[Intel-gfx] [PATCH v7 1/8] drm/i915: Convert requests to use struct fence
John Harrison
John.C.Harrison at Intel.com
Thu Apr 21 10:26:55 UTC 2016
On 21/04/2016 08:06, Maarten Lankhorst wrote:
> Op 20-04-16 om 19:09 schreef John.C.Harrison at Intel.com:
>> From: John Harrison <John.C.Harrison at 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.Harrison at Intel.com>
>> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
>> Reviewed-by: Jesse Barnes <jbarnes at 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
I don't understand what you mean? Are you referring to the
kmem_cache_free that frees the request object at the end of the above
function (which you have actually deleted from your reply)? Or are you
referring to something inside the i915_gem_request_remove_from_client()
call that your comments seem to be in reply to?
If you mean the free of the request itself, then the only usage of that
particular kmem_cache are within the driver mutex lock. Does that not
make it safe? If you mean the client remove, then where is the
kmem_cache_free in that call path?
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