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    Subject[PATCH 2/2] perf: Use hot regs with software sched switch/migrate events
    Date
    Scheduler's task migration events don't work because they always
    pass NULL regs perf_sw_event(). The event hence gets filtered
    in perf_swevent_add().

    Scheduler's context switches events use task_pt_regs() to get
    the context when the event occured which is a wrong thing to
    do as this won't give us the place in the kernel where we went
    to sleep but the place where we left userspace. The result is
    even more wrong if we switch from a kernel thread.

    Use the hot regs snapshot for both events as they belong to the
    non-interrupt/exception based events family. Unlike page faults
    or so that provide the regs matching the exact origin of the event,
    we need to save the current context.

    This makes the task migration event working and fix the context
    switch callchains and origin ip.

    Example: perf record -a -e cs

    Before:

    10.91% ksoftirqd/0 0 [k] 0000000000000000
    |
    --- (nil)
    perf_callchain
    perf_prepare_sample
    __perf_event_overflow
    perf_swevent_overflow
    perf_swevent_add
    perf_swevent_ctx_event
    do_perf_sw_event
    __perf_sw_event
    perf_event_task_sched_out
    schedule
    run_ksoftirqd
    kthread
    kernel_thread_helper

    After:

    23.77% hald-addon-stor [kernel.kallsyms] [k] schedule
    |
    --- schedule
    |
    |--60.00%-- schedule_timeout
    | wait_for_common
    | wait_for_completion
    | blk_execute_rq
    | scsi_execute
    | scsi_execute_req
    | sr_test_unit_ready
    | |
    | |--66.67%-- sr_media_change
    | | media_changed
    | | cdrom_media_changed
    | | sr_block_media_changed
    | | check_disk_change
    | | cdrom_open

    Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
    Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
    Cc: Paul Mackerras <paulus@samba.org>
    Cc: Ingo Molnar <mingo@elte.hu>
    Cc: David Miller <davem@davemloft.net>
    ---
    include/linux/perf_event.h | 21 ++++++++++++++-------
    kernel/perf_event.c | 4 +---
    2 files changed, 15 insertions(+), 10 deletions(-)

    diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
    index 9547703..c8e3754 100644
    --- a/include/linux/perf_event.h
    +++ b/include/linux/perf_event.h
    @@ -842,13 +842,6 @@ extern atomic_t perf_swevent_enabled[PERF_COUNT_SW_MAX];

    extern void __perf_sw_event(u32, u64, int, struct pt_regs *, u64);

    -static inline void
    -perf_sw_event(u32 event_id, u64 nr, int nmi, struct pt_regs *regs, u64 addr)
    -{
    - if (atomic_read(&perf_swevent_enabled[event_id]))
    - __perf_sw_event(event_id, nr, nmi, regs, addr);
    -}
    -
    extern void
    perf_arch_fetch_caller_regs(struct pt_regs *regs, unsigned long ip, int skip);

    @@ -887,6 +880,20 @@ static inline void perf_fetch_caller_regs(struct pt_regs *regs, int skip)
    return perf_arch_fetch_caller_regs(regs, ip, skip);
    }

    +static inline void
    +perf_sw_event(u32 event_id, u64 nr, int nmi, struct pt_regs *regs, u64 addr)
    +{
    + if (atomic_read(&perf_swevent_enabled[event_id])) {
    + struct pt_regs hot_regs;
    +
    + if (!regs) {
    + perf_fetch_caller_regs(&hot_regs, 1);
    + regs = &hot_regs;
    + }
    + __perf_sw_event(event_id, nr, nmi, regs, addr);
    + }
    +}
    +
    extern void __perf_event_mmap(struct vm_area_struct *vma);

    static inline void perf_event_mmap(struct vm_area_struct *vma)
    diff --git a/kernel/perf_event.c b/kernel/perf_event.c
    index fb3031c..bc7943c 100644
    --- a/kernel/perf_event.c
    +++ b/kernel/perf_event.c
    @@ -1164,11 +1164,9 @@ void perf_event_task_sched_out(struct task_struct *task,
    struct perf_event_context *ctx = task->perf_event_ctxp;
    struct perf_event_context *next_ctx;
    struct perf_event_context *parent;
    - struct pt_regs *regs;
    int do_switch = 1;

    - regs = task_pt_regs(task);
    - perf_sw_event(PERF_COUNT_SW_CONTEXT_SWITCHES, 1, 1, regs, 0);
    + perf_sw_event(PERF_COUNT_SW_CONTEXT_SWITCHES, 1, 1, NULL, 0);

    if (likely(!ctx || !cpuctx->task_ctx))
    return;
    --
    1.6.2.3


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