* [PATCH v4 1/1] doc: this_cpu_ops.txt: clarify remote accesses of per cpu areas
@ 2014-08-03 1:30 Pranith Kumar
2014-08-04 14:59 ` Christoph Lameter
0 siblings, 1 reply; 3+ messages in thread
From: Pranith Kumar @ 2014-08-03 1:30 UTC (permalink / raw)
To: Randy Dunlap, open list:DOCUMENTATION, open list; +Cc: Christoph Lameter
Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
CC: Christoph Lameter <cl@linux.com>
---
v4: more updates from Christoph Lameter
v3: clearly mention that remote write access is discouraged
v2: updated with comments from Christoph Lameter
Documentation/this_cpu_ops.txt | 174 ++++++++++++++++++++++++++++++++++-------
1 file changed, 146 insertions(+), 28 deletions(-)
diff --git a/Documentation/this_cpu_ops.txt b/Documentation/this_cpu_ops.txt
index 1a4ce7e..a94c017 100644
--- a/Documentation/this_cpu_ops.txt
+++ b/Documentation/this_cpu_ops.txt
@@ -2,33 +2,62 @@ this_cpu operations
-------------------
this_cpu operations are a way of optimizing access to per cpu
-variables associated with the *currently* executing processor through
-the use of segment registers (or a dedicated register where the cpu
-permanently stored the beginning of the per cpu area for a specific
-processor).
+variables associated with the *currently* executing processor. This is
+done through the use of segment registers (or a dedicated register where
+the cpu permanently stored the beginning of the per cpu area for a
+specific processor).
-The this_cpu operations add a per cpu variable offset to the processor
+this_cpu operations add a per cpu variable offset to the processor
specific percpu base and encode that operation in the instruction
operating on the per cpu variable.
-This means there are no atomicity issues between the calculation of
+This means that there are no atomicity issues between the calculation of
the offset and the operation on the data. Therefore it is not
-necessary to disable preempt or interrupts to ensure that the
+necessary to disable preemption or interrupts to ensure that the
processor is not changed between the calculation of the address and
the operation on the data.
Read-modify-write operations are of particular interest. Frequently
processors have special lower latency instructions that can operate
-without the typical synchronization overhead but still provide some
-sort of relaxed atomicity guarantee. The x86 for example can execute
-RMV (Read Modify Write) instructions like inc/dec/cmpxchg without the
+without the typical synchronization overhead, but still provide some
+sort of relaxed atomicity guarantees. The x86, for example, can execute
+RMW (Read Modify Write) instructions like inc/dec/cmpxchg without the
lock prefix and the associated latency penalty.
Access to the variable without the lock prefix is not synchronized but
synchronization is not necessary since we are dealing with per cpu
data specific to the currently executing processor. Only the current
processor should be accessing that variable and therefore there are no
-concurrency issues with other processors in the system.
+concurrency issues with other processors in the system. Please see the
+section "Remote access to per-CPU data" if you need remote access.
+
+The main use of the this_cpu operations has been to optimize counter
+operations.
+
+The following this_cpu() operations with implied preemption protection
+are defined. These operations can be used without worrying about
+preemption and interrupts.
+
+ this_cpu_add()
+ this_cpu_read(pcp)
+ this_cpu_write(pcp, val)
+ this_cpu_add(pcp, val)
+ this_cpu_and(pcp, val)
+ this_cpu_or(pcp, val)
+ this_cpu_add_return(pcp, val)
+ this_cpu_xchg(pcp, nval)
+ this_cpu_cmpxchg(pcp, oval, nval)
+ this_cpu_cmpxchg_double(pcp1, pcp2, oval1, oval2, nval1, nval2)
+ this_cpu_sub(pcp, val)
+ this_cpu_inc(pcp)
+ this_cpu_dec(pcp)
+ this_cpu_sub_return(pcp, val)
+ this_cpu_inc_return(pcp)
+ this_cpu_dec_return(pcp)
+
+
+Inner working of this_cpu operations
+------------------------------------
On x86 the fs: or the gs: segment registers contain the base of the
per cpu area. It is then possible to simply use the segment override
@@ -53,12 +82,11 @@ this_cpu_ops such sequence also required preempt disable/enable to
prevent the kernel from moving the thread to a different processor
while the calculation is performed.
-The main use of the this_cpu operations has been to optimize counter
-operations.
+Consider the following this_cpu operation
this_cpu_inc(x)
-results in the following single instruction (no lock prefix!)
+The above results in the following single instruction (no lock prefix!)
inc gs:[x]
@@ -100,11 +128,10 @@ Takes the offset of a per cpu variable (&x !) and returns the address
of the per cpu variable that belongs to the currently executing
processor. this_cpu_ptr avoids multiple steps that the common
get_cpu/put_cpu sequence requires. No processor number is
-available. Instead the offset of the local per cpu area is simply
+available. Instead, the offset of the local per cpu area is simply
added to the percpu offset.
-
Per cpu variables and offsets
-----------------------------
@@ -118,15 +145,16 @@ Therefore the use of x or &x outside of the context of per cpu
operations is invalid and will generally be treated like a NULL
pointer dereference.
-In the context of per cpu operations
+ DEFINE_PER_CPU(int, x);
- x is a per cpu variable. Most this_cpu operations take a cpu
- variable.
+In the context of per cpu operations the above implies that x is a per
+cpu variable. Most this_cpu operations take a cpu variable.
- &x is the *offset* a per cpu variable. this_cpu_ptr() takes
- the offset of a per cpu variable which makes this look a bit
- strange.
+ int __percpu *p = &x;
+&x and hence p is the *offset* of a per cpu variable. this_cpu_ptr()
+takes the offset of a per cpu variable which makes this look a bit
+strange.
Operations on a field of a per cpu structure
@@ -152,7 +180,7 @@ If we have an offset to struct s:
struct s __percpu *ps = &p;
- z = this_cpu_dec(ps->m);
+ this_cpu_dec(ps->m);
z = this_cpu_inc_return(ps->n);
@@ -172,16 +200,39 @@ if we do not make use of this_cpu ops later to manipulate fields:
Variants of this_cpu ops
-------------------------
-this_cpu ops are interrupt safe. Some architecture do not support
+this_cpu ops are interrupt safe. Some architectures do not support
these per cpu local operations. In that case the operation must be
replaced by code that disables interrupts, then does the operations
that are guaranteed to be atomic and then reenable interrupts. Doing
so is expensive. If there are other reasons why the scheduler cannot
change the processor we are executing on then there is no reason to
-disable interrupts. For that purpose the __this_cpu operations are
-provided. For example.
+disable interrupts. For that purpose the following __this_cpu operations
+are provided.
+
+These operations have no guarantee against concurrent interrupts or
+preemption. If a per cpu variable is not used in an interrupt context
+and the scheduler cannot preempt, then they are safe. If any interrupts
+still occur while an operation is in progress and if the interrupt too
+modifies the variable, then RMW actions can not be guaranteed to be
+safe.
+
+ __this_cpu_add()
+ __this_cpu_read(pcp)
+ __this_cpu_write(pcp, val)
+ __this_cpu_add(pcp, val)
+ __this_cpu_and(pcp, val)
+ __this_cpu_or(pcp, val)
+ __this_cpu_add_return(pcp, val)
+ __this_cpu_xchg(pcp, nval)
+ __this_cpu_cmpxchg(pcp, oval, nval)
+ __this_cpu_cmpxchg_double(pcp1, pcp2, oval1, oval2, nval1, nval2)
+ __this_cpu_sub(pcp, val)
+ __this_cpu_inc(pcp)
+ __this_cpu_dec(pcp)
+ __this_cpu_sub_return(pcp, val)
+ __this_cpu_inc_return(pcp)
+ __this_cpu_dec_return(pcp)
- __this_cpu_inc(x);
Will increment x and will not fallback to code that disables
interrupts on platforms that cannot accomplish atomicity through
@@ -189,7 +240,6 @@ address relocation and a Read-Modify-Write operation in the same
instruction.
-
&this_cpu_ptr(pp)->n vs this_cpu_ptr(&pp->n)
--------------------------------------------
@@ -202,4 +252,72 @@ with (). The second form also is consistent with the way
this_cpu_read() and friends are used.
+Remote access to per cpu data
+------------------------------
+
+Per cpu data structures are designed to be used by one cpu exclusively.
+If you use the variables as intended, this_cpu_ops() are guaranteed to
+be "atomic" as no other CPU has access to these data structures. In this
+context, a remote access means an access to a per cpu data structure
+from a CPU without using the this_cpu_* operations.
+
+There are special cases where you might need to access per cpu data
+structures remotely. One such case is to perform maintenance tasks of
+idle CPUs without having to wake them up using IPIs for power saving
+purposes. It is usually safe to do a remote read access and that is
+frequently done to summarize counters. Remote write access is the one
+which is problematic. Remote write accesses to per cpu data structures
+are highly discouraged until absolutely necessary. Please consider using
+an IPI to wake up the remote CPU and perform the update to its per cpu
+area.
+
+To access per-cpu data structure remotely, you need to convert the
+per-cpu offset to a pointer using this_cpu_ptr().
+
+ DEFINE_PER_CPU(struct data, datap);
+ struct data *p = per_cpu(&datap, cpu);
+
+p can now be passed to another CPU to be updated remotely.
+
+This makes it explicit that we are getting ready to access a percpu
+area.
+
+You can also do the following to convert the datap offset to an address
+
+ struct data *p = this_cpu_ptr(&datap);
+
+but, passing of pointers calculated via this_cpu_ptr to other cpus is
+unusual and should be avoided.
+
+Such remote accesses to per CPU data are not guaranteed to be "atomic"
+anymore. You will have to use atomic_t and rely on the standard atomic
+operations for these remote accesses to be atomic. An important thing to
+keep in mind while performing remote accesses is cache line sharing.
+Consider the following example
+
+
+ struct test {
+ atomic_t a;
+ int b;
+ };
+
+ DEFINE_PER_CPU(struct test, onecacheline);
+
+You cannot update the field 'a' remotely from one processor and expect
+this_cpu ops to work on the field b from the local CPU with atomic
+semantics. Care should be taken that such simultaneous access to data
+within the same cache line is avoided. Also costly synchronization is
+necessary when you are unsure of such cases. IPI is generally recommeded
+in such scenarios instead of remote write to per cpu areas.
+
+Even in cases where the remote writes are rare, please bear in
+mind that a remote write will evict the cache line from the processor
+that most likely will access it. If the processor wakes up and finds a
+missing local cache line of a per cpu area, its performance and hence
+the wake up times will be affected.
+
+Given these drawbacks, it is adviced to seriously consider the option of
+not using per cpu areas when there are considerable remote writes.
+
Christoph Lameter, April 3rd, 2013
+Pranith Kumar, Aug 2nd, 2014
--
1.9.1
^ permalink raw reply [flat|nested] 3+ messages in thread* Re: [PATCH v4 1/1] doc: this_cpu_ops.txt: clarify remote accesses of per cpu areas
2014-08-03 1:30 [PATCH v4 1/1] doc: this_cpu_ops.txt: clarify remote accesses of per cpu areas Pranith Kumar
@ 2014-08-04 14:59 ` Christoph Lameter
2014-08-04 15:30 ` Christoph Lameter
0 siblings, 1 reply; 3+ messages in thread
From: Christoph Lameter @ 2014-08-04 14:59 UTC (permalink / raw)
To: Pranith Kumar; +Cc: Randy Dunlap, open list:DOCUMENTATION, open list
On Sat, 2 Aug 2014, Pranith Kumar wrote:
> +Remote access to per cpu data
> +------------------------------
> +
> +Per cpu data structures are designed to be used by one cpu exclusively.
> +If you use the variables as intended, this_cpu_ops() are guaranteed to
> +be "atomic" as no other CPU has access to these data structures. In this
> +context, a remote access means an access to a per cpu data structure
> +from a CPU without using the this_cpu_* operations.
Access to the per cpu is valid in preempt safe sections by using
this_cpu_ptr() to calculate a basic addresa and then using this pointer
modifying per cpu variables.
> +There are special cases where you might need to access per cpu data
> +structures remotely. One such case is to perform maintenance tasks of
> +idle CPUs without having to wake them up using IPIs for power saving
> +purposes. It is usually safe to do a remote read access and that is
I am not sure what the "maintenance tasks" sentence is doing in here. This
looks like we are discussing read access. Maintenance implies write.
Regular remote access is a read acess to summarize counters and that
should be discussed first.
> +frequently done to summarize counters. Remote write access is the one
> +which is problematic. Remote write accesses to per cpu data structures
> +are highly discouraged until absolutely necessary. Please consider using
> +an IPI to wake up the remote CPU and perform the update to its per cpu
> +area.
> +
> +To access per-cpu data structure remotely, you need to convert the
> +per-cpu offset to a pointer using this_cpu_ptr().
> +
> + DEFINE_PER_CPU(struct data, datap);
> + struct data *p = per_cpu(&datap, cpu);
> +
> +p can now be passed to another CPU to be updated remotely.
Again this is not the typical way of doing things.
Remote cpus usually use per_cpu_ptr(percpustruct, cpu) to calculate the
adress of a variable in a remote percpu areas.
The approach presented here is not recommended because it implies the
adress of a local percpu struct being pushed to other cpus which may not
be aware of the nature of the storage area that they access. percpu_ptr()
makes it clear in a loop or so that we are performing remote percpu data
access now.
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH v4 1/1] doc: this_cpu_ops.txt: clarify remote accesses of per cpu areas
2014-08-04 14:59 ` Christoph Lameter
@ 2014-08-04 15:30 ` Christoph Lameter
0 siblings, 0 replies; 3+ messages in thread
From: Christoph Lameter @ 2014-08-04 15:30 UTC (permalink / raw)
To: Pranith Kumar; +Cc: Randy Dunlap, open list:DOCUMENTATION, open list
I updated the descriptions a bit more. Here is a patch with additional
changes to merge.
Subject: doc: Update description of this_cpu_ops
Update the description for per cpu operations to clarify use cases
if this_cpu operations and add considerations for remote access.
Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Signed-off-by: Christoph Lameter <cl@linux.com>
Index: linux/Documentation/this_cpu_ops.txt
===================================================================
--- linux.orig/Documentation/this_cpu_ops.txt 2014-08-04 10:03:30.669924060 -0500
+++ linux/Documentation/this_cpu_ops.txt 2014-08-04 10:29:08.860592601 -0500
@@ -2,26 +2,26 @@
-------------------
this_cpu operations are a way of optimizing access to per cpu
-variables associated with the *currently* executing processor through
-the use of segment registers (or a dedicated register where the cpu
-permanently stored the beginning of the per cpu area for a specific
-processor).
+variables associated with the *currently* executing processor. This is
+done through the use of segment registers (or a dedicated register where
+the cpu permanently stored the beginning of the per cpu area for a
+specific processor).
-The this_cpu operations add a per cpu variable offset to the processor
+this_cpu operations add a per cpu variable offset to the processor
specific percpu base and encode that operation in the instruction
operating on the per cpu variable.
-This means there are no atomicity issues between the calculation of
+This means that there are no atomicity issues between the calculation of
the offset and the operation on the data. Therefore it is not
-necessary to disable preempt or interrupts to ensure that the
+necessary to disable preemption or interrupts to ensure that the
processor is not changed between the calculation of the address and
the operation on the data.
Read-modify-write operations are of particular interest. Frequently
processors have special lower latency instructions that can operate
-without the typical synchronization overhead but still provide some
-sort of relaxed atomicity guarantee. The x86 for example can execute
-RMV (Read Modify Write) instructions like inc/dec/cmpxchg without the
+without the typical synchronization overhead, but still provide some
+sort of relaxed atomicity guarantees. The x86, for example, can execute
+RMW (Read Modify Write) instructions like inc/dec/cmpxchg without the
lock prefix and the associated latency penalty.
Access to the variable without the lock prefix is not synchronized but
@@ -30,6 +30,38 @@
processor should be accessing that variable and therefore there are no
concurrency issues with other processors in the system.
+Please note that accesses by remote processors to a per cpu area are
+exceptional situations and may impact performance and/or correctness
+(remote write operations) of local RMV operations via this_cpu_*.
+
+The main use of the this_cpu operations has been to optimize counter
+operations.
+
+The following this_cpu() operations with implied preemption protection
+are defined. These operations can be used without worrying about
+preemption and interrupts.
+
+ this_cpu_add()
+ this_cpu_read(pcp)
+ this_cpu_write(pcp, val)
+ this_cpu_add(pcp, val)
+ this_cpu_and(pcp, val)
+ this_cpu_or(pcp, val)
+ this_cpu_add_return(pcp, val)
+ this_cpu_xchg(pcp, nval)
+ this_cpu_cmpxchg(pcp, oval, nval)
+ this_cpu_cmpxchg_double(pcp1, pcp2, oval1, oval2, nval1, nval2)
+ this_cpu_sub(pcp, val)
+ this_cpu_inc(pcp)
+ this_cpu_dec(pcp)
+ this_cpu_sub_return(pcp, val)
+ this_cpu_inc_return(pcp)
+ this_cpu_dec_return(pcp)
+
+
+Inner working of this_cpu operations
+------------------------------------
+
On x86 the fs: or the gs: segment registers contain the base of the
per cpu area. It is then possible to simply use the segment override
to relocate a per cpu relative address to the proper per cpu area for
@@ -53,17 +85,16 @@
prevent the kernel from moving the thread to a different processor
while the calculation is performed.
-The main use of the this_cpu operations has been to optimize counter
-operations.
+Consider the following this_cpu operation:
this_cpu_inc(x)
-results in the following single instruction (no lock prefix!)
+The above results in the following single instruction (no lock prefix!)
inc gs:[x]
instead of the following operations required if there is no segment
-register.
+register:
int *y;
int cpu;
@@ -88,7 +119,7 @@
the same code paths. Since each processor has its own per cpu
variables no concurrent cacheline updates take place. The price that
has to be paid for this optimization is the need to add up the per cpu
-counters when the value of the counter is needed.
+counters when the value of a counter is needed.
Special operations:
@@ -100,9 +131,14 @@
of the per cpu variable that belongs to the currently executing
processor. this_cpu_ptr avoids multiple steps that the common
get_cpu/put_cpu sequence requires. No processor number is
-available. Instead the offset of the local per cpu area is simply
+available. Instead, the offset of the local per cpu area is simply
added to the percpu offset.
+Note that this operation is usually used in a code segment when
+preemption has been disabled. The pointer is then used to
+access local per cpu data in a critical section. When preemption
+is reenabled this pointer is usually no longer useful since it may
+no longer point to per cpu data of the current processor.
Per cpu variables and offsets
@@ -118,15 +154,16 @@
operations is invalid and will generally be treated like a NULL
pointer dereference.
-In the context of per cpu operations
+ DEFINE_PER_CPU(int, x);
- x is a per cpu variable. Most this_cpu operations take a cpu
- variable.
+In the context of per cpu operations the above implies that x is a per
+cpu variable. Most this_cpu operations take a cpu variable.
- &x is the *offset* a per cpu variable. this_cpu_ptr() takes
- the offset of a per cpu variable which makes this look a bit
- strange.
+ int __percpu *p = &x;
+&x and hence p is the *offset* of a per cpu variable. this_cpu_ptr()
+takes the offset of a per cpu variable which makes this look a bit
+strange.
Operations on a field of a per cpu structure
@@ -152,7 +189,7 @@
struct s __percpu *ps = &p;
- z = this_cpu_dec(ps->m);
+ this_cpu_dec(ps->m);
z = this_cpu_inc_return(ps->n);
@@ -172,16 +209,39 @@
Variants of this_cpu ops
-------------------------
-this_cpu ops are interrupt safe. Some architecture do not support
+this_cpu ops are interrupt safe. Some architectures do not support
these per cpu local operations. In that case the operation must be
replaced by code that disables interrupts, then does the operations
that are guaranteed to be atomic and then reenable interrupts. Doing
so is expensive. If there are other reasons why the scheduler cannot
change the processor we are executing on then there is no reason to
-disable interrupts. For that purpose the __this_cpu operations are
-provided. For example.
+disable interrupts. For that purpose the following __this_cpu operations
+are provided.
+
+These operations have no guarantee against concurrent interrupts or
+preemption. If a per cpu variable is not used in an interrupt context
+and the scheduler cannot preempt, then they are safe. If any interrupts
+still occur while an operation is in progress and if the interrupt too
+modifies the variable, then RMW actions can not be guaranteed to be
+safe.
+
+ __this_cpu_add()
+ __this_cpu_read(pcp)
+ __this_cpu_write(pcp, val)
+ __this_cpu_add(pcp, val)
+ __this_cpu_and(pcp, val)
+ __this_cpu_or(pcp, val)
+ __this_cpu_add_return(pcp, val)
+ __this_cpu_xchg(pcp, nval)
+ __this_cpu_cmpxchg(pcp, oval, nval)
+ __this_cpu_cmpxchg_double(pcp1, pcp2, oval1, oval2, nval1, nval2)
+ __this_cpu_sub(pcp, val)
+ __this_cpu_inc(pcp)
+ __this_cpu_dec(pcp)
+ __this_cpu_sub_return(pcp, val)
+ __this_cpu_inc_return(pcp)
+ __this_cpu_dec_return(pcp)
- __this_cpu_inc(x);
Will increment x and will not fallback to code that disables
interrupts on platforms that cannot accomplish atomicity through
@@ -189,12 +249,12 @@
instruction.
-
&this_cpu_ptr(pp)->n vs this_cpu_ptr(&pp->n)
--------------------------------------------
The first operation takes the offset and forms an address and then
-adds the offset of the n field.
+adds the offset of the n field. This may result in two add
+instructions emitted by the compiler.
The second one first adds the two offsets and then does the
relocation. IMHO the second form looks cleaner and has an easier time
@@ -202,4 +262,73 @@
this_cpu_read() and friends are used.
-Christoph Lameter, April 3rd, 2013
+Remote access to per cpu data
+------------------------------
+
+Per cpu data structures are designed to be used by one cpu exclusively.
+If you use the variables as intended, this_cpu_ops() are guaranteed to
+be "atomic" as no other CPU has access to these data structures.
+
+There are special cases where you might need to access per cpu data
+structures remotely. It is usually safe to do a remote read access
+and that is frequently done to summarize counters. Remote write access
+something which could be problematic because this_cpu ops do not
+have lock semantics. A remote write may interfere with a this_cpu
+RMV operation.
+
+Remote write accesses to percpu data structures are highly discouraged
+unless absolutely necessary. Please consider using an IPI to wake up
+the remote CPU and perform the update to its per cpu area.
+
+To access per-cpu data structure remotely, typically the per_cpu_ptr()
+function is used:
+
+
+ DEFINE_PER_CPU(struct data, datap);
+
+ struct data *p = per_cpu_ptr(&datap, cpu);
+
+This makes it explicit that we are getting ready to access a percpu
+area remotely.
+
+You can also do the following to convert the datap offset to an address
+
+ struct data *p = this_cpu_ptr(&datap);
+
+but, passing of pointers calculated via this_cpu_ptr to other cpus is
+unusual and should be avoided.
+
+Remote access are typically only for reading the status of another cpus
+per cppu data. Write accesses can cause unique problems due to the
+relaxed synchronization requirements for this_cpu operations.
+
+One example that illustrates some concerns with write operations is
+the following scenario that occurs because two per cpu variables
+share a cacheline but the relaxed synchronization is applied to
+only one process updating the cacheline.
+
+Consider the following example
+
+
+ struct test {
+ atomic_t a;
+ int b;
+ };
+
+ DEFINE_PER_CPU(struct test, onecacheline);
+
+There is some concern about what would happen if the field 'a' is updated
+remotely from one processor and the local processor would use this_cpu ops
+to update field b. Care should be taken that such simultaneous accesses to
+data within the same cache line are avoided. Also costly synchronization
+may be necessary. IPIs are generally recommeded in such scenarios instead
+of a remote write to the per cpu area of another processor.
+
+Even in cases where the remote writes are rare, please bear in
+mind that a remote write will evict the cache line from the processor
+that most likely will access it. If the processor wakes up and finds a
+missing local cache line of a per cpu area, its performance and hence
+the wake up times will be affected.
+
+Christoph Lameter, August 4th, 2014
+Pranith Kumar, Aug 2nd, 2014
^ permalink raw reply [flat|nested] 3+ messages in thread
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2014-08-04 15:30 ` Christoph Lameter
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