This is a continuation of the previous discussion which was here: https://lore.kernel.org/dri-devel/[email protected]/
Work is now converted to a much simpler approach by adding real-time scheduling workqueues based on Tejun's feedback To re-cap, when an userspace thread submits GPU work, due how the DRM scheduler uses workqueues to feed the GPU, and regardless of the GPU rendering context priority, or the CPU scheduling priority of the userspace thread itself, the use of workqueues adds (a lot of) latency to the submit path. When CPU is busy with enough backround load this translates to severe latency spikes measured as time between userspace submitting work and GPU actually being given that work to execute. With the panthor workqueue upgraded to use WQ_HIGHPRI and varying the CPU priority of the submit thread, the test program from https://gitlab.freedesktop.org/panfrost/linux/-/work_items/49 reproduces these kind of latencies: . N RT M 27 28 32 95% 163 246 809 98% 924 991 1882 Legend: M = Median submit latency in us 95% = Percentile latency in us . = Userspace submit thread SCHED_OTHER N = -||= nice -1 RT = -||- FIFO 1 Upgrading the panthor workqueues so that the realtime GPU priority queue uses WQ_RTPRI, submit latency becomes completely controlled with the median of 14us and 95 and 98-th percentiles at 23us and 25us respectively. Important to note is that VK_QUEUE_GLOBAL_PRIORITY_REALTIME already required the userspace to have CAP_SYS_NICE, meaning access to real-time workqueues is effectively also guared behind this capability. The implementation of WQ_RTPRI itself also adds some limites on the maximum number of worker threads in order to prevent scheduling starvation. Cc: Boris Brezillon <[email protected]> Cc: Chia-I Wu <[email protected]> Cc: Liviu Dudau <[email protected]> Cc: Matthew Brost <[email protected]> Cc: Steven Price <[email protected]> Cc: Tejun Heo <[email protected]> Tvrtko Ursulin (2): workqueue: Add support for real-time workers drm/panthor: Create per queue priority workqueues drivers/gpu/drm/panthor/panthor_sched.c | 38 +++++- include/linux/workqueue.h | 22 +++- kernel/workqueue.c | 154 ++++++++++++++++++------ 3 files changed, 164 insertions(+), 50 deletions(-) -- 2.54.0
