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drm/i915: Pull i915_gem_active into the i915_active family

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Looking forward, we need to break the struct_mutex dependency on
i915_gem_active. In the meantime, external use of i915_gem_active is
quite beguiling, little do new users suspect that it implies a barrier
as each request it tracks must be ordered wrt the previous one. As one
of many, it can be used to track activity across multiple timelines, a
shared fence, which fits our unordered request submission much better. We
need to steer external users away from the singular, exclusive fence
imposed by i915_gem_active to i915_active instead. As part of that
process, we move i915_gem_active out of i915_request.c into
i915_active.c to start separating the two concepts, and rename it to
i915_active_request (both to tie it to the concept of tracking just one
request, and to give it a longer, less appealing name).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190205130005.2807-5-chris@chris-wilson.co.uk
This commit is contained in:
Chris Wilson
2019-02-05 13:00:05 +00:00
parent 5f5c139d69
commit 21950ee7cc
21 changed files with 480 additions and 508 deletions
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383
drivers/gpu/drm/i915/i915_request.h
View File

@@ -403,387 +403,4 @@ static inline void i915_request_mark_complete(struct i915_request *rq)
void i915_retire_requests(struct drm_i915_private *i915);
/*
* We treat requests as fences. This is not be to confused with our
* "fence registers" but pipeline synchronisation objects ala GL_ARB_sync.
* We use the fences to synchronize access from the CPU with activity on the
* GPU, for example, we should not rewrite an object's PTE whilst the GPU
* is reading them. We also track fences at a higher level to provide
* implicit synchronisation around GEM objects, e.g. set-domain will wait
* for outstanding GPU rendering before marking the object ready for CPU
* access, or a pageflip will wait until the GPU is complete before showing
* the frame on the scanout.
*
* In order to use a fence, the object must track the fence it needs to
* serialise with. For example, GEM objects want to track both read and
* write access so that we can perform concurrent read operations between
* the CPU and GPU engines, as well as waiting for all rendering to
* complete, or waiting for the last GPU user of a "fence register". The
* object then embeds a #i915_gem_active to track the most recent (in
* retirement order) request relevant for the desired mode of access.
* The #i915_gem_active is updated with i915_gem_active_set() to track the
* most recent fence request, typically this is done as part of
* i915_vma_move_to_active().
*
* When the #i915_gem_active completes (is retired), it will
* signal its completion to the owner through a callback as well as mark
* itself as idle (i915_gem_active.request == NULL). The owner
* can then perform any action, such as delayed freeing of an active
* resource including itself.
*/
struct i915_gem_active;
typedef void (*i915_gem_retire_fn)(struct i915_gem_active *,
struct i915_request *);
struct i915_gem_active {
struct i915_request __rcu *request;
struct list_head link;
i915_gem_retire_fn retire;
};
void i915_gem_retire_noop(struct i915_gem_active *,
struct i915_request *request);
/**
* init_request_active - prepares the activity tracker for use
* @active - the active tracker
* @func - a callback when then the tracker is retired (becomes idle),
* can be NULL
*
* init_request_active() prepares the embedded @active struct for use as
* an activity tracker, that is for tracking the last known active request
* associated with it. When the last request becomes idle, when it is retired
* after completion, the optional callback @func is invoked.
*/
static inline void
init_request_active(struct i915_gem_active *active,
i915_gem_retire_fn retire)
{
RCU_INIT_POINTER(active->request, NULL);
INIT_LIST_HEAD(&active->link);
active->retire = retire ?: i915_gem_retire_noop;
}
/**
* i915_gem_active_set - updates the tracker to watch the current request
* @active - the active tracker
* @request - the request to watch
*
* i915_gem_active_set() watches the given @request for completion. Whilst
* that @request is busy, the @active reports busy. When that @request is
* retired, the @active tracker is updated to report idle.
*/
static inline void
i915_gem_active_set(struct i915_gem_active *active,
struct i915_request *request)
{
list_move(&active->link, &request->active_list);
rcu_assign_pointer(active->request, request);
}
/**
* i915_gem_active_set_retire_fn - updates the retirement callback
* @active - the active tracker
* @fn - the routine called when the request is retired
* @mutex - struct_mutex used to guard retirements
*
* i915_gem_active_set_retire_fn() updates the function pointer that
* is called when the final request associated with the @active tracker
* is retired.
*/
static inline void
i915_gem_active_set_retire_fn(struct i915_gem_active *active,
i915_gem_retire_fn fn,
struct mutex *mutex)
{
lockdep_assert_held(mutex);
active->retire = fn ?: i915_gem_retire_noop;
}
static inline struct i915_request *
__i915_gem_active_peek(const struct i915_gem_active *active)
{
/*
* Inside the error capture (running with the driver in an unknown
* state), we want to bend the rules slightly (a lot).
*
* Work is in progress to make it safer, in the meantime this keeps
* the known issue from spamming the logs.
*/
return rcu_dereference_protected(active->request, 1);
}
/**
* i915_gem_active_raw - return the active request
* @active - the active tracker
*
* i915_gem_active_raw() returns the current request being tracked, or NULL.
* It does not obtain a reference on the request for the caller, so the caller
* must hold struct_mutex.
*/
static inline struct i915_request *
i915_gem_active_raw(const struct i915_gem_active *active, struct mutex *mutex)
{
return rcu_dereference_protected(active->request,
lockdep_is_held(mutex));
}
/**
* i915_gem_active_peek - report the active request being monitored
* @active - the active tracker
*
* i915_gem_active_peek() returns the current request being tracked if
* still active, or NULL. It does not obtain a reference on the request
* for the caller, so the caller must hold struct_mutex.
*/
static inline struct i915_request *
i915_gem_active_peek(const struct i915_gem_active *active, struct mutex *mutex)
{
struct i915_request *request;
request = i915_gem_active_raw(active, mutex);
if (!request || i915_request_completed(request))
return NULL;
return request;
}
/**
* i915_gem_active_get - return a reference to the active request
* @active - the active tracker
*
* i915_gem_active_get() returns a reference to the active request, or NULL
* if the active tracker is idle. The caller must hold struct_mutex.
*/
static inline struct i915_request *
i915_gem_active_get(const struct i915_gem_active *active, struct mutex *mutex)
{
return i915_request_get(i915_gem_active_peek(active, mutex));
}
/**
* __i915_gem_active_get_rcu - return a reference to the active request
* @active - the active tracker
*
* __i915_gem_active_get() returns a reference to the active request, or NULL
* if the active tracker is idle. The caller must hold the RCU read lock, but
* the returned pointer is safe to use outside of RCU.
*/
static inline struct i915_request *
__i915_gem_active_get_rcu(const struct i915_gem_active *active)
{
/*
* Performing a lockless retrieval of the active request is super
* tricky. SLAB_TYPESAFE_BY_RCU merely guarantees that the backing
* slab of request objects will not be freed whilst we hold the
* RCU read lock. It does not guarantee that the request itself
* will not be freed and then *reused*. Viz,
*
* Thread A Thread B
*
* rq = active.request
* retire(rq) -> free(rq);
* (rq is now first on the slab freelist)
* active.request = NULL
*
* rq = new submission on a new object
* ref(rq)
*
* To prevent the request from being reused whilst the caller
* uses it, we take a reference like normal. Whilst acquiring
* the reference we check that it is not in a destroyed state
* (refcnt == 0). That prevents the request being reallocated
* whilst the caller holds on to it. To check that the request
* was not reallocated as we acquired the reference we have to
* check that our request remains the active request across
* the lookup, in the same manner as a seqlock. The visibility
* of the pointer versus the reference counting is controlled
* by using RCU barriers (rcu_dereference and rcu_assign_pointer).
*
* In the middle of all that, we inspect whether the request is
* complete. Retiring is lazy so the request may be completed long
* before the active tracker is updated. Querying whether the
* request is complete is far cheaper (as it involves no locked
* instructions setting cachelines to exclusive) than acquiring
* the reference, so we do it first. The RCU read lock ensures the
* pointer dereference is valid, but does not ensure that the
* seqno nor HWS is the right one! However, if the request was
* reallocated, that means the active tracker's request was complete.
* If the new request is also complete, then both are and we can
* just report the active tracker is idle. If the new request is
* incomplete, then we acquire a reference on it and check that
* it remained the active request.
*
* It is then imperative that we do not zero the request on
* reallocation, so that we can chase the dangling pointers!
* See i915_request_alloc().
*/
do {
struct i915_request *request;
request = rcu_dereference(active->request);
if (!request || i915_request_completed(request))
return NULL;
/*
* An especially silly compiler could decide to recompute the
* result of i915_request_completed, more specifically
* re-emit the load for request->fence.seqno. A race would catch
* a later seqno value, which could flip the result from true to
* false. Which means part of the instructions below might not
* be executed, while later on instructions are executed. Due to
* barriers within the refcounting the inconsistency can't reach
* past the call to i915_request_get_rcu, but not executing
* that while still executing i915_request_put() creates
* havoc enough. Prevent this with a compiler barrier.
*/
barrier();
request = i915_request_get_rcu(request);
/*
* What stops the following rcu_access_pointer() from occurring
* before the above i915_request_get_rcu()? If we were
* to read the value before pausing to get the reference to
* the request, we may not notice a change in the active
* tracker.
*
* The rcu_access_pointer() is a mere compiler barrier, which
* means both the CPU and compiler are free to perform the
* memory read without constraint. The compiler only has to
* ensure that any operations after the rcu_access_pointer()
* occur afterwards in program order. This means the read may
* be performed earlier by an out-of-order CPU, or adventurous
* compiler.
*
* The atomic operation at the heart of
* i915_request_get_rcu(), see dma_fence_get_rcu(), is
* atomic_inc_not_zero() which is only a full memory barrier
* when successful. That is, if i915_request_get_rcu()
* returns the request (and so with the reference counted
* incremented) then the following read for rcu_access_pointer()
* must occur after the atomic operation and so confirm
* that this request is the one currently being tracked.
*
* The corresponding write barrier is part of
* rcu_assign_pointer().
*/
if (!request || request == rcu_access_pointer(active->request))
return rcu_pointer_handoff(request);
i915_request_put(request);
} while (1);
}
/**
* i915_gem_active_get_unlocked - return a reference to the active request
* @active - the active tracker
*
* i915_gem_active_get_unlocked() returns a reference to the active request,
* or NULL if the active tracker is idle. The reference is obtained under RCU,
* so no locking is required by the caller.
*
* The reference should be freed with i915_request_put().
*/
static inline struct i915_request *
i915_gem_active_get_unlocked(const struct i915_gem_active *active)
{
struct i915_request *request;
rcu_read_lock();
request = __i915_gem_active_get_rcu(active);
rcu_read_unlock();
return request;
}
/**
* i915_gem_active_isset - report whether the active tracker is assigned
* @active - the active tracker
*
* i915_gem_active_isset() returns true if the active tracker is currently
* assigned to a request. Due to the lazy retiring, that request may be idle
* and this may report stale information.
*/
static inline bool
i915_gem_active_isset(const struct i915_gem_active *active)
{
return rcu_access_pointer(active->request);
}
/**
* i915_gem_active_wait - waits until the request is completed
* @active - the active request on which to wait
* @flags - how to wait
* @timeout - how long to wait at most
* @rps - userspace client to charge for a waitboost
*
* i915_gem_active_wait() waits until the request is completed before
* returning, without requiring any locks to be held. Note that it does not
* retire any requests before returning.
*
* This function relies on RCU in order to acquire the reference to the active
* request without holding any locks. See __i915_gem_active_get_rcu() for the
* glory details on how that is managed. Once the reference is acquired, we
* can then wait upon the request, and afterwards release our reference,
* free of any locking.
*
* This function wraps i915_request_wait(), see it for the full details on
* the arguments.
*
* Returns 0 if successful, or a negative error code.
*/
static inline int
i915_gem_active_wait(const struct i915_gem_active *active, unsigned int flags)
{
struct i915_request *request;
long ret = 0;
request = i915_gem_active_get_unlocked(active);
if (request) {
ret = i915_request_wait(request, flags, MAX_SCHEDULE_TIMEOUT);
i915_request_put(request);
}
return ret < 0 ? ret : 0;
}
/**
* i915_gem_active_retire - waits until the request is retired
* @active - the active request on which to wait
*
* i915_gem_active_retire() waits until the request is completed,
* and then ensures that at least the retirement handler for this
* @active tracker is called before returning. If the @active
* tracker is idle, the function returns immediately.
*/
static inline int __must_check
i915_gem_active_retire(struct i915_gem_active *active,
struct mutex *mutex)
{
struct i915_request *request;
long ret;
request = i915_gem_active_raw(active, mutex);
if (!request)
return 0;
ret = i915_request_wait(request,
I915_WAIT_INTERRUPTIBLE | I915_WAIT_LOCKED,
MAX_SCHEDULE_TIMEOUT);
if (ret < 0)
return ret;
list_del_init(&active->link);
RCU_INIT_POINTER(active->request, NULL);
active->retire(active, request);
return 0;
}
#define for_each_active(mask, idx) \
for (; mask ? idx = ffs(mask) - 1, 1 : 0; mask &= ~BIT(idx))
#endif /* I915_REQUEST_H */