Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx> writes: > +Rob, Eric, Mark and more > > Hi, > > On Fri, 5 Apr 2019 16:20:45 +0100 > Steven Price <steven.price@xxxxxxx> wrote: > >> On 04/04/2019 16:20, Boris Brezillon wrote: >> > Hello, >> > >> > This patch adds new ioctls to expose GPU counters to userspace. >> > These will be used by the mesa driver (should be posted soon). >> > >> > A few words about the implementation: I followed the VC4/Etnaviv model >> > where perf counters are retrieved on a per-job basis. This allows one >> > to have get accurate results when there are users using the GPU >> > concurrently. >> > AFAICT, the mali kbase is using a different approach where several >> > users can register a performance monitor but with no way to have fined >> > grained control over what job/GPU-context to track. >> >> mali_kbase submits overlapping jobs. The jobs on slot 0 and slot 1 can >> be from different contexts (address spaces), and mali_kbase also fully >> uses the _NEXT registers. So there can be a job from one context >> executing on slot 0 and a job from a different context waiting in the >> _NEXT registers. (And the same for slot 1). This means that there's no >> (visible) gap between the first job finishing and the second job >> starting. Early versions of the driver even had a throttle to avoid >> interrupt storms (see JOB_IRQ_THROTTLE) which would further delay the >> IRQ - but thankfully that's gone. >> >> The upshot is that it's basically impossible to measure "per-job" >> counters when running at full speed. Because multiple jobs are running >> and the driver doesn't actually know when one ends and the next starts. >> >> Since one of the primary use cases is to draw pretty graphs of the >> system load [1], this "per-job" information isn't all that relevant (and >> minimal performance overhead is important). And if you want to monitor >> just one application it is usually easiest to ensure that it is the only >> thing running. >> >> [1] >> https://developer.arm.com/tools-and-software/embedded/arm-development-studio/components/streamline-performance-analyzer >> >> > This design choice comes at a cost: every time the perfmon context >> > changes (the perfmon context is the list of currently active >> > perfmons), the driver has to add a fence to prevent new jobs from >> > corrupting counters that will be dumped by previous jobs. >> > >> > Let me know if that's an issue and if you think we should approach >> > things differently. >> >> It depends what you expect to do with the counters. Per-job counters are >> certainly useful sometimes. But serialising all jobs can mess up the >> thing you are trying to measure the performance of. > > I finally found some time to work on v2 this morning, and it turns out > implementing global perf monitors as done in mali_kbase means rewriting > almost everything (apart from the perfcnt layout stuff). I'm not against > doing that, but I'd like to be sure this is really what we want. > > Eric, Rob, any opinion on that? Is it acceptable to expose counters > through the pipe_query/AMD_perfmon interface if we don't have this > job (or at least draw call) granularity? If not, should we keep the > solution I'm proposing here to make sure counters values are accurate, > or should we expose perf counters through a non-standard API? You should definitely not count perf results from someone else's context against your own! People doing perf analysis will expect slight performance changes (like missing bin/render parallelism between contexts) when doing perf queries, but they will be absolutely lost if their non-texturing job starts showing texturing results from some unrelated context.
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