• Peter Zijlstra's avatar
    perf: Rewrite core context handling · bd275681
    Peter Zijlstra authored
    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 PMU
     - S390 cpum_cf PMU
     - PowerPC trace_imc PMU
    
    *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                            |
                                      perf_event ---> pmu ----------------'
    
    With the new design, perf_event_context will hold all events for all
    pmus in the (respective pinned/flexible) rbtrees. This can be achieved
    by adding pmu to rbtree key:
    
      {cpu, pmu, cgroup, group_index}
    
    Each perf_event_context carries a list of perf_event_pmu_context which
    is used to hold per-pmu-per-context state. For example, 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.
    
    Additionally, 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.
    
    Further optimizations to current implementation are possible. For
    example, ctx_resched() can be optimized to reschedule only single pmu
    events.
    
    Much thanks to Ravi for picking this up and pushing it towards
    completion.
    Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
    Co-developed-by: default avatarRavi Bangoria <ravi.bangoria@amd.com>
    Signed-off-by: default avatarRavi Bangoria <ravi.bangoria@amd.com>
    Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
    Link: https://lkml.kernel.org/r/20221008062424.313-1-ravi.bangoria@amd.com
    bd275681
brs.c 9.84 KB