs

		served by this struct rcu_node.  This can be helpful

		served by this struct rcu_node.  This can be helpful
@@ -431,8 +265,9 @@ rcu_bh: 
  6 np=120834 qsp=9902 cbr=0 cng=0 gpc=6 gps=3 nf=2 nn=110921

  6 np=120834 qsp=9902 cbr=0 cng=0 gpc=6 gps=3 nf=2 nn=110921

  7 np=144888 qsp=26336 cbr=0 cng=0 gpc=8 gps=2 nf=0 nn=118542

  7 np=144888 qsp=26336 cbr=0 cng=0 gpc=8 gps=2 nf=0 nn=118542





As always, this is once again split into "rcu" and "rcu_bh" portions.

As always, this is once again split into "rcu_sched" and "rcu_bh"

The fields are as follows:

portions, with CONFIG_TREE_PREEMPT_RCU kernels having an additional

"rcu_preempt" section.  The fields are as follows:





o	"np" is the number of times that __rcu_pending() has been invoked

o	"np" is the number of times that __rcu_pending() has been invoked

	for the corresponding flavor of RCU.

	for the corresponding flavor of RCU.