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    Subject[for-next][PATCH 09/23] tracing/timerlat: Notify IRQ new max latency only if stop tracing is set
    From: Daniel Bristot de Oliveira <bristot@kernel.org>

    Currently, the notification of a new max latency is sent from
    timerlat's IRQ handler anytime a new max latency is found.

    While this behavior is not wrong, the send IPI overhead itself
    will increase the thread latency and that is not the desired
    effect (tracing overhead).

    Moreover, the thread will notify a new max latency again because
    the thread latency as it is always higher than the IRQ latency
    that woke it up.

    The only case in which it is helpful to notify a new max latency
    from IRQ is when stop tracing (for the IRQ) is set, as in this
    case, the thread will not be dispatched.

    Notify a new max latency from the IRQ handler only if stop tracing is
    set for the IRQ handler.

    Link: https://lkml.kernel.org/r/2c2d9a56c0886c8402ba320de32856cbbb10c2bb.1652175637.git.bristot@kernel.org

    Cc: Juri Lelli <juri.lelli@redhat.com>
    Cc: Ingo Molnar <mingo@redhat.com>
    Reported-by: Clark Williams <williams@redhat.com>
    Fixes: a955d7eac177 ("trace: Add timerlat tracer")
    Signed-off-by: Daniel Bristot de Oliveira <bristot@kernel.org>
    Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
    ---
    kernel/trace/trace_osnoise.c | 9 +++++----
    1 file changed, 5 insertions(+), 4 deletions(-)

    diff --git a/kernel/trace/trace_osnoise.c b/kernel/trace/trace_osnoise.c
    index e9ae1f33a7f0..6494ca27ea6f 100644
    --- a/kernel/trace/trace_osnoise.c
    +++ b/kernel/trace/trace_osnoise.c
    @@ -1578,11 +1578,12 @@ static enum hrtimer_restart timerlat_irq(struct hrtimer *timer)

    trace_timerlat_sample(&s);

    - notify_new_max_latency(diff);
    -
    - if (osnoise_data.stop_tracing)
    - if (time_to_us(diff) >= osnoise_data.stop_tracing)
    + if (osnoise_data.stop_tracing) {
    + if (time_to_us(diff) >= osnoise_data.stop_tracing) {
    osnoise_stop_tracing();
    + notify_new_max_latency(diff);
    + }
    + }

    wake_up_process(tlat->kthread);

    --
    2.35.1
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