diff options
| -rw-r--r-- | drivers/gpu/drm/i915/i915_perf.c | 163 |
1 files changed, 163 insertions, 0 deletions
diff --git a/drivers/gpu/drm/i915/i915_perf.c b/drivers/gpu/drm/i915/i915_perf.c index 1a87fe967439..95512824922b 100644 --- a/drivers/gpu/drm/i915/i915_perf.c +++ b/drivers/gpu/drm/i915/i915_perf.c @@ -24,6 +24,169 @@ * Robert Bragg <robert@sixbynine.org> */ + +/** + * DOC: i915 Perf, streaming API for GPU metrics + * + * Gen graphics supports a large number of performance counters that can help + * driver and application developers understand and optimize their use of the + * GPU. + * + * This i915 perf interface enables userspace to configure and open a file + * descriptor representing a stream of GPU metrics which can then be read() as + * a stream of sample records. + * + * The interface is particularly suited to exposing buffered metrics that are + * captured by DMA from the GPU, unsynchronized with and unrelated to the CPU. + * + * Streams representing a single context are accessible to applications with a + * corresponding drm file descriptor, such that OpenGL can use the interface + * without special privileges. Access to system-wide metrics requires root + * privileges by default, unless changed via the dev.i915.perf_event_paranoid + * sysctl option. + * + * + * The interface was initially inspired by the core Perf infrastructure but + * some notable differences are: + * + * i915 perf file descriptors represent a "stream" instead of an "event"; where + * a perf event primarily corresponds to a single 64bit value, while a stream + * might sample sets of tightly-coupled counters, depending on the + * configuration. For example the Gen OA unit isn't designed to support + * orthogonal configurations of individual counters; it's configured for a set + * of related counters. Samples for an i915 perf stream capturing OA metrics + * will include a set of counter values packed in a compact HW specific format. + * The OA unit supports a number of different packing formats which can be + * selected by the user opening the stream. Perf has support for grouping + * events, but each event in the group is configured, validated and + * authenticated individually with separate system calls. + * + * i915 perf stream configurations are provided as an array of u64 (key,value) + * pairs, instead of a fixed struct with multiple miscellaneous config members, + * interleaved with event-type specific members. + * + * i915 perf doesn't support exposing metrics via an mmap'd circular buffer. + * The supported metrics are being written to memory by the GPU unsynchronized + * with the CPU, using HW specific packing formats for counter sets. Sometimes + * the constraints on HW configuration require reports to be filtered before it + * would be acceptable to expose them to unprivileged applications - to hide + * the metrics of other processes/contexts. For these use cases a read() based + * interface is a good fit, and provides an opportunity to filter data as it + * gets copied from the GPU mapped buffers to userspace buffers. + * + * + * Some notes regarding Linux Perf: + * -------------------------------- + * + * The first prototype of this driver was based on the core perf + * infrastructure, and while we did make that mostly work, with some changes to + * perf, we found we were breaking or working around too many assumptions baked + * into perf's currently cpu centric design. + * + * In the end we didn't see a clear benefit to making perf's implementation and + * interface more complex by changing design assumptions while we knew we still + * wouldn't be able to use any existing perf based userspace tools. + * + * Also considering the Gen specific nature of the Observability hardware and + * how userspace will sometimes need to combine i915 perf OA metrics with + * side-band OA data captured via MI_REPORT_PERF_COUNT commands; we're + * expecting the interface to be used by a platform specific userspace such as + * OpenGL or tools. This is to say; we aren't inherently missing out on having + * a standard vendor/architecture agnostic interface by not using perf. + * + * + * For posterity, in case we might re-visit trying to adapt core perf to be + * better suited to exposing i915 metrics these were the main pain points we + * hit: + * + * - The perf based OA PMU driver broke some significant design assumptions: + * + * Existing perf pmus are used for profiling work on a cpu and we were + * introducing the idea of _IS_DEVICE pmus with different security + * implications, the need to fake cpu-related data (such as user/kernel + * registers) to fit with perf's current design, and adding _DEVICE records + * as a way to forward device-specific status records. + * + * The OA unit writes reports of counters into a circular buffer, without + * involvement from the CPU, making our PMU driver the first of a kind. + * + * Given the way we were periodically forward data from the GPU-mapped, OA + * buffer to perf's buffer, those bursts of sample writes looked to perf like + * we were sampling too fast and so we had to subvert its throttling checks. + * + * Perf supports groups of counters and allows those to be read via + * transactions internally but transactions currently seem designed to be + * explicitly initiated from the cpu (say in response to a userspace read()) + * and while we could pull a report out of the OA buffer we can't + * trigger a report from the cpu on demand. + * + * Related to being report based; the OA counters are configured in HW as a + * set while perf generally expects counter configurations to be orthogonal. + * Although counters can be associated with a group leader as they are + * opened, there's no clear precedent for being able to provide group-wide + * configuration attributes (for example we want to let userspace choose the + * OA unit report format used to capture all counters in a set, or specify a + * GPU context to filter metrics on). We avoided using perf's grouping + * feature and forwarded OA reports to userspace via perf's 'raw' sample + * field. This suited our userspace well considering how coupled the counters + * are when dealing with normalizing. It would be inconvenient to split + * counters up into separate events, only to require userspace to recombine + * them. For Mesa it's also convenient to be forwarded raw, periodic reports + * for combining with the side-band raw reports it captures using + * MI_REPORT_PERF_COUNT commands. + * + * _ As a side note on perf's grouping feature; there was also some concern + * that using PERF_FORMAT_GROUP as a way to pack together counter values + * would quite drastically inflate our sample sizes, which would likely + * lower the effective sampling resolutions we could use when the available + * memory bandwidth is limited. + * + * With the OA unit's report formats, counters are packed together as 32 + * or 40bit values, with the largest report size being 256 bytes. + * + * PERF_FORMAT_GROUP values are 64bit, but there doesn't appear to be a + * documented ordering to the values, implying PERF_FORMAT_ID must also be + * used to add a 64bit ID before each value; giving 16 bytes per counter. + * + * Related to counter orthogonality; we can't time share the OA unit, while + * event scheduling is a central design idea within perf for allowing + * userspace to open + enable more events than can be configured in HW at any + * one time. The OA unit is not designed to allow re-configuration while in + * use. We can't reconfigure the OA unit without losing internal OA unit + * state which we can't access explicitly to save and restore. Reconfiguring + * the OA unit is also relatively slow, involving ~100 register writes. From + * userspace Mesa also depends on a stable OA configuration when emitting + * MI_REPORT_PERF_COUNT commands and importantly the OA unit can't be + * disabled while there are outstanding MI_RPC commands lest we hang the + * command streamer. + * + * The contents of sample records aren't extensible by device drivers (i.e. + * the sample_type bits). As an example; Sourab Gupta had been looking to + * attach GPU timestamps to our OA samples. We were shoehorning OA reports + * into sample records by using the 'raw' field, but it's tricky to pack more + * than one thing into this field because events/core.c currently only lets a + * pmu give a single raw data pointer plus len which will be copied into the + * ring buffer. To include more than the OA report we'd have to copy the + * report into an intermediate larger buffer. I'd been considering allowing a + * vector of data+len values to be specified for copying the raw data, but + * it felt like a kludge to being using the raw field for this purpose. + * + * - It felt like our perf based PMU was making some technical compromises + * just for the sake of using perf: + * + * perf_event_open() requires events to either relate to a pid or a specific + * cpu core, while our device pmu related to neither. Events opened with a + * pid will be automatically enabled/disabled according to the scheduling of + * that process - so not appropriate for us. When an event is related to a + * cpu id, perf ensures pmu methods will be invoked via an inter process + * interrupt on that core. To avoid invasive changes our userspace opened OA + * perf events for a specific cpu. This was workable but it meant the + * majority of the OA driver ran in atomic context, including all OA report + * forwarding, which wasn't really necessary in our case and seems to make + * our locking requirements somewhat complex as we handled the interaction + * with the rest of the i915 driver. + */ + #include <linux/anon_inodes.h> #include <linux/sizes.h> |