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Documentation / filesystems / gfs2-glocks.txt




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Based on kernel version 3.16. Page generated on 2014-08-06 21:39 EST.

1	                   Glock internal locking rules
2	                  ------------------------------
3	
4	This documents the basic principles of the glock state machine
5	internals. Each glock (struct gfs2_glock in fs/gfs2/incore.h)
6	has two main (internal) locks:
7	
8	 1. A spinlock (gl_spin) which protects the internal state such
9	    as gl_state, gl_target and the list of holders (gl_holders)
10	 2. A non-blocking bit lock, GLF_LOCK, which is used to prevent other
11	    threads from making calls to the DLM, etc. at the same time. If a
12	    thread takes this lock, it must then call run_queue (usually via the
13	    workqueue) when it releases it in order to ensure any pending tasks
14	    are completed.
15	
16	The gl_holders list contains all the queued lock requests (not
17	just the holders) associated with the glock. If there are any
18	held locks, then they will be contiguous entries at the head
19	of the list. Locks are granted in strictly the order that they
20	are queued, except for those marked LM_FLAG_PRIORITY which are
21	used only during recovery, and even then only for journal locks.
22	
23	There are three lock states that users of the glock layer can request,
24	namely shared (SH), deferred (DF) and exclusive (EX). Those translate
25	to the following DLM lock modes:
26	
27	Glock mode    | DLM lock mode
28	------------------------------
29	    UN        |    IV/NL  Unlocked (no DLM lock associated with glock) or NL
30	    SH        |    PR     (Protected read)
31	    DF        |    CW     (Concurrent write)
32	    EX        |    EX     (Exclusive)
33	
34	Thus DF is basically a shared mode which is incompatible with the "normal"
35	shared lock mode, SH. In GFS2 the DF mode is used exclusively for direct I/O
36	operations. The glocks are basically a lock plus some routines which deal
37	with cache management. The following rules apply for the cache:
38	
39	Glock mode   |  Cache data | Cache Metadata | Dirty Data | Dirty Metadata
40	--------------------------------------------------------------------------
41	    UN       |     No      |       No       |     No     |      No
42	    SH       |     Yes     |       Yes      |     No     |      No
43	    DF       |     No      |       Yes      |     No     |      No
44	    EX       |     Yes     |       Yes      |     Yes    |      Yes
45	
46	These rules are implemented using the various glock operations which
47	are defined for each type of glock. Not all types of glocks use
48	all the modes. Only inode glocks use the DF mode for example.
49	
50	Table of glock operations and per type constants:
51	
52	Field            | Purpose
53	----------------------------------------------------------------------------
54	go_xmote_th      | Called before remote state change (e.g. to sync dirty data)
55	go_xmote_bh      | Called after remote state change (e.g. to refill cache)
56	go_inval         | Called if remote state change requires invalidating the cache
57	go_demote_ok     | Returns boolean value of whether its ok to demote a glock
58	                 | (e.g. checks timeout, and that there is no cached data)
59	go_lock          | Called for the first local holder of a lock
60	go_unlock        | Called on the final local unlock of a lock
61	go_dump          | Called to print content of object for debugfs file, or on
62	                 | error to dump glock to the log.
63	go_type          | The type of the glock, LM_TYPE_.....
64	go_callback	 | Called if the DLM sends a callback to drop this lock
65	go_flags	 | GLOF_ASPACE is set, if the glock has an address space
66	                 | associated with it
67	
68	The minimum hold time for each lock is the time after a remote lock
69	grant for which we ignore remote demote requests. This is in order to
70	prevent a situation where locks are being bounced around the cluster
71	from node to node with none of the nodes making any progress. This
72	tends to show up most with shared mmaped files which are being written
73	to by multiple nodes. By delaying the demotion in response to a
74	remote callback, that gives the userspace program time to make
75	some progress before the pages are unmapped.
76	
77	There is a plan to try and remove the go_lock and go_unlock callbacks
78	if possible, in order to try and speed up the fast path though the locking.
79	Also, eventually we hope to make the glock "EX" mode locally shared
80	such that any local locking will be done with the i_mutex as required
81	rather than via the glock.
82	
83	Locking rules for glock operations:
84	
85	Operation     |  GLF_LOCK bit lock held |  gl_spin spinlock held
86	-----------------------------------------------------------------
87	go_xmote_th   |       Yes               |       No
88	go_xmote_bh   |       Yes               |       No
89	go_inval      |       Yes               |       No
90	go_demote_ok  |       Sometimes         |       Yes
91	go_lock       |       Yes               |       No
92	go_unlock     |       Yes               |       No
93	go_dump       |       Sometimes         |       Yes
94	go_callback   |       Sometimes (N/A)   |       Yes
95	
96	N.B. Operations must not drop either the bit lock or the spinlock
97	if its held on entry. go_dump and do_demote_ok must never block.
98	Note that go_dump will only be called if the glock's state
99	indicates that it is caching uptodate data.
100	
101	Glock locking order within GFS2:
102	
103	 1. i_mutex (if required)
104	 2. Rename glock (for rename only)
105	 3. Inode glock(s)
106	    (Parents before children, inodes at "same level" with same parent in
107	     lock number order)
108	 4. Rgrp glock(s) (for (de)allocation operations)
109	 5. Transaction glock (via gfs2_trans_begin) for non-read operations
110	 6. Page lock  (always last, very important!)
111	
112	There are two glocks per inode. One deals with access to the inode
113	itself (locking order as above), and the other, known as the iopen
114	glock is used in conjunction with the i_nlink field in the inode to
115	determine the lifetime of the inode in question. Locking of inodes
116	is on a per-inode basis. Locking of rgrps is on a per rgrp basis.
117	In general we prefer to lock local locks prior to cluster locks.
118	
119	                            Glock Statistics
120	                           ------------------
121	
122	The stats are divided into two sets: those relating to the
123	super block and those relating to an individual glock. The
124	super block stats are done on a per cpu basis in order to
125	try and reduce the overhead of gathering them. They are also
126	further divided by glock type. All timings are in nanoseconds.
127	
128	In the case of both the super block and glock statistics,
129	the same information is gathered in each case. The super
130	block timing statistics are used to provide default values for
131	the glock timing statistics, so that newly created glocks
132	should have, as far as possible, a sensible starting point.
133	The per-glock counters are initialised to zero when the
134	glock is created. The per-glock statistics are lost when
135	the glock is ejected from memory.
136	
137	The statistics are divided into three pairs of mean and
138	variance, plus two counters. The mean/variance pairs are
139	smoothed exponential estimates and the algorithm used is
140	one which will be very familiar to those used to calculation
141	of round trip times in network code. See "TCP/IP Illustrated,
142	Volume 1", W. Richard Stevens, sect 21.3, "Round-Trip Time Measurement",
143	p. 299 and onwards. Also, Volume 2, Sect. 25.10, p. 838 and onwards.
144	Unlike the TCP/IP Illustrated case, the mean and variance are
145	not scaled, but are in units of integer nanoseconds.
146	
147	The three pairs of mean/variance measure the following
148	things:
149	
150	 1. DLM lock time (non-blocking requests)
151	 2. DLM lock time (blocking requests)
152	 3. Inter-request time (again to the DLM)
153	
154	A non-blocking request is one which will complete right
155	away, whatever the state of the DLM lock in question. That
156	currently means any requests when (a) the current state of
157	the lock is exclusive, i.e. a lock demotion (b) the requested
158	state is either null or unlocked (again, a demotion) or (c) the
159	"try lock" flag is set. A blocking request covers all the other
160	lock requests.
161	
162	There are two counters. The first is there primarily to show
163	how many lock requests have been made, and thus how much data
164	has gone into the mean/variance calculations. The other counter
165	is counting queuing of holders at the top layer of the glock
166	code. Hopefully that number will be a lot larger than the number
167	of dlm lock requests issued.
168	
169	So why gather these statistics? There are several reasons
170	we'd like to get a better idea of these timings:
171	
172	1. To be able to better set the glock "min hold time"
173	2. To spot performance issues more easily
174	3. To improve the algorithm for selecting resource groups for
175	allocation (to base it on lock wait time, rather than blindly
176	using a "try lock")
177	
178	Due to the smoothing action of the updates, a step change in
179	some input quantity being sampled will only fully be taken
180	into account after 8 samples (or 4 for the variance) and this
181	needs to be carefully considered when interpreting the
182	results.
183	
184	Knowing both the time it takes a lock request to complete and
185	the average time between lock requests for a glock means we
186	can compute the total percentage of the time for which the
187	node is able to use a glock vs. time that the rest of the
188	cluster has its share. That will be very useful when setting
189	the lock min hold time.
190	
191	Great care has been taken to ensure that we
192	measure exactly the quantities that we want, as accurately
193	as possible. There are always inaccuracies in any
194	measuring system, but I hope this is as accurate as we
195	can reasonably make it.
196	
197	Per sb stats can be found here:
198	/sys/kernel/debug/gfs2/<fsname>/sbstats
199	Per glock stats can be found here:
200	/sys/kernel/debug/gfs2/<fsname>/glstats
201	
202	Assuming that debugfs is mounted on /sys/kernel/debug and also
203	that <fsname> is replaced with the name of the gfs2 filesystem
204	in question.
205	
206	The abbreviations used in the output as are follows:
207	
208	srtt     - Smoothed round trip time for non-blocking dlm requests
209	srttvar  - Variance estimate for srtt
210	srttb    - Smoothed round trip time for (potentially) blocking dlm requests
211	srttvarb - Variance estimate for srttb
212	sirt     - Smoothed inter-request time (for dlm requests)
213	sirtvar  - Variance estimate for sirt
214	dlm      - Number of dlm requests made (dcnt in glstats file)
215	queue    - Number of glock requests queued (qcnt in glstats file)
216	
217	The sbstats file contains a set of these stats for each glock type (so 8 lines
218	for each type) and for each cpu (one column per cpu). The glstats file contains
219	a set of these stats for each glock in a similar format to the glocks file, but
220	using the format mean/variance for each of the timing stats.
221	
222	The gfs2_glock_lock_time tracepoint prints out the current values of the stats
223	for the glock in question, along with some addition information on each dlm
224	reply that is received:
225	
226	status - The status of the dlm request
227	flags  - The dlm request flags
228	tdiff  - The time taken by this specific request
229	(remaining fields as per above list)
230	
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