Re: [PATCH 08/17] drm/i915: Drop spinlocks around adding to the client request list

John Harrison
2016-09-02T10:30:37Z

On 22/08/2016 09:03, Chris Wilson wrote:
> Adding to the tail of the client request list as the only other user is
> in the throttle ioctl that iterates forwards over the list. It only
> needs protection against deletion of a request as it reads it, it simply
> won't see a new request added to the end of the list, or it would be too
> early and rejected. We can further reduce the number of spinlocks
> required when throttling by removing stale requests from the client_list
> as we throttle.
>
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> ---
> drivers/gpu/drm/i915/i915_debugfs.c | 2 +-
> drivers/gpu/drm/i915/i915_gem.c | 14 ++++++------
> drivers/gpu/drm/i915/i915_gem_execbuffer.c | 14 +++++++-----
> drivers/gpu/drm/i915/i915_gem_request.c | 34 ++++++------------------------
> drivers/gpu/drm/i915/i915_gem_request.h | 4 +---
> 5 files changed, 23 insertions(+), 45 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_debugfs.c b/drivers/gpu/drm/i915/i915_debugfs.c
> index 086053fa2820..996744708f31 100644
> --- a/drivers/gpu/drm/i915/i915_debugfs.c
> +++ b/drivers/gpu/drm/i915/i915_debugfs.c
> @@ -480,7 +480,7 @@ static int i915_gem_object_info(struct seq_file *m, void* data)
> mutex_lock(&dev->struct_mutex);
> request = list_first_entry_or_null(&file_priv->mm.request_list,
> struct drm_i915_gem_request,
> - client_list);
> + client_link);
> rcu_read_lock();
> task = pid_task(request && request->ctx->pid ?
> request->ctx->pid : file->pid,
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 7b8abda541e6..e432211e8b24 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -3673,16 +3673,14 @@ i915_gem_ring_throttle(struct drm_device *dev, struct drm_file *file)
> return -EIO;
>
> spin_lock(&file_priv->mm.lock);
> - list_for_each_entry(request, &file_priv->mm.request_list, client_list) {
> + list_for_each_entry(request, &file_priv->mm.request_list, client_link) {
> if (time_after_eq(request->emitted_jiffies, recent_enough))
> break;
>
> - /*
> - * Note that the request might not have been submitted yet.
> - * In which case emitted_jiffies will be zero.
> - */
> - if (!request->emitted_jiffies)
> - continue;
> + if (target) {
> + list_del(&target->client_link);
> + target->file_priv = NULL;
> + }
>
> target = request;
> }
> @@ -4639,7 +4637,7 @@ void i915_gem_release(struct drm_device *dev, struct drm_file *file)
> * file_priv.
> */
> spin_lock(&file_priv->mm.lock);
> - list_for_each_entry(request, &file_priv->mm.request_list, client_list)
> + list_for_each_entry(request, &file_priv->mm.request_list, client_link)
> request->file_priv = NULL;
> spin_unlock(&file_priv->mm.lock);
>
> diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> index 125fb38eff40..5689445b1cd3 100644
> --- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> +++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> @@ -1421,6 +1421,14 @@ out:
> return vma;
> }
>
> +static void
> +add_to_client(struct drm_i915_gem_request *req,
> + struct drm_file *file)
> +{
> + req->file_priv = file->driver_priv;
> + list_add_tail(&req->client_link, &req->file_priv->mm.request_list);
> +}
> +
> static int
> execbuf_submit(struct i915_execbuffer_params *params,
> struct drm_i915_gem_execbuffer2 *args,
> @@ -1512,6 +1520,7 @@ execbuf_submit(struct i915_execbuffer_params *params,
> trace_i915_gem_ring_dispatch(params->request, params->dispatch_flags);
>
> i915_gem_execbuffer_move_to_active(vmas, params->request);
> + add_to_client(params->request, params->file);
>
> return 0;
> }
> @@ -1808,10 +1817,6 @@ i915_gem_do_execbuffer(struct drm_device *dev, void *data,
> */
> params->request->batch = params->batch;
>
> - ret = i915_gem_request_add_to_client(params->request, file);
> - if (ret)
> - goto err_request;
> -
> /*
> * Save assorted stuff away to pass through to *_submission().
> * NB: This data should be 'persistent' and not local as it will
> @@ -1825,7 +1830,6 @@ i915_gem_do_execbuffer(struct drm_device *dev, void *data,
> params->ctx = ctx;
>
> ret = execbuf_submit(params, args, &eb->vmas);
> -err_request:
> __i915_add_request(params->request, ret == 0);
>
> err_batch_unpin:
> diff --git a/drivers/gpu/drm/i915/i915_gem_request.c b/drivers/gpu/drm/i915/i915_gem_request.c
> index 1a215320cefb..bf62427a35b7 100644
> --- a/drivers/gpu/drm/i915/i915_gem_request.c
> +++ b/drivers/gpu/drm/i915/i915_gem_request.c
> @@ -115,42 +115,20 @@ const struct fence_ops i915_fence_ops = {
> .timeline_value_str = i915_fence_timeline_value_str,
> };
>
> -int i915_gem_request_add_to_client(struct drm_i915_gem_request *req,
> - struct drm_file *file)
> -{
> - struct drm_i915_private *dev_private;
> - struct drm_i915_file_private *file_priv;
> -
> - WARN_ON(!req || !file || req->file_priv);
> -
> - if (!req || !file)
> - return -EINVAL;
> -
> - if (req->file_priv)
> - return -EINVAL;
> -
> - dev_private = req->i915;
> - file_priv = file->driver_priv;
> -
> - spin_lock(&file_priv->mm.lock);
> - req->file_priv = file_priv;
> - list_add_tail(&req->client_list, &file_priv->mm.request_list);
> - spin_unlock(&file_priv->mm.lock);
> -
> - return 0;
> -}
> -
> static inline void
> i915_gem_request_remove_from_client(struct drm_i915_gem_request *request)
> {
> - struct drm_i915_file_private *file_priv = request->file_priv;
> + struct drm_i915_file_private *file_priv;
>
> + file_priv = request->file_priv;
> if (!file_priv)
> return;
>
> spin_lock(&file_priv->mm.lock);
> - list_del(&request->client_list);
> - request->file_priv = NULL;
> + if (request->file_priv) {
Why check for request->file_priv again? The block above will exit if it
is null. There surely can't be a race with remove_from_client being
called concurrently with add_to_client? Especially as add_to_client no
longer takes the spin_lock anyway.

> + list_del(&request->client_link);
> + request->file_priv = NULL;
> + }
> spin_unlock(&file_priv->mm.lock);
> }
>
> diff --git a/drivers/gpu/drm/i915/i915_gem_request.h b/drivers/gpu/drm/i915/i915_gem_request.h
> index 6c72bd8d9423..9d5a66bfc509 100644
> --- a/drivers/gpu/drm/i915/i915_gem_request.h
> +++ b/drivers/gpu/drm/i915/i915_gem_request.h
> @@ -132,7 +132,7 @@ struct drm_i915_gem_request {
>
> struct drm_i915_file_private *file_priv;
> /** file_priv list entry for this request */
> - struct list_head client_list;
> + struct list_head client_link;
>
> /**
> * The ELSP only accepts two elements at a time, so we queue
> @@ -167,8 +167,6 @@ static inline bool fence_is_i915(struct fence *fence)
> struct drm_i915_gem_request * __must_check
> i915_gem_request_alloc(struct intel_engine_cs *engine,
> struct i915_gem_context *ctx);
> -int i915_gem_request_add_to_client(struct drm_i915_gem_request *req,
> - struct drm_file *file);
> void i915_gem_request_retire_upto(struct drm_i915_gem_request *req);
>
> static inline u32