RE: [PATCH 1/6] eal: add static per-lcore memory allocation facility

["Morten Brørup" <mb@smartsharesystems.com>]

> From: Mattias Rönnblom [mailto:mattias.ronnblom@ericsson.com]
> Sent: Tuesday, 10 September 2024 09.04
> 
> Introduce DPDK per-lcore id variables, or lcore variables for short.

Throughout the descriptions and comments,
please replace "lcore id" with "lcore" (e.g. "per-lcore variables"),
when referring to the lcore, and not the index of the lcore.
(Your intention might be to highlight that it only covers threads with an lcore id,
but if that wasn't the case, you would refer to them as "threads" not "lcores".)
Except, of course, when referring to an actual lcore id, e.g. lcore_id function parameters.

Paraphrasing:
Consider the type of what you are referring to;
use "lcore" if its type is "thread", and
use "lcore id" if its type is "int".

I might be wrong here, but please think hard about it.

> 
> An lcore variable has one value for every current and future lcore
> id-equipped thread.
> 
> The primary <rte_lcore_var.h> use case is for statically allocating
> small, frequently-accessed data structures, for which one instance
> should exist for each lcore.
> 
> Lcore variables are similar to thread-local storage (TLS, e.g., C11
> _Thread_local), but decoupling the values' life time with that of the
> threads.
> 
> Lcore variables are also similar in terms of functionality provided by
> FreeBSD kernel's DPCPU_*() family of macros and the associated
> build-time machinery. DPCPU uses linker scripts, which effectively
> prevents the reuse of its, otherwise seemingly viable, approach.
> 
> The currently-prevailing way to solve the same problem as lcore
> variables is to keep a module's per-lcore data as RTE_MAX_LCORE-sized
> array of cache-aligned, RTE_CACHE_GUARDed structs. The benefit of
> lcore variables over this approach is that data related to the same
> lcore now is close (spatially, in memory), rather than data used by
> the same module, which in turn avoid excessive use of padding,
> polluting caches with unused data.
> 
> Signed-off-by: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
> Acked-by: Morten Brørup <mb@smartsharesystems.com>
> 
> --

> +++ b/doc/api/doxy-api-index.md
> @@ -99,6 +99,7 @@ The public API headers are grouped by topics:
>    [interrupts](@ref rte_interrupts.h),
>    [launch](@ref rte_launch.h),
>    [lcore](@ref rte_lcore.h),
> +  [lcore-varible](@ref rte_lcore_var.h),

Typo: varible -> variable


> +++ b/doc/guides/rel_notes/release_24_11.rst
> @@ -55,6 +55,20 @@ New Features
>       Also, make sure to start the actual text at the margin.
>       =======================================================
> 
> +* **Added EAL per-lcore static memory allocation facility.**
> +
> +    Added EAL API <rte_lcore_var.h> for statically allocating small,
> +    frequently-accessed data structures, for which one instance should
> +    exist for each lcore.
> +
> +    With lcore variables, data is organized spatially on a per-lcore
> +    basis, rather than per library or PMD, avoiding the need for cache
> +    aligning (or RTE_CACHE_GUARDing) data structures, which in turn
> +    reduces CPU cache internal fragmentation, improving performance.
> +
> +    Lcore variables are similar to thread-local storage (TLS, e.g.,
> +    C11 _Thread_local), but decoupling the values' life time from that
> +    of the threads.

When referring to TLS, you might want to clarify that lcore variables are not instantiated for unregistered threads.


> +static void *lcore_buffer;
> +static size_t offset = RTE_MAX_LCORE_VAR;
> +
> +static void *
> +lcore_var_alloc(size_t size, size_t align)
> +{
> +	void *handle;
> +	void *value;
> +
> +	offset = RTE_ALIGN_CEIL(offset, align);
> +
> +	if (offset + size > RTE_MAX_LCORE_VAR) {
> +		lcore_buffer = aligned_alloc(RTE_CACHE_LINE_SIZE,
> +					     LCORE_BUFFER_SIZE);
> +		RTE_VERIFY(lcore_buffer != NULL);
> +
> +		offset = 0;
> +	}

To determine if the lcore_buffer memory should be allocated, why not just check if lcore_buffer == NULL?
Then offset wouldn't need an initial value of RTE_MAX_LCORE_VAR.

> +
> +	handle = RTE_PTR_ADD(lcore_buffer, offset);
> +
> +	offset += size;
> +
> +	RTE_LCORE_VAR_FOREACH_VALUE(value, handle)
> +		memset(value, 0, size);
> +
> +	EAL_LOG(DEBUG, "Allocated %"PRIuPTR" bytes of per-lcore data with a "
> +		"%"PRIuPTR"-byte alignment", size, align);
> +
> +	return handle;
> +}


> +/**
> + * @file
> + *
> + * RTE Per-lcore id variables

Suggest mentioning the short form too, e.g.:
"RTE Per-lcore id variables (RTE Lcore variables)"

> + *
> + * This API provides a mechanism to create and access per-lcore id
> + * variables in a space- and cycle-efficient manner.
> + *
> + * A per-lcore id variable (or lcore variable for short) has one value
> + * for each EAL thread and registered non-EAL thread.

And service thread.

> + * There is one
> + * copy for each current and future lcore id-equipped thread, with the

"one copy" -> "one instance"

> + * total number of copies amounting to @c RTE_MAX_LCORE. The value of

"copies" -> "instances"

> + * an lcore variable for a particular lcore id is independent from
> + * other values (for other lcore ids) within the same lcore variable.
> + *
> + * In order to access the values of an lcore variable, a handle is
> + * used. The type of the handle is a pointer to the value's type
> + * (e.g., for @c uint32_t lcore variable, the handle is a
> + * <code>uint32_t *</code>. The handler type is used to inform the

Typo: "handler" -> "handle", I think :-/
Found this typo multiple times; search-replace.

> + * access macros the type of the values. A handle may be passed
> + * between modules and threads just like any pointer, but its value
> + * must be treated as a an opaque identifier. An allocated handle
> + * never has the value NULL.
> + *
> + * @b Creation
> + *
> + * An lcore variable is created in two steps:
> + *  1. Define a lcore variable handle by using @ref RTE_LCORE_VAR_HANDLE.
> + *  2. Allocate lcore variable storage and initialize the handle with
> + *     a unique identifier by @ref RTE_LCORE_VAR_ALLOC or
> + *     @ref RTE_LCORE_VAR_INIT. Allocation generally occurs the time of
> + *     module initialization, but may be done at any time.
> + *
> + * An lcore variable is not tied to the owning thread's lifetime. It's
> + * available for use by any thread immediately after having been
> + * allocated, and continues to be available throughout the lifetime of
> + * the EAL.
> + *
> + * Lcore variables cannot and need not be freed.
> + *
> + * @b Access
> + *
> + * The value of any lcore variable for any lcore id may be accessed
> + * from any thread (including unregistered threads), but it should
> + * only be *frequently* read from or written to by the owner.
> + *
> + * Values of the same lcore variable but owned by to different lcore

Typo: to -> two

> + * ids may be frequently read or written by the owners without risking
> + * false sharing.
> + *
> + * An appropriate synchronization mechanism (e.g., atomic loads and
> + * stores) should employed to assure there are no data races between
> + * the owning thread and any non-owner threads accessing the same
> + * lcore variable instance.
> + *
> + * The value of the lcore variable for a particular lcore id is
> + * accessed using @ref RTE_LCORE_VAR_LCORE_VALUE.
> + *
> + * A common pattern is for an EAL thread or a registered non-EAL
> + * thread to access its own lcore variable value. For this purpose, a
> + * short-hand exists in the form of @ref RTE_LCORE_VAR_VALUE.
> + *
> + * Although the handle (as defined by @ref RTE_LCORE_VAR_HANDLE) is a
> + * pointer with the same type as the value, it may not be directly
> + * dereferenced and must be treated as an opaque identifier.
> + *
> + * Lcore variable handles and value pointers may be freely passed
> + * between different threads.
> + *
> + * @b Storage
> + *
> + * An lcore variable's values may by of a primitive type like @c int,

Two typos: "values may by" -> "value may be"

> + * but would more typically be a @c struct.
> + *
> + * The lcore variable handle introduces a per-variable (not
> + * per-value/per-lcore id) overhead of @c sizeof(void *) bytes, so
> + * there are some memory footprint gains to be made by organizing all
> + * per-lcore id data for a particular module as one lcore variable
> + * (e.g., as a struct).
> + *
> + * An application may choose to define an lcore variable handle, which
> + * it then it goes on to never allocate.
> + *
> + * The size of a lcore variable's value must be less than the DPDK
> + * build-time constant @c RTE_MAX_LCORE_VAR.
> + *
> + * The lcore variable are stored in a series of lcore buffers, which
> + * are allocated from the libc heap. Heap allocation failures are
> + * treated as fatal.
> + *
> + * Lcore variables should generally *not* be @ref __rte_cache_aligned
> + * and need *not* include a @ref RTE_CACHE_GUARD field, since the use
> + * of these constructs are designed to avoid false sharing. In the
> + * case of an lcore variable instance, the thread most recently
> + * accessing nearby data structures should almost-always the lcore

Missing word: should almost-always *be* the lcore variables' owner.


> + * variables' owner. Adding padding will increase the effective memory
> + * working set size, potentially reducing performance.
> + *
> + * Lcore variable values take on an initial value of zero.
> + *
> + * @b Example
> + *
> + * Below is an example of the use of an lcore variable:
> + *
> + * @code{.c}
> + * struct foo_lcore_state {
> + *         int a;
> + *         long b;
> + * };
> + *
> + * static RTE_LCORE_VAR_HANDLE(struct foo_lcore_state, lcore_states);
> + *
> + * long foo_get_a_plus_b(void)
> + * {
> + *         struct foo_lcore_state *state = RTE_LCORE_VAR_VALUE(lcore_states);
> + *
> + *         return state->a + state->b;
> + * }
> + *
> + * RTE_INIT(rte_foo_init)
> + * {
> + *         RTE_LCORE_VAR_ALLOC(lcore_states);
> + *
> + *         struct foo_lcore_state *state;
> + *         RTE_LCORE_VAR_FOREACH_VALUE(state, lcore_states) {
> + *                 (initialize 'state')

Consider: (initialize 'state') -> /* initialize 'state' */

> + *         }
> + *
> + *         (other initialization)

Consider: (other initialization) -> /* other initialization */

> + * }
> + * @endcode
> + *
> + *
> + * @b Alternatives
> + *
> + * Lcore variables are designed to replace a pattern exemplified below:
> + * @code{.c}
> + * struct __rte_cache_aligned foo_lcore_state {
> + *         int a;
> + *         long b;
> + *         RTE_CACHE_GUARD;
> + * };
> + *
> + * static struct foo_lcore_state lcore_states[RTE_MAX_LCORE];
> + * @endcode
> + *
> + * This scheme is simple and effective, but has one drawback: the data
> + * is organized so that objects related to all lcores for a particular
> + * module is kept close in memory. At a bare minimum, this forces the
> + * use of cache-line alignment to avoid false sharing. With CPU

Consider adding: use of *padding to* cache-line alignment
My point here is:
This sentence should somehow include the word "padding".
This paragraph is not only aboud cache line alignment, it is primarily about padding.

> + * hardware prefetching and memory loads resulting from speculative
> + * execution (functions which seemingly are getting more eager faster
> + * than they are getting more intelligent), one or more "guard" cache
> + * lines may be required to separate one lcore's data from another's.
> + *
> + * Lcore variables has the upside of working with, not against, the

Typo: has -> have

> + * CPU's assumptions and for example next-line prefetchers may well
> + * work the way its designers intended (i.e., to the benefit, not
> + * detriment, of system performance).
> + *
> + * Another alternative to @ref rte_lcore_var.h is the @ref
> + * rte_per_lcore.h API, which make use of thread-local storage (TLS,

Typo: make -> makes

> + * e.g., GCC __thread or C11 _Thread_local). The main differences
> + * between by using the various forms of TLS (e.g., @ref
> + * RTE_DEFINE_PER_LCORE or _Thread_local) and the use of lcore
> + * variables are:
> + *
> + *   * The existence and non-existence of a thread-local variable
> + *     instance follow that of particular thread's. The data cannot be

Typo: "thread's" -> "threads", I think. :-/

> + *     accessed before the thread has been created, nor after it has
> + *     exited. As a result, thread-local variables must initialized in

Missing word: must *be* initialized

> + *     a "lazy" manner (e.g., at the point of thread creation). Lcore
> + *     variables may be accessed immediately after having been
> + *     allocated (which may be prior any thread beyond the main
> + *     thread is running).
> + *   * A thread-local variable is duplicated across all threads in the
> + *     process, including unregistered non-EAL threads (i.e.,
> + *     "regular" threads). For DPDK applications heavily relying on
> + *     multi-threading (in conjunction to DPDK's "one thread per core"
> + *     pattern), either by having many concurrent threads or
> + *     creating/destroying threads at a high rate, an excessive use of
> + *     thread-local variables may cause inefficiencies (e.g.,
> + *     increased thread creation overhead due to thread-local storage
> + *     initialization or increased total RAM footprint usage). Lcore
> + *     variables *only* exist for threads with an lcore id.
> + *   * If data in thread-local storage may be shared between threads
> + *     (i.e., can a pointer to a thread-local variable be passed to
> + *     and successfully dereferenced by non-owning thread) depends on
> + *     the details of the TLS implementation. With GCC __thread and
> + *     GCC _Thread_local, such data sharing is supported. In the C11
> + *     standard, the result of accessing another thread's
> + *     _Thread_local object is implementation-defined. Lcore variable
> + *     instances may be accessed reliably by any thread.
> + */
> +
> +#include <stddef.h>
> +#include <stdalign.h>
> +
> +#include <rte_common.h>
> +#include <rte_config.h>
> +#include <rte_lcore.h>
> +
> +#ifdef __cplusplus
> +extern "C" {
> +#endif
> +
> +/**
> + * Given the lcore variable type, produces the type of the lcore
> + * variable handle.
> + */
> +#define RTE_LCORE_VAR_HANDLE_TYPE(type)		\
> +	type *
> +
> +/**
> + * Define a lcore variable handle.

Typo: "a lcore" -> "an lcore"
Found this typo multiple times; search-replace "a lcore".

> + *
> + * This macro defines a variable which is used as a handle to access
> + * the various per-lcore id instances of a per-lcore id variable.

Suggest:
"the various per-lcore id instances of a per-lcore id variable" ->
"the various instances of a per-lcore id variable"

> + *
> + * The aim with this macro is to make clear at the point of
> + * declaration that this is an lcore handler, rather than a regular
> + * pointer.
> + *
> + * Add @b static as a prefix in case the lcore variable are only to be

Typo: are -> is

> + * accessed from a particular translation unit.
> + */
> +#define RTE_LCORE_VAR_HANDLE(type, name)	\
> +	RTE_LCORE_VAR_HANDLE_TYPE(type) name
> +
> +/**
> + * Allocate space for an lcore variable, and initialize its handle.
> + *
> + * The values of the lcore variable are initialized to zero.

Consider adding: "the lcore variable *instances* are initialized"
Found this typo multiple times; search-replace.

> + */
> +#define RTE_LCORE_VAR_ALLOC_SIZE_ALIGN(handle, size, align)	\
> +	handle = rte_lcore_var_alloc(size, align)
> +
> +/**
> + * Allocate space for an lcore variable, and initialize its handle,
> + * with values aligned for any type of object.
> + *
> + * The values of the lcore variable are initialized to zero.
> + */
> +#define RTE_LCORE_VAR_ALLOC_SIZE(handle, size)	\
> +	RTE_LCORE_VAR_ALLOC_SIZE_ALIGN(handle, size, 0)
> +
> +/**
> + * Allocate space for an lcore variable of the size and alignment
> requirements
> + * suggested by the handler pointer type, and initialize its handle.
> + *
> + * The values of the lcore variable are initialized to zero.
> + */
> +#define RTE_LCORE_VAR_ALLOC(handle)					\
> +	RTE_LCORE_VAR_ALLOC_SIZE_ALIGN(handle, sizeof(*(handle)),	\
> +				       alignof(typeof(*(handle))))
> +
> +/**
> + * Allocate an explicitly-sized, explicitly-aligned lcore variable by
> + * means of a @ref RTE_INIT constructor.
> + *
> + * The values of the lcore variable are initialized to zero.
> + */
> +#define RTE_LCORE_VAR_INIT_SIZE_ALIGN(name, size, align)		\
> +	RTE_INIT(rte_lcore_var_init_ ## name)				\
> +	{								\
> +		RTE_LCORE_VAR_ALLOC_SIZE_ALIGN(name, size, align);	\
> +	}
> +
> +/**
> + * Allocate an explicitly-sized lcore variable by means of a @ref
> + * RTE_INIT constructor.
> + *
> + * The values of the lcore variable are initialized to zero.
> + */
> +#define RTE_LCORE_VAR_INIT_SIZE(name, size)		\
> +	RTE_LCORE_VAR_INIT_SIZE_ALIGN(name, size, 0)
> +
> +/**
> + * Allocate an lcore variable by means of a @ref RTE_INIT constructor.
> + *
> + * The values of the lcore variable are initialized to zero.
> + */
> +#define RTE_LCORE_VAR_INIT(name)					\
> +	RTE_INIT(rte_lcore_var_init_ ## name)				\
> +	{								\
> +		RTE_LCORE_VAR_ALLOC(name);				\
> +	}
> +
> +/**
> + * Get void pointer to lcore variable instance with the specified
> + * lcore id.
> + *
> + * @param lcore_id
> + *   The lcore id specifying which of the @c RTE_MAX_LCORE value
> + *   instances should be accessed. The lcore id need not be valid
> + *   (e.g., may be @ref LCORE_ID_ANY), but in such a case, the pointer
> + *   is also not valid (and thus should not be dereferenced).
> + * @param handle
> + *   The lcore variable handle.
> + */
> +static inline void *
> +rte_lcore_var_lcore_ptr(unsigned int lcore_id, void *handle)
> +{
> +	return RTE_PTR_ADD(handle, lcore_id * RTE_MAX_LCORE_VAR);
> +}
> +
> +/**
> + * Get pointer to lcore variable instance with the specified lcore id.
> + *
> + * @param lcore_id
> + *   The lcore id specifying which of the @c RTE_MAX_LCORE value
> + *   instances should be accessed. The lcore id need not be valid
> + *   (e.g., may be @ref LCORE_ID_ANY), but in such a case, the pointer
> + *   is also not valid (and thus should not be dereferenced).
> + * @param handle
> + *   The lcore variable handle.
> + */
> +#define RTE_LCORE_VAR_LCORE_VALUE(lcore_id, handle)			\
> +	((typeof(handle))rte_lcore_var_lcore_ptr(lcore_id, handle))
> +
> +/**
> + * Get pointer to lcore variable instance of the current thread.
> + *
> + * May only be used by EAL threads and registered non-EAL threads.
> + */
> +#define RTE_LCORE_VAR_VALUE(handle) \
> +	RTE_LCORE_VAR_LCORE_VALUE(rte_lcore_id(), handle)
> +
> +/**
> + * Iterate over each lcore id's value for a lcore variable.
> + *
> + * @param value
> + *   A pointer set successivly set to point to lcore variable value

"set successivly set" -> "successivly set"

Thinking out loud, ignore at your preference:
During the RFC discussions, the term used for referring to an lcore variable was discussed;
we considered "pointer", but settled for "value".
Perhaps "instance" would be usable in comments like like the one describing this function...
"A pointer set successivly set to point to lcore variable value" ->
"A pointer set successivly set to point to lcore variable instance".
I don't know.


> + *   corresponding to every lcore id (up to @c RTE_MAX_LCORE).
> + * @param handle
> + *   The lcore variable handle.
> + */
> +#define RTE_LCORE_VAR_FOREACH_VALUE(value, handle)			\
> +	for (unsigned int lcore_id =					\
> +		     (((value) = RTE_LCORE_VAR_LCORE_VALUE(0, handle)), 0); \
> +	     lcore_id < RTE_MAX_LCORE;					\
> +	     lcore_id++, (value) = RTE_LCORE_VAR_LCORE_VALUE(lcore_id, handle))
> +
> +/**
> + * Allocate space in the per-lcore id buffers for a lcore variable.
> + *
> + * The pointer returned is only an opaque identifer of the variable. To
> + * get an actual pointer to a particular instance of the variable use
> + * @ref RTE_LCORE_VAR_VALUE or @ref RTE_LCORE_VAR_LCORE_VALUE.
> + *
> + * The lcore variable values' memory is set to zero.
> + *
> + * The allocation is always successful, barring a fatal exhaustion of
> + * the per-lcore id buffer space.
> + *
> + * rte_lcore_var_alloc() is not multi-thread safe.
> + *
> + * @param size
> + *   The size (in bytes) of the variable's per-lcore id value. Must be > 0.
> + * @param align
> + *   If 0, the values will be suitably aligned for any kind of type
> + *   (i.e., alignof(max_align_t)). Otherwise, the values will be aligned
> + *   on a multiple of *align*, which must be a power of 2 and equal or
> + *   less than @c RTE_CACHE_LINE_SIZE.
> + * @return
> + *   The id of the variable, stored in a void pointer value. The value

"id" -> "handle"

> + *   is always non-NULL.
> + */
> +__rte_experimental
> +void *
> +rte_lcore_var_alloc(size_t size, size_t align);
> +
> +#ifdef __cplusplus
> +}
> +#endif
> +
> +#endif /* _RTE_LCORE_VAR_H_ */
> diff --git a/lib/eal/version.map b/lib/eal/version.map
> index e3ff412683..5f5a3522c0 100644
> --- a/lib/eal/version.map
> +++ b/lib/eal/version.map
> @@ -396,6 +396,9 @@ EXPERIMENTAL {
> 
>  	# added in 24.03
>  	rte_vfio_get_device_info; # WINDOWS_NO_EXPORT
> +
> +	rte_lcore_var_alloc;
> +	rte_lcore_var;

No such function: rte_lcore_var

>  };
> 
>  INTERNAL {
> --
> 2.34.1