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All the requests at the same priority are executed in FIFO order. They do not need to be stored in the rbtree themselves, as they are a simple list within a level. If we move the requests at one priority into a list, we can then reduce the rbtree to the set of priorities. This should keep the height of the rbtree small, as the number of active priorities can not exceed the number of active requests and should be typically only a few. Currently, we have ~2k possible different priority levels, that may increase to allow even more fine grained selection. Allocating those in advance seems a waste (and may be impossible), so we opt for allocating upon first use, and freeing after its requests are depleted. To avoid the possibility of an allocation failure causing us to lose a request, we preallocate the default priority (0) and bump any request to that priority if we fail to allocate it the appropriate plist. Having a request (that is ready to run, so not leading to corruption) execute out-of-order is better than leaking the request (and its dependency tree) entirely. There should be a benefit to reducing execlists_dequeue() to principally using a simple list (and reducing the frequency of both rbtree iteration and balancing on erase) but for typical workloads, request coalescing should be small enough that we don't notice any change. The main gain is from improving PI calls to schedule, and the explicit list within a level should make request unwinding simpler (we just need to insert at the head of the list rather than the tail and not have to make the rbtree search more complicated). v2: Avoid use-after-free when deleting a depleted priolist v3: Michał found the solution to handling the allocation failure gracefully. If we disable all priority scheduling following the allocation failure, those requests will be executed in fifo and we will ensure that this request and its dependencies are in strict fifo (even when it doesn't realise it is only a single list). Normal scheduling is restored once we know the device is idle, until the next failure! Suggested-by: Michał Wajdeczko <michal.wajdeczko@intel.com> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk> Cc: Michał Winiarski <michal.winiarski@intel.com> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com> Reviewed-by: Michał Winiarski <michal.winiarski@intel.com> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com> Link: http://patchwork.freedesktop.org/patch/msgid/20170517121007.27224-8-chris@chris-wilson.co.uk |
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Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.