File:  [NetBSD Developer Wiki] / wikisrc / projects / project / kernel_continuations.mdwn
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Wed Feb 8 14:18:15 2012 UTC (9 years ago) by plunky
Branches: MAIN
CVS tags: HEAD
add missing words

    1: [[!template id=project
    3: title="Kernel continuations"
    5: contact="""
    6: [tech-kern](,
    7: [board](,
    8: [core](
    9: """
   11: category="kernel"
   12: difficulty="hard"
   13: funded="The NetBSD Foundation"
   15: description="""
   16: This project proposal is a subtask of [[smp_networking]] and is elegible
   17: for funding independently.
   19: The goal of this project is to implement continuations at the kernel level.
   20: Most of the pieces are already available in the kernel, so this can be
   21: reworded as: combine *callouts*, *softints*, and *workqueues* into a single
   22: framework.  Continuations are meant to be cheap; very cheap.
   24: Please note that the main goal of this project is to simplify the
   25: implementation of [[SMP networking|smp_networking]], so care must be taken
   26: in the design of the interface to support all the features required for
   27: this other project.
   29: The proposed interface looks like the following.  This interface is mostly
   30: derived from the `callout(9)` API and is a superset of the softint(9) API.
   31: The most significant change is that workqueue items are not tied to a
   32: specific kernel thread.
   34: * `kcont_t *kcont_create(kcont_wq_t *wq, kmutex_t *lock, void
   35:   (*func)(void *, kcont_t *), void *arg, int flags);`
   37:   A `wq` must be supplied.  It may be one returned by
   38:   `kcont_workqueue_acquire` or a predefined workqueue such as (sorted from
   39:   highest priority to lowest):
   41:   * `wq_softserial`, `wq_softnet`, `wq_softbio`, `wq_softclock`
   42:   * `wq_prihigh`, `wq_primedhigh`, `wq_primedlow`, `wq_prilow`
   44:   `lock`, if non-NULL, should be locked before calling `func(arg)` and
   45:   released afterwards.  However, if the lock is released and/or destroyed
   46:   before the called function returns, then, before returning,
   47:   `kcont_set_mutex` must be called with either a new mutex to be released
   48:   or `NULL`.  If acquiring lock would block, other pending kernel
   49:   continuations which depend on other locks may be dispatched in the
   50:   meantime.  However, all continuations sharing the same set of `{ wq, lock,
   51:   [ci] }` need to be processed in the order they were scheduled.
   53:   `flags` must be 0.  This field is just provided for extensibility.
   55: * `int kcont_schedule(kcont_t *kc, struct cpu_info *ci, int nticks);`
   57:   If the continuation is marked as *INVOKING*, an error of `EBUSY` should
   58:   be returned.  If `nticks` is 0, the continuation is marked as *INVOKING*
   59:   while *EXPIRED* and *PENDING* are cleared, and the continuation is
   60:   scheduled to be invoked without delay.  Otherwise, the continuation is
   61:   marked as *PENDING* while *EXPIRED* status is cleared, and the timer
   62:   reset to `nticks`.  Once the timer expires, the continuation is marked as
   63:   *EXPIRED* and *INVOKING*, and the *PENDING* status is cleared.  If `ci`
   64:   is non-NULL, the continuation is invoked on the specified CPU if the
   65:   continuations's workqueue has per-cpu queues.  If that workqueue does not
   66:   provide per-cpu queues, an error of `ENOENT` is returned.  Otherwise when
   67:   `ci` is `NULL`, the continuation is invoked on either the current CPU or
   68:   the next available CPU depending on whether the continuation's workqueue
   69:   has per-cpu queues or not, respectively.
   71: * `void kcont_destroy(kcont_t *kc);`
   73: * `kmutex_t *kcont_getmutex(kcont_t *kc);`
   75:   Returns the lock currently associated with the continuation `kc`.
   77: * `void kcont_setarg(kcont_t *kc, void *arg);`
   79:   Updates `arg` in the continuation `kc`.  If no lock is associated with
   80:   the continuation, then `arg` may be changed at any time; however, if the
   81:   continuation is being invoked, it may not pick up the change.  Otherwise,
   82:   `kcont_setarg` must only be called when the associated lock is locked.
   84: * `kmutex_t *kcont_setmutex(kcont_t *kc, kmutex_t *lock);`
   86:   Updates the lock associated with the continuation `kc` and returns the
   87:   previous lock.  If no lock is currently associated with the continuation,
   88:   then calling this function with a lock other than NULL will trigger an
   89:   assertion failure.  Otherwise, `kcont_setmutex` must be called only when
   90:   the existing lock (which will be replaced) is locked.  If
   91:   `kcont_setmutex` is called as a result of the invokation of func, then
   92:   after kcont_setmutex has been called but before func returns, the
   93:   replaced lock must have been released, and the replacement lock, if
   94:   non-NULL, must be locked upon return.
   96: * `void kcont_setfunc(kcont_t *kc, void (*func)(void *), void *arg);`
   98:   Updates `func` and `arg` in the continuation `kc`.  If no lock is
   99:   associated with the continuation, then only arg may be changed.
  100:   Otherwise, `kcont_setfunc` must be called only when the associated lock
  101:   is locked.
  103: * `bool kcont_stop(kcont_t *kc);`
  105:   The `kcont_stop function` stops the timer associated the continuation
  106:   handle kc.  The *PENDING* and *EXPIRED* status for the continuation
  107:   handle is cleared.  It is safe to call `kcont_stop` on a continuation
  108:   handle that is not pending, so long as it is initialized.  `kcont_stop`
  109:   will return a non-zero value if the continuation was *EXPIRED*.
  111: * `bool kcont_pending(kcont_t *kc);`
  113:   The `kcont_pending` function tests the *PENDING* status of the
  114:   continuation handle `kc`.  A *PENDING* continuation is one who's timer
  115:   has been started and has not expired.  Note that it is possible for a
  116:   continuation's timer to have expired without being invoked if the
  117:   continuation's lock could not be acquired or there are higher priority
  118:   threads preventing its invokation.  Note that it is only safe to test
  119:   *PENDING* status when holding the continuation's lock.
  121: * `bool kcont_expired(kcont_t *kc);`
  123:   Tests to see if the continuation's function has been invoked since the
  124:   last `kcont_schedule`.
  126: * `bool kcont_active(kcont_t *kc);`
  128: * `bool kcont_invoking(kcont_t *kc);`
  130:   Tests the *INVOKING* status of the handle `kc`.  This flag is set just
  131:   before a continuation's function is being called.  Since the scheduling
  132:   of the worker threads may induce delays, other pending higher-priority
  133:   code may run before the continuation function is allowed to run.  This
  134:   may create a race condition if this higher-priority code deallocates
  135:   storage containing one or more continuation structures whose continuation
  136:   functions are about to be run.  In such cases, one technique to prevent
  137:   references to deallocated storage would be to test whether any
  138:   continuation functions are in the *INVOKING* state using
  139:   `kcont_invoking`, and if so, to mark the data structure and defer storage
  140:   deallocation until the continuation function is allowed to run.  For this
  141:   handshake protocol to work, the continuation function will have to use
  142:   the `kcont_ack` function to clear this flag.
  144: * `bool kcont_ack(kcont_t *kc);`
  146:   Clears the *INVOKING* state in the continuation handle `kc`.  This is
  147:   used in situations where it is necessary to protect against the race
  148:   condition described under `kcont_invoking`.
  150: * `kcont_wq_t *kcont_workqueue_acquire(pri_t pri, int flags);`
  152:   Returns a workqueue that matches the specified criteria.  Thus if
  153:   multiple requesters ask for the same criteria, they are all returned the
  154:   same workqueue.  `pri` specifies the priority at which the kernel thread
  155:   which empties the workqueue should run.
  157:   If `flags` is 0 then the standard operation is required.  However, the
  158:   following flag(s) may be bitwise ORed together:
  160:   * `WQ_PERCPU` specifies that the workqueue should have a separate queue
  161:     for each CPU, thus allowing continuations to be invoked on specific CPUs.
  163: * `int kcont_workqueue_release(kcont_wq_t *wq);`
  165:   Releases an acquired workqueue.  On the last release, the workqueue's
  166:   resources are freed and the workqueue is destroyed.
  167: """
  168: ]]
  170: [[!tag smp_networking]]

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