drm/i915/guc: Delay disabling guc_id scheduling for better hysteresis
Add a delay, configurable via debugfs (default 34ms), to disable scheduling of a context after the pin count goes to zero. Disable scheduling is a costly operation as it requires synchronizing with the GuC. So the idea is that a delay allows the user to resubmit something before doing this operation. This delay is only done if the context isn't closed and less than a given threshold (default is 3/4) of the guc_ids are in use. Alan Previn: Matt Brost first introduced this patch back in Oct 2021. However no real world workload with measured performance impact was available to prove the intended results. Today, this series is being republished in response to a real world workload that benefited greatly from it along with measured performance improvement. Workload description: 36 containers were created on a DG2 device where each container was performing a combination of 720p 3d game rendering and 30fps video encoding. The workload density was configured in a way that guaranteed each container to ALWAYS be able to render and encode no less than 30fps with a predefined maximum render + encode latency time. That means the totality of all 36 containers and their workloads were not saturating the engines to their max (in order to maintain just enough headrooom to meet the min fps and max latencies of incoming container submissions). Problem statement: It was observed that the CPU core processing the i915 soft IRQ work was experiencing severe load. Using tracelogs and an instrumentation patch to count specific i915 IRQ events, it was confirmed that the majority of the CPU cycles were caused by the gen11_other_irq_handler() -> guc_irq_handler() code path. The vast majority of the cycles was determined to be processing a specific G2H IRQ: i.e. INTEL_GUC_ACTION_SCHED_CONTEXT_MODE_DONE. These IRQs are sent by GuC in response to i915 KMD sending H2G requests: INTEL_GUC_ACTION_SCHED_CONTEXT_MODE_SET. Those H2G requests are sent whenever a context goes idle so that we can unpin the context from GuC. The high CPU utilization % symptom was limiting density scaling. Root Cause Analysis: Because the incoming execution buffers were spread across 36 different containers (each with multiple contexts) but the system in totality was NOT saturated to the max, it was assumed that each context was constantly idling between submissions. This was causing a thrashing of unpinning contexts from GuC at one moment, followed quickly by repinning them due to incoming workload the very next moment. These event-pairs were being triggered across multiple contexts per container, across all containers at the rate of > 30 times per sec per context. Metrics: When running this workload without this patch, we measured an average of ~69K INTEL_GUC_ACTION_SCHED_CONTEXT_MODE_DONE events every 10 seconds or ~10 million times over ~25+ mins. With this patch, the count reduced to ~480 every 10 seconds or about ~28K over ~10 mins. The improvement observed is ~99% for the average counts per 10 seconds. Design awareness: Selftest impact. As temporary WA disable this feature for the selftests. Selftests are very timing sensitive and any change in timing can cause failure. A follow up patch will fixup the selftests to understand this delay. Design awareness: Race between guc_request_alloc and guc_context_close. If a context close is issued while there is a request submission in flight and a delayed schedule disable is pending, guc_context_close and guc_request_alloc will race to cancel the delayed disable. To close the race, make sure that guc_request_alloc waits for guc_context_close to finish running before checking any state. Design awareness: GT Reset event. If a gt reset is triggered, as preparation steps, add an additional step to ensure all contexts that have a pending delay-disable-schedule task be flushed of it. Move them directly into the closed state after cancelling the worker. This is okay because the existing flow flushes all yet-to-arrive G2H's dropping them anyway. Signed-off-by:Matthew Brost <matthew.brost@intel.com> Signed-off-by:
Alan Previn <alan.previn.teres.alexis@intel.com> Signed-off-by:
Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com> Reviewed-by:
John Harrison <John.C.Harrison@Intel.com> Signed-off-by:
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