SPANK(8)

NAME
SPANK - Slurm Plug-in Architecture for Node and job (K)control

DESCRIPTION
This manual briefly describes the capabilities of the Slurm Plug-in Architecture for Node and job Kontrol (SPANK) as well as the SPANK configuration file: (By default: plugstack.conf.)

SPANK provides a very generic interface for stackable plug-ins which may be used to dynamically modify the job launch code in Slurm. SPANK plugins may be built without access to Slurm source code. They need only be compiled against Slurm's spank.h header file, added to the SPANK config file plugstack.conf, and they will be loaded at runtime during the next job launch. Thus, the SPANK infrastructure provides administrators and other developers a low cost, low effort ability to dynamically modify the runtime behavior of Slurm job launch.

NOTE: All SPANK plugins should be recompiled when upgrading Slurm to a new major release. The SPANK API is not guaranteed to be ABI compatible between major releases. Any SPANK plugin linking to any of the Slurm libraries should be carefully checked as the Slurm APIs and headers can change between major releases.

SPANK PLUGINS
SPANK plugins are loaded in up to five separate contexts during a Slurm job. Briefly, the five contexts are:

local
    In local context, the plugin is loaded by srun. (i.e. the "local" part of a parallel job).

remote
    In remote context, the plugin is loaded by slurmstepd. (i.e. the "remote" part of a parallel job).

allocator
    In allocator context, the plugin is loaded in one of the job allocation utilities salloc, sbatch or scrontab.

slurmd
    In slurmd context, the plugin is loaded in the slurmd daemon itself. NOTE: Plugins loaded in slurmd context persist for the entire time slurmd is running, so if configuration is changed or plugins are updated, slurmd must be restarted for the changes to take effect.

job_script
    In the job_script context, plugins are loaded in the context of the job prolog or epilog. NOTE: Plugins are loaded in job_script context on each run on the job prolog or epilog, in a separate address space from plugins in slurmd context. This means there is no state shared between this context and other contexts, or even between one call to slurm_spank_job_prolog or slurm_spank_job_epilog and subsequent calls.

In local context, only the init, exit, init_post_opt, and local_user_init functions are called. In allocator context, only the init, exit, and init_post_opt functions are called. Similarly, in slurmd context, only the init and slurmd_exit callbacks are active, and in the job_script context, only the job_prolog and job_epilog callbacks are used. Plugins may query the context in which they are running with the spank_context and spank_remote functions defined in spank.h.

SPANK plugins may be called from multiple points during the Slurm job launch. A plugin may define the following functions:

slurm_spank_init
    Called just after plugins are loaded. In remote context, this is just after job step is initialized. This function is called before any plugin option processing.

slurm_spank_job_prolog
    Called at the same time as the job prolog. If this function returns a non-zero value and the SPANK plugin that contains it is required in the plugstack.conf, the node that this is run on will be drained.

slurm_spank_init_post_opt
    Called at the same point as slurm_spank_init, but after all user options to the plugin have been processed. The reason that the init and init_post_opt callbacks are separated is so that plugins can process system-wide options specified in plugstack.conf in the init callback, then process user options, and finally take some action in slurm_spank_init_post_opt if necessary. In the case of a heterogeneous job, slurm_spank_init is invoked once per job component.

slurm_spank_local_user_init
    Called in local (srun) context only after all options have been processed. This is called after the job ID and step IDs are available. This happens in srun after the allocation is made, but before tasks are launched.

slurm_spank_user_init
    Called after privileges are temporarily dropped. (remote context only)

slurm_spank_task_init_privileged
    Called for each task just after fork, but before all elevated privileges are dropped. (remote context only)

slurm_spank_task_init
    Called for each task just before execve (2). If you are restricting memory with cgroups, memory allocated here will be in the job's cgroup. (remote context only)

slurm_spank_task_post_fork
    Called for each task from parent process after fork (2) is complete. Due to the fact that slurmd does not exec any tasks until all tasks have completed fork (2), this call is guaranteed to run before the user task is executed. (remote context only)

slurm_spank_task_exit
    Called for each task as its exit status is collected by Slurm. (remote context only)

slurm_spank_exit
    Called once just before slurmstepd exits in remote context. In local context, called before srun exits.

slurm_spank_job_epilog
    Called at the same time as the job epilog. If this function returns a non-zero value and the SPANK plugin that contains it is required in the plugstack.conf, the node that this is run on will be drained.

slurm_spank_slurmd_exit
    Called in slurmd when the daemon is shut down.

All of these functions have the same prototype, for example:
   int slurm_spank_init (spank_t spank, int ac, char *argv[])

Where spank is the SPANK handle which must be passed back to Slurm when the plugin calls functions like spank_get_item and spank_getenv. Configured arguments (See CONFIGURATION below) are passed in the argument vector argv with argument count ac.

SPANK plugins can query the current list of supported slurm_spank symbols to determine if the current version supports a given plugin hook. This may be useful because the list of plugin symbols may grow in the future. The query is done using the spank_symbol_supported function, which has the following prototype:
    int spank_symbol_supported (const char *sym);

The return value is 1 if the symbol is supported, 0 if not.

SPANK plugins do not have direct access to internally defined Slurm data structures. Instead, information about the currently executing job is obtained via the spank_get_item function call.
  spank_err_t spank_get_item (spank_t spank, spank_item_t item, ...);
The spank_get_item call must be passed the current SPANK handle as well as the item requested, which is defined by the passed spank_item_t. A variable number of pointer arguments are also passed, depending on which item was requested by the plugin. A list of the valid values for item is kept in the spank.h header file. Some examples are:

S_JOB_UID
    User id for running job. (uid_t *) is third arg of spank_get_item

S_JOB_STEPID
    Job step id for running job. (uint32_t *) is third arg of spank_get_item.

S_TASK_EXIT_STATUS
    Exit status for exited task. Only valid from slurm_spank_task_exit. (int *) is third arg of spank_get_item.

S_JOB_ARGV
    Complete job command line. Third and fourth args to spank_get_item are (int *, char ***).

See spank.h for more details.

SPANK functions in the local and allocator environment should use the getenv, setenv, and unsetenv functions to view and modify the job's environment. SPANK functions in the remote environment should use the spank_getenv, spank_setenv, and spank_unsetenv functions to view and modify the job's environment. spank_getenv searches the job's environment for the environment variable var and copies the current value into a buffer buf of length len. spank_setenv allows a SPANK plugin to set or overwrite a variable in the job's environment, and spank_unsetenv unsets an environment variable in the job's environment. The prototypes are:
 spank_err_t spank_getenv (spank_t spank, const char *var,
		           char *buf, int len);
 spank_err_t spank_setenv (spank_t spank, const char *var,
		           const char *val, int overwrite);
 spank_err_t spank_unsetenv (spank_t spank, const char *var);

These are only necessary in remote context since modifications of the standard process environment using setenv (3), getenv (3), and unsetenv (3) may be used in local context.

Functions are also available from within the SPANK plugins to establish environment variables to be exported to the Slurm PrologSlurmctld, Prolog, Epilog and EpilogSlurmctld programs (the so-called job control environment). The name of environment variables established by these calls will be prepended with the string SPANK_ in order to avoid any security implications of arbitrary environment variable control. (After all, the job control scripts do run as root or the Slurm user.).

These functions are available from local context only.
  spank_err_t spank_job_control_getenv(spank_t spank, const char *var,
		             char *buf, int len);
  spank_err_t spank_job_control_setenv(spank_t spank, const char *var,
		             const char *val, int overwrite);
  spank_err_t spank_job_control_unsetenv(spank_t spank, const char *var);

See spank.h for more information.

Many of the described SPANK functions available to plugins return errors via the spank_err_t error type. On success, the return value will be set to ESPANK_SUCCESS, while on failure, the return value will be set to one of many error values defined in spank.h. The SPANK interface provides a simple function
  const char * spank_strerror(spank_err_t err);
which may be used to translate a spank_err_t value into its string representation.

The slurm_spank_log function can be used to print messages back to the user at an error level. This is to keep users from having to rely on the slurm_error function, which can be confusing because it prepends "error:" to every message.

SPANK OPTIONS

SPANK plugins also have an interface through which they may define and implement extra job options. These options are made available to the user through Slurm commands such as srun(1), salloc(1), and sbatch(1). If the option is specified by the user, its value is forwarded and registered with the plugin in slurmd when the job is run. In this way, SPANK plugins may dynamically provide new options and functionality to Slurm.

Each option registered by a plugin to Slurm takes the form of a struct spank_option which is declared in spank.h as
   struct spank_option {
      char *         name;
      char *         arginfo;
      char *         usage;
      int            has_arg;
      int            val;
      spank_opt_cb_f cb;
   };
Where

name
    is the name of the option. Its length is limited to SPANK_OPTION_MAXLEN defined in spank.h.

arginfo
    is a description of the argument to the option, if the option does take an argument.

usage
    is a short description of the option suitable for --help output.

has_arg
    0 if option takes no argument, 1 if option takes an argument, and 2 if the option takes an optional argument. (See getopt_long (3)).

val A plugin-local value to return to the option callback function.

cb  A callback function that is invoked when the plugin option is registered with Slurm. spank_opt_cb_f is typedef'd in spank.h as

      typedef int (*spank_opt_cb_f) (int val, const char *optarg,
    		                 int remote);
    Where val is the value of the val field in the spank_option struct, optarg is the supplied argument if applicable, and remote is 0 if the function is being called from the "local" host (e.g. host where srun or sbatch/salloc are invoked) or 1 from the "remote" host (host where slurmd/slurmstepd run) but only executed by slurmstepd (remote context) if the option was registered for such context.

Plugin options may be registered with Slurm using the spank_option_register function. This function is only valid when called from the plugin's slurm_spank_init handler, and registers one option at a time. The prototype is
   spank_err_t spank_option_register (spank_t sp,
		   struct spank_option *opt);
This function will return ESPANK_SUCCESS on successful registration of an option, or ESPANK_BAD_ARG for errors including invalid spank_t handle, or when the function is not called from the slurm_spank_init function. All options need to be registered from all contexts in which they will be used. For instance, if an option is only used in local (srun) and remote (slurmd) contexts, then spank_option_register should only be called from within those contexts. For example:
   if (spank_context() != S_CTX_ALLOCATOR)
      spank_option_register (sp, opt);
If, however, the option is used in all contexts, the spank_option_register needs to be called everywhere.

In addition to spank_option_register, plugins may also export options to Slurm by defining a table of struct spank_option with the symbol name spank_options. This method, however, is not supported for use with sbatch and salloc (allocator context), thus the use of spank_option_register is preferred. When using the spank_options table, the final element in the array must be filled with zeros. A SPANK_OPTIONS_TABLE_END macro is provided in spank.h for this purpose.

When an option is provided by the user on the local side, either by command line options or by environment variables, Slurm will immediately invoke the option's callback with remote=0. This is meant for the plugin to do local sanity checking of the option before the value is sent to the remote side during job launch. If the argument the user specified is invalid, the plugin should issue an error and issue a non-zero return code from the callback. The plugin should be able to handle cases where the spank option is set multiple times through environment variables and command line options. Environment variables are processed before command line options.

On the remote side, options and their arguments are registered just after SPANK plugins are loaded and before the spank_init handler is called. This allows plugins to modify behavior of all plugin functionality based on the value of user-provided options.

As an alternative to use of an option callback and global variable, plugins can use the spank_option_getopt option to check for supplied options after option processing. This function has the prototype:
   spank_err_t spank_option_getopt(spank_t sp,
       struct spank_option *opt, char **optargp);
This function returns ESPANK_SUCCESS if the option defined in the struct spank_option opt has been used by the user. If optargp is non-NULL then it is set to any option argument passed (if the option takes an argument). The use of this method is required to process options in job_script context (slurm_spank_job_prolog and slurm_spank_job_epilog). This function is valid in the following contexts: slurm_spank_job_prolog, slurm_spank_local_user_init, slurm_spank_user_init, slurm_spank_task_init_privileged, slurm_spank_task_init, slurm_spank_task_exit, and slurm_spank_job_epilog.

CONFIGURATION

The default SPANK plug-in stack configuration file is plugstack.conf in the same directory as slurm.conf(5), though this may be changed via the Slurm config parameter PlugStackConfig. Normally the plugstack.conf file should be identical on all nodes of the cluster. The config file lists SPANK plugins, one per line, along with whether the plugin is required or optional, and any global arguments that are to be passed to the plugin for runtime configuration. Comments are preceded with '#' and extend to the end of the line. If the configuration file is missing or empty, it will simply be ignored.

NOTE: The SPANK plugins need to be installed on the machines that execute slurmd (compute nodes) as well as on the machines that execute job allocation utilities such as salloc, sbatch, etc (login nodes).

The format of each non-comment line in the configuration file is:
  required/optional   plugin   arguments
For example:
  optional /usr/lib/slurm/test.so
Tells slurmd to load the plugin test.so passing no arguments. If a SPANK plugin is required, then failure of any of the plugin's functions will cause slurmd, or the job allocator command to terminate the job, while optional plugins only cause a warning.

If a fully-qualified path is not specified for a plugin, then the currently configured PluginDir in slurm.conf(5) is searched.

SPANK plugins are stackable, meaning that more than one plugin may be placed into the config file. The plugins will simply be called in order, one after the other, and appropriate action taken on failure given that state of the plugin's optional flag.

Additional config files or directories of config files may be included in plugstack.conf with the include keyword. The include keyword must appear on its own line, and takes a glob as its parameter, so multiple files may be included from one include line. For example, the following syntax will load all config files in the /etc/slurm/plugstack.conf.d directory, in local collation order:
  include /etc/slurm/plugstack.conf.d/*
which might be considered a more flexible method for building up a spank plugin stack.

The SPANK config file is re-read on each job launch, so editing the config file will not affect running jobs. However care should be taken so that a partially edited config file is not read by a launching job.

Errors
When SPANK plugin results in a non-zero result, the following changes will result:
Command | Function | Context | Exitcode | Drains Node | Fails job
srun | slurm_spank_init | local | 1 | no | yes
srun | slurm_spank_init_post_opt | local | 1 | no | yes
srun | slurm_spank_local_user_init | local | 1 | no | yes
srun | slurm_spank_user_init | remote | 0 | no | no
srun | slurm_spank_task_init_privileged | remote | 1 | no | yes
srun | slurm_spank_task_post_fork | remote | 0 | no | no
srun | slurm_spank_task_init | remote | 1 | no | yes
srun | slurm_spank_task_exit | remote | 0 | no | no
srun | slurm_spank_exit | local | 0 | no | yes
salloc | slurm_spank_init | allocator | 1 | no | yes
salloc | slurm_spank_init_post_opt | allocator | 1 | no | yes
salloc | slurm_spank_user_init | remote | 1 | no | yes
salloc | slurm_spank_task_init_privileged | remote | 1 | no | yes
salloc | slurm_spank_task_post_fork | remote | 1 | no | yes
salloc | slurm_spank_task_init | remote | 1 | no | yes
salloc | slurm_spank_task_exit | remote | 0 | no | no
salloc | slurm_spank_exit | allocator | 0 | no | yes
sbatch | slurm_spank_init | allocator | 1 | no | yes
sbatch | slurm_spank_init_post_opt | allocator | 1 | no | yes
sbatch | slurm_spank_user_init | remote | 1 | yes | yes
sbatch | slurm_spank_task_init_privileged | remote | 1 | no | yes
sbatch | slurm_spank_task_post_fork | remote | 1 | yes | yes
sbatch | slurm_spank_task_init | remote | 1 | no | yes
sbatch | slurm_spank_task_exit | remote | 0 | no | no
sbatch | slurm_spank_exit | allocator | 0 | no | no
scrontab | slurm_spank_init | allocator | 1 | no | no
scrontab | slurm_spank_exit | allocator | 0 | no | no
NOTE: The behavior for ProctrackType=proctrack/pgid may result in timeouts for slurm_spank_task_post_fork with remote context on failure.

COPYING
Portions copyright (C) 2010-2022 SchedMD LLC. Copyright (C) 2006 The Regents of the University of California. Produced at Lawrence Livermore National Laboratory (cf, DISCLAIMER). CODE-OCEC-09-009. All rights reserved.

This file is part of Slurm, a resource management program. For details, see <https://slurm.schedmd.com/>.

Slurm is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

Slurm is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

FILES
/etc/slurm/slurm.conf - Slurm configuration file.
/etc/slurm/plugstack.conf - SPANK configuration file.
/usr/include/slurm/spank.h - SPANK header file.

SEE ALSO

srun(1), slurm.conf(5)