On 10/03/2020 00:13, Chris Wilson wrote:
Quoting Tvrtko Ursulin (2020-03-09 23:26:34)
On 09/03/2020 21:34, Chris Wilson wrote:
Quoting Tvrtko Ursulin (2020-03-09 18:31:18)
+struct i915_drm_client *
+i915_drm_client_add(struct i915_drm_clients *clients, struct task_struct *task)
+{
+ struct i915_drm_client *client;
+ int ret;
+
+ client = kzalloc(sizeof(*client), GFP_KERNEL);
+ if (!client)
+ return ERR_PTR(-ENOMEM);
+
+ kref_init(&client->kref);
+ client->clients = clients;
+
+ ret = mutex_lock_interruptible(&clients->lock);
+ if (ret)
+ goto err_id;
+ ret = xa_alloc_cyclic(&clients->xarray, &client->id, client,
+ xa_limit_32b, &clients->next_id, GFP_KERNEL);
So what's next_id used for that explains having the over-arching mutex?
It's to give out client id's "cyclically" - before I apparently
misunderstood what xa_alloc_cyclic is supposed to do - I thought after
giving out id 1 it would give out 2 next, even if 1 was returned to the
pool in the meantime. But it doesn't, I need to track the start point
for the next search with "next".
Ok. A requirement of the API for the external counter.
I want this to make intel_gpu_top's life easier, so it doesn't have to
deal with id recycling for all practical purposes.
Fair enough. I only worry about the radix nodes and sparse ids :)
I only found in docs that it should be efficient when the data is
"densely clustered". And given that does appear based on a tree like
structure I thought that means a few clusters of ids should be okay. But
maybe in practice we would have more than a few clusters. I guess that
could indeed be the case.. hm.. Maybe I could use a list and keep
pointer to last entry. When u32 next wraps I reset to list head.
Downside is any search for next free id potentially has to walk over one
used up cluster. May be passable apart for IGT-type stress tests.
And a peek into xa implementation told me the internal lock is not
protecting "next.
See xa_alloc_cyclic(), seems to cover __xa_alloc_cycle (where *next is
manipulated) under the xa_lock.
Ha, true, not sure how I went past top-level and forgot what's in there. :)
Regards,
Tvrtko
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