Messages in this thread | | | Date | Mon, 31 Jan 2022 10:13:09 +0530 | Subject | Re: [RFC v2] perf: Rewrite core context handling | From | Ravi Bangoria <> |
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On 13-Jan-22 7:17 PM, Ravi Bangoria wrote: > From: Peter Zijlstra <peterz@infradead.org> > > This is the 2nd version of RFC originally posted by Peter[1]. > > There have been various issues and limitations with the way perf uses > (task) contexts to track events. Most notable is the single hardware > PMU task context, which has resulted in a number of yucky things (both > proposed and merged). > > Notably: > - HW breakpoint PMU > - ARM big.little PMU / Intel ADL PMU > - Intel Branch Monitoring PMU > - AMD IBS > > Current design: > --------------- > Currently we have a per task and per cpu perf_event_contexts: > > task_struct::perf_events_ctxp[] <-> perf_event_context <-> perf_cpu_context > ^ | ^ | > `---------------------------------' | `--> pmu > v ^ > perf_event ------' > > Each task has an array of pointers to a perf_event_context. Each > perf_event_context has a direct relation to a PMU and a group of > events for that PMU. The task related perf_event_context's have a > pointer back to that task. > > Each PMU has a per-cpu pointer to a per-cpu perf_cpu_context, which > includes a perf_event_context, which again has a direct relation to > that PMU, and a group of events for that PMU. > > The perf_cpu_context also tracks which task context is currently > associated with that CPU and includes a few other things like the > hrtimer for rotation etc. > > Each perf_event is then associated with its PMU and one > perf_event_context. > > Proposed design: > ---------------- > New design proposed by this patch reduce to a single task context and > a single CPU context but adds some intermediate data-structures: > > task_struct::perf_event_ctxp -> perf_event_context <- perf_cpu_context > ^ | ^ ^ > `---------------------------------' | | > | | perf_cpu_pmu_context > | `----. ^ > | | | > | v v > | ,--> perf_event_pmu_context > | | ^ > | | | > v v v > perf_event ---> pmu > > With new design, perf_event_context will hold all pmu events in the > respective(pinned/flexible) rbtrees. This can be achieved by adding > pmu to rbtree key: > > {cpu, pmu, cgroup_id, group_index} > > Each perf_event_context carry a list of perf_event_pmu_context which > is used to hold per-pmu-per-context state. For ex, it keeps track of > currently active events for that pmu, a pmu specific task_ctx_data, > a flag to tell whether rotation is required or not etc. > > Similarly perf_cpu_pmu_context is used to hold per-pmu-per-cpu state > like hrtimer details to drive the event rotation, a pointer to > perf_event_pmu_context of currently running task and some other > ancillary information. > > Each perf_event is associated to it's pmu, perf_event_context and > perf_event_pmu_context. > > Original RFC -> RFC v2: > ----------------------- > In addition to porting the patch to latest (v5.16-rc6) kernel, here > are some of the major changes between two revisions: > > - There were quite a bit of fundamental changes since original patch. > Most notably a rbtree key has changed from {cpu,group_index} to > {cpu,cgroup_id,group_index}. Adding a pmu key in between as proposed > in original patch is not straight forward as it will break cgroup > specific optimization. Hence we need to iterate over all pmu_ctx > for a given ctx and call visit_groups_merge() one by one. > - Enabled cgroup support (CGROUP_PERF). > - Some changes wrt multiplexing events as with new design the rotation > happens at cgroup subtree unlike at pmu subtree in original patch. > > Because of additional complexity above changes bring in, I thought to > get initial review about the overall approach before starting to make it > upstream ready. Hence this patch just provides an idea of the direction > we will head toward. Many loose ends in the patch rightnow. Like, I've > not paid much attention to synchronization related aspects. Similarly, > some of the issues marked in original patch (XXX) haven't been fixed. > > A simple perf stat/record/top survives with the patch but machine > crashes with first run of perf test (stale cpc->task_epc causing the > crash). Lockdep is also screaming a lot :)
Hi Peter, can you please review this.
Thanks, Ravi
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