Op 20-04-16 om 19:09 schreef john.c.harri...@intel.com:
> From: John Harrison <john.c.harri...@intel.com>
>
> There is a construct in the linux kernel called 'struct fence' that is
> intended to keep track of work that is executed on hardware. I.e. it
> solves the basic problem that the drivers 'struct
> drm_i915_gem_request' is trying to address. The request structure does
> quite a lot more than simply track the execution progress so is very
> definitely still required. However, the basic completion status side
> could be updated to use the ready made fence implementation and gain
> all the advantages that provides.
>
> This patch makes the first step of integrating a struct fence into the
> request. It replaces the explicit reference count with that of the
> fence. It also replaces the 'is completed' test with the fence's
> equivalent. Currently, that simply chains on to the original request
> implementation. A future patch will improve this.
>
> v3: Updated after review comments by Tvrtko Ursulin. Added fence
> context/seqno pair to the debugfs request info. Renamed fence 'driver
> name' to just 'i915'. Removed BUG_ONs.
>
> v5: Changed seqno format in debugfs to %x rather than %u as that is
> apparently the preferred appearance. Line wrapped some long lines to
> keep the style checker happy.
>
> v6: Updated to newer nigthly and resolved conflicts. The biggest issue
> was with the re-worked busy spin precursor to waiting on a request. In
> particular, the addition of a 'request_started' helper function. This
> has no corresponding concept within the fence framework. However, it
> is only ever used in one place and the whole point of that place is to
> always directly read the seqno for absolutely lowest latency possible.
> So the simple solution is to just make the seqno test explicit at that
> point now rather than later in the series (it was previously being
> done anyway when fences become interrupt driven).
>
> v7: Rebased to newer nightly - lots of ring -> engine renaming and
> interface change to get_seqno().
>
> For: VIZ-5190
> Signed-off-by: John Harrison <john.c.harri...@intel.com>
> Cc: Tvrtko Ursulin <tvrtko.ursu...@intel.com>
> Reviewed-by: Jesse Barnes <jbar...@virtuousgeek.org>
> ---
> drivers/gpu/drm/i915/i915_debugfs.c | 5 ++-
> drivers/gpu/drm/i915/i915_drv.h | 51 ++++++++++-------------
> drivers/gpu/drm/i915/i915_gem.c | 72
> +++++++++++++++++++++++++++++----
> drivers/gpu/drm/i915/intel_lrc.c | 1 +
> drivers/gpu/drm/i915/intel_ringbuffer.c | 1 +
> drivers/gpu/drm/i915/intel_ringbuffer.h | 3 ++
> 6 files changed, 94 insertions(+), 39 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_debugfs.c
> b/drivers/gpu/drm/i915/i915_debugfs.c
> index 2d11b49..6917515 100644
> --- a/drivers/gpu/drm/i915/i915_debugfs.c
> +++ b/drivers/gpu/drm/i915/i915_debugfs.c
> @@ -707,11 +707,12 @@ static int i915_gem_request_info(struct seq_file *m,
> void *data)
> task = NULL;
> if (req->pid)
> task = pid_task(req->pid, PIDTYPE_PID);
> - seq_printf(m, " %x @ %d: %s [%d]\n",
> + seq_printf(m, " %x @ %d: %s [%d], fence = %x:%x\n",
> req->seqno,
> (int) (jiffies - req->emitted_jiffies),
> task ? task->comm : "<unknown>",
> - task ? task->pid : -1);
> + task ? task->pid : -1,
> + req->fence.context, req->fence.seqno);
> rcu_read_unlock();
> }
>
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index d1e6e58..e5f49f3 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -54,6 +54,7 @@
> #include <linux/pm_qos.h>
> #include "intel_guc.h"
> #include "intel_dpll_mgr.h"
> +#include <linux/fence.h>
>
> /* General customization:
> */
> @@ -2242,7 +2243,17 @@ void i915_gem_track_fb(struct drm_i915_gem_object *old,
> * initial reference taken using kref_init
> */
> struct drm_i915_gem_request {
> - struct kref ref;
> + /**
> + * Underlying object for implementing the signal/wait stuff.
> + * NB: Never call fence_later() or return this fence object to user
> + * land! Due to lazy allocation, scheduler re-ordering, pre-emption,
> + * etc., there is no guarantee at all about the validity or
> + * sequentiality of the fence's seqno! It is also unsafe to let
> + * anything outside of the i915 driver get hold of the fence object
> + * as the clean up when decrementing the reference count requires
> + * holding the driver mutex lock.
> + */
> + struct fence fence;
>
> /** On Which ring this request was generated */
> struct drm_i915_private *i915;
> @@ -2328,7 +2339,13 @@ struct drm_i915_gem_request * __must_check
> i915_gem_request_alloc(struct intel_engine_cs *engine,
> struct intel_context *ctx);
> void i915_gem_request_cancel(struct drm_i915_gem_request *req);
> -void i915_gem_request_free(struct kref *req_ref);
> +
> +static inline bool i915_gem_request_completed(struct drm_i915_gem_request
> *req,
> + bool lazy_coherency)
> +{
> + return fence_is_signaled(&req->fence);
> +}
> +
> int i915_gem_request_add_to_client(struct drm_i915_gem_request *req,
> struct drm_file *file);
>
> @@ -2348,7 +2365,7 @@ static inline struct drm_i915_gem_request *
> i915_gem_request_reference(struct drm_i915_gem_request *req)
> {
> if (req)
> - kref_get(&req->ref);
> + fence_get(&req->fence);
> return req;
> }
>
> @@ -2356,7 +2373,7 @@ static inline void
> i915_gem_request_unreference(struct drm_i915_gem_request *req)
> {
> WARN_ON(!mutex_is_locked(&req->engine->dev->struct_mutex));
> - kref_put(&req->ref, i915_gem_request_free);
> + fence_put(&req->fence);
> }
>
> static inline void
> @@ -2368,7 +2385,7 @@ i915_gem_request_unreference__unlocked(struct
> drm_i915_gem_request *req)
> return;
>
> dev = req->engine->dev;
> - if (kref_put_mutex(&req->ref, i915_gem_request_free,
> &dev->struct_mutex))
> + if (kref_put_mutex(&req->fence.refcount, fence_release,
> &dev->struct_mutex))
> mutex_unlock(&dev->struct_mutex);
> }
>
> @@ -2385,12 +2402,6 @@ static inline void i915_gem_request_assign(struct
> drm_i915_gem_request **pdst,
> }
>
> /*
> - * XXX: i915_gem_request_completed should be here but currently needs the
> - * definition of i915_seqno_passed() which is below. It will be moved in
> - * a later patch when the call to i915_seqno_passed() is obsoleted...
> - */
> -
> -/*
> * A command that requires special handling by the command parser.
> */
> struct drm_i915_cmd_descriptor {
> @@ -3055,24 +3066,6 @@ i915_seqno_passed(uint32_t seq1, uint32_t seq2)
> return (int32_t)(seq1 - seq2) >= 0;
> }
>
> -static inline bool i915_gem_request_started(struct drm_i915_gem_request *req,
> - bool lazy_coherency)
> -{
> - if (!lazy_coherency && req->engine->irq_seqno_barrier)
> - req->engine->irq_seqno_barrier(req->engine);
> - return i915_seqno_passed(req->engine->get_seqno(req->engine),
> - req->previous_seqno);
> -}
> -
> -static inline bool i915_gem_request_completed(struct drm_i915_gem_request
> *req,
> - bool lazy_coherency)
> -{
> - if (!lazy_coherency && req->engine->irq_seqno_barrier)
> - req->engine->irq_seqno_barrier(req->engine);
> - return i915_seqno_passed(req->engine->get_seqno(req->engine),
> - req->seqno);
> -}
> -
> int __must_check i915_gem_get_seqno(struct drm_device *dev, u32 *seqno);
> int __must_check i915_gem_set_seqno(struct drm_device *dev, u32 seqno);
>
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index ebef03b..1add29a9 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -1183,6 +1183,7 @@ static int __i915_spin_request(struct
> drm_i915_gem_request *req, int state)
> {
> unsigned long timeout;
> unsigned cpu;
> + uint32_t seqno;
>
> /* When waiting for high frequency requests, e.g. during synchronous
> * rendering split between the CPU and GPU, the finite amount of time
> @@ -1198,12 +1199,14 @@ static int __i915_spin_request(struct
> drm_i915_gem_request *req, int state)
> return -EBUSY;
>
> /* Only spin if we know the GPU is processing this request */
> - if (!i915_gem_request_started(req, true))
> + seqno = req->engine->get_seqno(req->engine);
> + if (!i915_seqno_passed(seqno, req->previous_seqno))
> return -EAGAIN;
>
> timeout = local_clock_us(&cpu) + 5;
> while (!need_resched()) {
> - if (i915_gem_request_completed(req, true))
> + seqno = req->engine->get_seqno(req->engine);
> + if (i915_seqno_passed(seqno, req->seqno))
> return 0;
>
> if (signal_pending_state(state, current))
> @@ -1215,7 +1218,10 @@ static int __i915_spin_request(struct
> drm_i915_gem_request *req, int state)
> cpu_relax_lowlatency();
> }
>
> - if (i915_gem_request_completed(req, false))
> + if (req->engine->irq_seqno_barrier)
> + req->engine->irq_seqno_barrier(req->engine);
> + seqno = req->engine->get_seqno(req->engine);
> + if (i915_seqno_passed(seqno, req->seqno))
> return 0;
>
> return -EAGAIN;
> @@ -2721,12 +2727,14 @@ static void i915_set_reset_status(struct
> drm_i915_private *dev_priv,
> }
> }
>
> -void i915_gem_request_free(struct kref *req_ref)
> +static void i915_gem_request_free(struct fence *req_fence)
> {
> - struct drm_i915_gem_request *req = container_of(req_ref,
> - typeof(*req), ref);
> + struct drm_i915_gem_request *req = container_of(req_fence,
> + typeof(*req), fence);
> struct intel_context *ctx = req->ctx;
>
> + WARN_ON(!mutex_is_locked(&req->engine->dev->struct_mutex));
> +
> if (req->file_priv)
> i915_gem_request_remove_from_client(req);
>
>
Is kmem_cache_free rcu-safe?
I don't think it is, and that would cause some hard to debug issues.
Adding SLAB_DESTROY_BY_RCU to flags wouldn't do what you would expect here,
so your best bet would be to do a call_rcu(&fence->rcu,
wrapper_for_kmem_cache_free);
~Maarten
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