The padata parallel execution mechanism

Date:December 2019

Padata is a mechanism by which the kernel can farm jobs out to be done in parallel on multiple CPUs while retaining their ordering. It was developed for use with the IPsec code, which needs to be able to perform encryption and decryption on large numbers of packets without reordering those packets. The crypto developers made a point of writing padata in a sufficiently general fashion that it could be put to other uses as well.

Usage

Initializing

The first step in using padata is to set up a padata_instance structure for overall control of how jobs are to be run:

#include <linux/padata.h>

struct padata_instance *padata_alloc_possible(const char *name);

'name' simply identifies the instance.

There are functions for enabling and disabling the instance:

int padata_start(struct padata_instance *pinst);
void padata_stop(struct padata_instance *pinst);

These functions are setting or clearing the "PADATA_INIT" flag; if that flag is not set, other functions will refuse to work. padata_start() returns zero on success (flag set) or -EINVAL if the padata cpumask contains no active CPU (flag not set). padata_stop() clears the flag and blocks until the padata instance is unused.

Finally, complete padata initialization by allocating a padata_shell:

struct padata_shell *padata_alloc_shell(struct padata_instance *pinst);

A padata_shell is used to submit a job to padata and allows a series of such jobs to be serialized independently. A padata_instance may have one or more padata_shells associated with it, each allowing a separate series of jobs.

Modifying cpumasks

The CPUs used to run jobs can be changed in two ways, programatically with padata_set_cpumask() or via sysfs. The former is defined:

int padata_set_cpumask(struct padata_instance *pinst, int cpumask_type,
                       cpumask_var_t cpumask);

Here cpumask_type is one of PADATA_CPU_PARALLEL or PADATA_CPU_SERIAL, where a parallel cpumask describes which processors will be used to execute jobs submitted to this instance in parallel and a serial cpumask defines which processors are allowed to be used as the serialization callback processor. cpumask specifies the new cpumask to use.

There may be sysfs files for an instance's cpumasks. For example, pcrypt's live in /sys/kernel/pcrypt/<instance-name>. Within an instance's directory there are two files, parallel_cpumask and serial_cpumask, and either cpumask may be changed by echoing a bitmask into the file, for example:

echo f > /sys/kernel/pcrypt/pencrypt/parallel_cpumask

Reading one of these files shows the user-supplied cpumask, which may be different from the 'usable' cpumask.

Padata maintains two pairs of cpumasks internally, the user-supplied cpumasks and the 'usable' cpumasks. (Each pair consists of a parallel and a serial cpumask.) The user-supplied cpumasks default to all possible CPUs on instance allocation and may be changed as above. The usable cpumasks are always a subset of the user-supplied cpumasks and contain only the online CPUs in the user-supplied masks; these are the cpumasks padata actually uses. So it is legal to supply a cpumask to padata that contains offline CPUs. Once an offline CPU in the user-supplied cpumask comes online, padata is going to use it.

Changing the CPU masks are expensive operations, so it should not be done with great frequency.

Running A Job

Actually submitting work to the padata instance requires the creation of a padata_priv structure, which represents one job:

struct padata_priv {
    /* Other stuff here... */
    void                    (*parallel)(struct padata_priv *padata);
    void                    (*serial)(struct padata_priv *padata);
};

This structure will almost certainly be embedded within some larger structure specific to the work to be done. Most of its fields are private to padata, but the structure should be zeroed at initialisation time, and the parallel() and serial() functions should be provided. Those functions will be called in the process of getting the work done as we will see momentarily.

The submission of the job is done with:

int padata_do_parallel(struct padata_shell *ps,
                       struct padata_priv *padata, int *cb_cpu);

The ps and padata structures must be set up as described above; cb_cpu points to the preferred CPU to be used for the final callback when the job is done; it must be in the current instance's CPU mask (if not the cb_cpu pointer is updated to point to the CPU actually chosen). The return value from padata_do_parallel() is zero on success, indicating that the job is in progress. -EBUSY means that somebody, somewhere else is messing with the instance's CPU mask, while -EINVAL is a complaint about cb_cpu not being in the serial cpumask, no online CPUs in the parallel or serial cpumasks, or a stopped instance.

Each job submitted to padata_do_parallel() will, in turn, be passed to exactly one call to the above-mentioned parallel() function, on one CPU, so true parallelism is achieved by submitting multiple jobs. parallel() runs with software interrupts disabled and thus cannot sleep. The parallel() function gets the padata_priv structure pointer as its lone parameter; information about the actual work to be done is probably obtained by using container_of() to find the enclosing structure.

Note that parallel() has no return value; the padata subsystem assumes that parallel() will take responsibility for the job from this point. The job need not be completed during this call, but, if parallel() leaves work outstanding, it should be prepared to be called again with a new job before the previous one completes.

Serializing Jobs

When a job does complete, parallel() (or whatever function actually finishes the work) should inform padata of the fact with a call to:

void padata_do_serial(struct padata_priv *padata);

At some point in the future, padata_do_serial() will trigger a call to the serial() function in the padata_priv structure. That call will happen on the CPU requested in the initial call to padata_do_parallel(); it, too, is run with local software interrupts disabled. Note that this call may be deferred for a while since the padata code takes pains to ensure that jobs are completed in the order in which they were submitted.

Destroying

Cleaning up a padata instance predictably involves calling the three free functions that correspond to the allocation in reverse:

void padata_free_shell(struct padata_shell *ps);
void padata_stop(struct padata_instance *pinst);
void padata_free(struct padata_instance *pinst);

It is the user's responsibility to ensure all outstanding jobs are complete before any of the above are called.

Interface

struct padata_priv

Represents one job

Definition

struct padata_priv {
  struct list_head        list;
  struct parallel_data    *pd;
  int cb_cpu;
  int cpu;
  unsigned int            seq_nr;
  int info;
  void (*parallel)(struct padata_priv *padata);
  void (*serial)(struct padata_priv *padata);
};

Members

list
List entry, to attach to the padata lists.
pd
Pointer to the internal control structure.
cb_cpu
Callback cpu for serializatioon.
cpu
Cpu for parallelization.
seq_nr
Sequence number of the parallelized data object.
info
Used to pass information from the parallel to the serial function.
parallel
Parallel execution function.
serial
Serial complete function.
struct padata_list

one per work type per CPU

Definition

struct padata_list {
  struct list_head        list;
  spinlock_t lock;
};

Members

list
List head.
lock
List lock.
struct padata_serial_queue

The percpu padata serial queue

Definition

struct padata_serial_queue {
  struct padata_list    serial;
  struct work_struct    work;
  struct parallel_data *pd;
};

Members

serial
List to wait for serialization after reordering.
work
work struct for serialization.
pd
Backpointer to the internal control structure.
struct padata_parallel_queue

The percpu padata parallel queue

Definition

struct padata_parallel_queue {
  struct padata_list    parallel;
  struct padata_list    reorder;
  struct work_struct    work;
  atomic_t num_obj;
};

Members

parallel
List to wait for parallelization.
reorder
List to wait for reordering after parallel processing.
work
work struct for parallelization.
num_obj
Number of objects that are processed by this cpu.
struct padata_cpumask

The cpumasks for the parallel/serial workers

Definition

struct padata_cpumask {
  cpumask_var_t pcpu;
  cpumask_var_t cbcpu;
};

Members

pcpu
cpumask for the parallel workers.
cbcpu
cpumask for the serial (callback) workers.
struct parallel_data

Internal control structure, covers everything that depends on the cpumask in use.

Definition

struct parallel_data {
  struct padata_shell             *ps;
  struct padata_parallel_queue    __percpu *pqueue;
  struct padata_serial_queue      __percpu *squeue;
  atomic_t refcnt;
  atomic_t seq_nr;
  unsigned int                    processed;
  int cpu;
  struct padata_cpumask           cpumask;
  struct work_struct              reorder_work;
  spinlock_t ____cacheline_aligned lock;
};

Members

ps
padata_shell object.
pqueue
percpu padata queues used for parallelization.
squeue
percpu padata queues used for serialuzation.
refcnt
Number of objects holding a reference on this parallel_data.
seq_nr
Sequence number of the parallelized data object.
processed
Number of already processed objects.
cpu
Next CPU to be processed.
cpumask
The cpumasks in use for parallel and serial workers.
reorder_work
work struct for reordering.
lock
Reorder lock.
struct padata_shell

Wrapper around struct parallel_data, its purpose is to allow the underlying control structure to be replaced on the fly using RCU.

Definition

struct padata_shell {
  struct padata_instance          *pinst;
  struct parallel_data __rcu      *pd;
  struct parallel_data            *opd;
  struct list_head                list;
};

Members

pinst
padat instance.
pd
Actual parallel_data structure which may be substituted on the fly.
opd
Pointer to old pd to be freed by padata_replace.
list
List entry in padata_instance list.
struct padata_instance

The overall control structure.

Definition

struct padata_instance {
  struct hlist_node                node;
  struct workqueue_struct         *parallel_wq;
  struct workqueue_struct         *serial_wq;
  struct list_head                pslist;
  struct padata_cpumask           cpumask;
  struct padata_cpumask           rcpumask;
  struct kobject                   kobj;
  struct mutex                     lock;
  u8 flags;
#define PADATA_INIT     1;
#define PADATA_RESET    2;
#define PADATA_INVALID  4;
};

Members

node
Used by CPU hotplug.
parallel_wq
The workqueue used for parallel work.
serial_wq
The workqueue used for serial work.
pslist
List of padata_shell objects attached to this instance.
cpumask
User supplied cpumasks for parallel and serial works.
rcpumask
Actual cpumasks based on user cpumask and cpu_online_mask.
kobj
padata instance kernel object.
lock
padata instance lock.
flags
padata flags.
int padata_do_parallel(struct padata_shell * ps, struct padata_priv * padata, int * cb_cpu)

padata parallelization function

Parameters

struct padata_shell * ps
padatashell
struct padata_priv * padata
object to be parallelized
int * cb_cpu
pointer to the CPU that the serialization callback function should run on. If it's not in the serial cpumask of pinst (i.e. cpumask.cbcpu), this function selects a fallback CPU and if none found, returns -EINVAL.

Description

The parallelization callback function will run with BHs off.

Note

Every object which is parallelized by padata_do_parallel must be seen by padata_do_serial.

Return

0 on success or else negative error code.

void padata_do_serial(struct padata_priv * padata)

padata serialization function

Parameters

struct padata_priv * padata
object to be serialized.

Description

padata_do_serial must be called for every parallelized object. The serialization callback function will run with BHs off.

int padata_set_cpumask(struct padata_instance * pinst, int cpumask_type, cpumask_var_t cpumask)

Sets specified by cpumask_type cpumask to the value equivalent to cpumask.

Parameters

struct padata_instance * pinst
padata instance
int cpumask_type
PADATA_CPU_SERIAL or PADATA_CPU_PARALLEL corresponding to parallel and serial cpumasks respectively.
cpumask_var_t cpumask
the cpumask to use

Return

0 on success or negative error code

int padata_start(struct padata_instance * pinst)

start the parallel processing

Parameters

struct padata_instance * pinst
padata instance to start

Return

0 on success or negative error code

void padata_stop(struct padata_instance * pinst)

stop the parallel processing

Parameters

struct padata_instance * pinst
padata instance to stop
struct padata_instance * padata_alloc(const char * name, const struct cpumask * pcpumask, const struct cpumask * cbcpumask)

allocate and initialize a padata instance and specify cpumasks for serial and parallel workers.

Parameters

const char * name
used to identify the instance
const struct cpumask * pcpumask
cpumask that will be used for padata parallelization
const struct cpumask * cbcpumask
cpumask that will be used for padata serialization

Return

new instance on success, NULL on error

struct padata_instance * padata_alloc_possible(const char * name)

Allocate and initialize padata instance. Use the cpu_possible_mask for serial and parallel workers.

Parameters

const char * name
used to identify the instance

Return

new instance on success, NULL on error

void padata_free(struct padata_instance * pinst)

free a padata instance

Parameters

struct padata_instance * pinst
padata instance to free
struct padata_shell * padata_alloc_shell(struct padata_instance * pinst)

Allocate and initialize padata shell.

Parameters

struct padata_instance * pinst
Parent padata_instance object.

Return

new shell on success, NULL on error

void padata_free_shell(struct padata_shell * ps)

free a padata shell

Parameters

struct padata_shell * ps
padata shell to free