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Documentation / this_cpu_ops.txt




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Based on kernel version 3.19. Page generated on 2015-02-13 21:22 EST.

1	this_cpu operations
2	-------------------
3	
4	this_cpu operations are a way of optimizing access to per cpu
5	variables associated with the *currently* executing processor. This is
6	done through the use of segment registers (or a dedicated register where
7	the cpu permanently stored the beginning of the per cpu	area for a
8	specific processor).
9	
10	this_cpu operations add a per cpu variable offset to the processor
11	specific per cpu base and encode that operation in the instruction
12	operating on the per cpu variable.
13	
14	This means that there are no atomicity issues between the calculation of
15	the offset and the operation on the data. Therefore it is not
16	necessary to disable preemption or interrupts to ensure that the
17	processor is not changed between the calculation of the address and
18	the operation on the data.
19	
20	Read-modify-write operations are of particular interest. Frequently
21	processors have special lower latency instructions that can operate
22	without the typical synchronization overhead, but still provide some
23	sort of relaxed atomicity guarantees. The x86, for example, can execute
24	RMW (Read Modify Write) instructions like inc/dec/cmpxchg without the
25	lock prefix and the associated latency penalty.
26	
27	Access to the variable without the lock prefix is not synchronized but
28	synchronization is not necessary since we are dealing with per cpu
29	data specific to the currently executing processor. Only the current
30	processor should be accessing that variable and therefore there are no
31	concurrency issues with other processors in the system.
32	
33	Please note that accesses by remote processors to a per cpu area are
34	exceptional situations and may impact performance and/or correctness
35	(remote write operations) of local RMW operations via this_cpu_*.
36	
37	The main use of the this_cpu operations has been to optimize counter
38	operations.
39	
40	The following this_cpu() operations with implied preemption protection
41	are defined. These operations can be used without worrying about
42	preemption and interrupts.
43	
44		this_cpu_read(pcp)
45		this_cpu_write(pcp, val)
46		this_cpu_add(pcp, val)
47		this_cpu_and(pcp, val)
48		this_cpu_or(pcp, val)
49		this_cpu_add_return(pcp, val)
50		this_cpu_xchg(pcp, nval)
51		this_cpu_cmpxchg(pcp, oval, nval)
52		this_cpu_cmpxchg_double(pcp1, pcp2, oval1, oval2, nval1, nval2)
53		this_cpu_sub(pcp, val)
54		this_cpu_inc(pcp)
55		this_cpu_dec(pcp)
56		this_cpu_sub_return(pcp, val)
57		this_cpu_inc_return(pcp)
58		this_cpu_dec_return(pcp)
59	
60	
61	Inner working of this_cpu operations
62	------------------------------------
63	
64	On x86 the fs: or the gs: segment registers contain the base of the
65	per cpu area. It is then possible to simply use the segment override
66	to relocate a per cpu relative address to the proper per cpu area for
67	the processor. So the relocation to the per cpu base is encoded in the
68	instruction via a segment register prefix.
69	
70	For example:
71	
72		DEFINE_PER_CPU(int, x);
73		int z;
74	
75		z = this_cpu_read(x);
76	
77	results in a single instruction
78	
79		mov ax, gs:[x]
80	
81	instead of a sequence of calculation of the address and then a fetch
82	from that address which occurs with the per cpu operations. Before
83	this_cpu_ops such sequence also required preempt disable/enable to
84	prevent the kernel from moving the thread to a different processor
85	while the calculation is performed.
86	
87	Consider the following this_cpu operation:
88	
89		this_cpu_inc(x)
90	
91	The above results in the following single instruction (no lock prefix!)
92	
93		inc gs:[x]
94	
95	instead of the following operations required if there is no segment
96	register:
97	
98		int *y;
99		int cpu;
100	
101		cpu = get_cpu();
102		y = per_cpu_ptr(&x, cpu);
103		(*y)++;
104		put_cpu();
105	
106	Note that these operations can only be used on per cpu data that is
107	reserved for a specific processor. Without disabling preemption in the
108	surrounding code this_cpu_inc() will only guarantee that one of the
109	per cpu counters is correctly incremented. However, there is no
110	guarantee that the OS will not move the process directly before or
111	after the this_cpu instruction is executed. In general this means that
112	the value of the individual counters for each processor are
113	meaningless. The sum of all the per cpu counters is the only value
114	that is of interest.
115	
116	Per cpu variables are used for performance reasons. Bouncing cache
117	lines can be avoided if multiple processors concurrently go through
118	the same code paths.  Since each processor has its own per cpu
119	variables no concurrent cache line updates take place. The price that
120	has to be paid for this optimization is the need to add up the per cpu
121	counters when the value of a counter is needed.
122	
123	
124	Special operations:
125	-------------------
126	
127		y = this_cpu_ptr(&x)
128	
129	Takes the offset of a per cpu variable (&x !) and returns the address
130	of the per cpu variable that belongs to the currently executing
131	processor.  this_cpu_ptr avoids multiple steps that the common
132	get_cpu/put_cpu sequence requires. No processor number is
133	available. Instead, the offset of the local per cpu area is simply
134	added to the per cpu offset.
135	
136	Note that this operation is usually used in a code segment when
137	preemption has been disabled. The pointer is then used to
138	access local per cpu data in a critical section. When preemption
139	is re-enabled this pointer is usually no longer useful since it may
140	no longer point to per cpu data of the current processor.
141	
142	
143	Per cpu variables and offsets
144	-----------------------------
145	
146	Per cpu variables have *offsets* to the beginning of the per cpu
147	area. They do not have addresses although they look like that in the
148	code. Offsets cannot be directly dereferenced. The offset must be
149	added to a base pointer of a per cpu area of a processor in order to
150	form a valid address.
151	
152	Therefore the use of x or &x outside of the context of per cpu
153	operations is invalid and will generally be treated like a NULL
154	pointer dereference.
155	
156		DEFINE_PER_CPU(int, x);
157	
158	In the context of per cpu operations the above implies that x is a per
159	cpu variable. Most this_cpu operations take a cpu variable.
160	
161		int __percpu *p = &x;
162	
163	&x and hence p is the *offset* of a per cpu variable. this_cpu_ptr()
164	takes the offset of a per cpu variable which makes this look a bit
165	strange.
166	
167	
168	Operations on a field of a per cpu structure
169	--------------------------------------------
170	
171	Let's say we have a percpu structure
172	
173		struct s {
174			int n,m;
175		};
176	
177		DEFINE_PER_CPU(struct s, p);
178	
179	
180	Operations on these fields are straightforward
181	
182		this_cpu_inc(p.m)
183	
184		z = this_cpu_cmpxchg(p.m, 0, 1);
185	
186	
187	If we have an offset to struct s:
188	
189		struct s __percpu *ps = &p;
190	
191		this_cpu_dec(ps->m);
192	
193		z = this_cpu_inc_return(ps->n);
194	
195	
196	The calculation of the pointer may require the use of this_cpu_ptr()
197	if we do not make use of this_cpu ops later to manipulate fields:
198	
199		struct s *pp;
200	
201		pp = this_cpu_ptr(&p);
202	
203		pp->m--;
204	
205		z = pp->n++;
206	
207	
208	Variants of this_cpu ops
209	-------------------------
210	
211	this_cpu ops are interrupt safe. Some architectures do not support
212	these per cpu local operations. In that case the operation must be
213	replaced by code that disables interrupts, then does the operations
214	that are guaranteed to be atomic and then re-enable interrupts. Doing
215	so is expensive. If there are other reasons why the scheduler cannot
216	change the processor we are executing on then there is no reason to
217	disable interrupts. For that purpose the following __this_cpu operations
218	are provided.
219	
220	These operations have no guarantee against concurrent interrupts or
221	preemption. If a per cpu variable is not used in an interrupt context
222	and the scheduler cannot preempt, then they are safe. If any interrupts
223	still occur while an operation is in progress and if the interrupt too
224	modifies the variable, then RMW actions can not be guaranteed to be
225	safe.
226	
227		__this_cpu_read(pcp)
228		__this_cpu_write(pcp, val)
229		__this_cpu_add(pcp, val)
230		__this_cpu_and(pcp, val)
231		__this_cpu_or(pcp, val)
232		__this_cpu_add_return(pcp, val)
233		__this_cpu_xchg(pcp, nval)
234		__this_cpu_cmpxchg(pcp, oval, nval)
235		__this_cpu_cmpxchg_double(pcp1, pcp2, oval1, oval2, nval1, nval2)
236		__this_cpu_sub(pcp, val)
237		__this_cpu_inc(pcp)
238		__this_cpu_dec(pcp)
239		__this_cpu_sub_return(pcp, val)
240		__this_cpu_inc_return(pcp)
241		__this_cpu_dec_return(pcp)
242	
243	
244	Will increment x and will not fall-back to code that disables
245	interrupts on platforms that cannot accomplish atomicity through
246	address relocation and a Read-Modify-Write operation in the same
247	instruction.
248	
249	
250	&this_cpu_ptr(pp)->n vs this_cpu_ptr(&pp->n)
251	--------------------------------------------
252	
253	The first operation takes the offset and forms an address and then
254	adds the offset of the n field. This may result in two add
255	instructions emitted by the compiler.
256	
257	The second one first adds the two offsets and then does the
258	relocation.  IMHO the second form looks cleaner and has an easier time
259	with (). The second form also is consistent with the way
260	this_cpu_read() and friends are used.
261	
262	
263	Remote access to per cpu data
264	------------------------------
265	
266	Per cpu data structures are designed to be used by one cpu exclusively.
267	If you use the variables as intended, this_cpu_ops() are guaranteed to
268	be "atomic" as no other CPU has access to these data structures.
269	
270	There are special cases where you might need to access per cpu data
271	structures remotely. It is usually safe to do a remote read access
272	and that is frequently done to summarize counters. Remote write access
273	something which could be problematic because this_cpu ops do not
274	have lock semantics. A remote write may interfere with a this_cpu
275	RMW operation.
276	
277	Remote write accesses to percpu data structures are highly discouraged
278	unless absolutely necessary. Please consider using an IPI to wake up
279	the remote CPU and perform the update to its per cpu area.
280	
281	To access per-cpu data structure remotely, typically the per_cpu_ptr()
282	function is used:
283	
284	
285		DEFINE_PER_CPU(struct data, datap);
286	
287		struct data *p = per_cpu_ptr(&datap, cpu);
288	
289	This makes it explicit that we are getting ready to access a percpu
290	area remotely.
291	
292	You can also do the following to convert the datap offset to an address
293	
294		struct data *p = this_cpu_ptr(&datap);
295	
296	but, passing of pointers calculated via this_cpu_ptr to other cpus is
297	unusual and should be avoided.
298	
299	Remote access are typically only for reading the status of another cpus
300	per cpu data. Write accesses can cause unique problems due to the
301	relaxed synchronization requirements for this_cpu operations.
302	
303	One example that illustrates some concerns with write operations is
304	the following scenario that occurs because two per cpu variables
305	share a cache-line but the relaxed synchronization is applied to
306	only one process updating the cache-line.
307	
308	Consider the following example
309	
310	
311		struct test {
312			atomic_t a;
313			int b;
314		};
315	
316		DEFINE_PER_CPU(struct test, onecacheline);
317	
318	There is some concern about what would happen if the field 'a' is updated
319	remotely from one processor and the local processor would use this_cpu ops
320	to update field b. Care should be taken that such simultaneous accesses to
321	data within the same cache line are avoided. Also costly synchronization
322	may be necessary. IPIs are generally recommended in such scenarios instead
323	of a remote write to the per cpu area of another processor.
324	
325	Even in cases where the remote writes are rare, please bear in
326	mind that a remote write will evict the cache line from the processor
327	that most likely will access it. If the processor wakes up and finds a
328	missing local cache line of a per cpu area, its performance and hence
329	the wake up times will be affected.
330	
331	Christoph Lameter, August 4th, 2014
332	Pranith Kumar, Aug 2nd, 2014
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