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    SubjectRe: [PATCH v2 1/4] ftrace: Implement fs notification for preempt/irqsoff tracers
    On Wed, May 01, 2019 at 10:36:47PM +0200, Viktor Rosendahl wrote:
    > This patch implements the feature that the trace file, e.g.
    > /sys/kernel/debug/tracing/trace will receive notifications through
    > the fsnotify framework when a new trace is available.
    >
    > This makes it possible to implement a user space program that can,
    > with equal probability, obtain traces of latencies that occur
    > immediately after each other in spite of the fact that the
    > preempt/irqsoff tracers operate in overwrite mode.
    >
    > Signed-off-by: Viktor Rosendahl <viktor.rosendahl@gmail.com>

    I agree with the general idea, but I don't really like how it is done in the
    patch.

    We do have a notification mechanism already in the form of trace_pipe. Can we
    not improve that in some way to be notified of a new trace data? In theory,
    the trace_pipe does fit into the description in the documentation: "Reads
    from this file will block until new data is retrieved"

    More comment below:

    > ---
    > kernel/trace/Kconfig | 10 ++++++++++
    > kernel/trace/trace.c | 31 +++++++++++++++++++++++++++++--
    > kernel/trace/trace.h | 5 +++++
    > kernel/trace/trace_irqsoff.c | 35 +++++++++++++++++++++++++++++++++++
    > 4 files changed, 79 insertions(+), 2 deletions(-)
    >
    > diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
    > index 8bd1d6d001d7..35e5fd3224f6 100644
    > --- a/kernel/trace/Kconfig
    > +++ b/kernel/trace/Kconfig
    > @@ -234,6 +234,16 @@ config PREEMPT_TRACER
    > enabled. This option and the irqs-off timing option can be
    > used together or separately.)
    >
    > + config PREEMPTIRQ_FSNOTIFY
    > + bool "Generate fsnotify events for the latency tracers"
    > + default n
    > + depends on (IRQSOFF_TRACER || PREEMPT_TRACER) && FSNOTIFY
    > + help
    > + This option will enable the generation of fsnotify events for the
    > + trace file. This makes it possible for userspace to be notified about
    > + modification of /sys/kernel/debug/tracing/trace through the inotify
    > + interface.

    Does this have to be a CONFIG option? If prefer if the code automatically
    does the notification and it is always enabled. I don't see any drawbacks of
    that.

    > +
    > config SCHED_TRACER
    > bool "Scheduling Latency Tracer"
    > select GENERIC_TRACER
    > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
    > index ca1ee656d6d8..ebefb8d4e072 100644
    > --- a/kernel/trace/trace.c
    > +++ b/kernel/trace/trace.c
    > @@ -44,6 +44,8 @@
    > #include <linux/trace.h>
    > #include <linux/sched/clock.h>
    > #include <linux/sched/rt.h>
    > +#include <linux/fsnotify.h>
    > +#include <linux/workqueue.h>
    >
    > #include "trace.h"
    > #include "trace_output.h"
    > @@ -8191,6 +8193,32 @@ static __init void create_trace_instances(struct dentry *d_tracer)
    > return;
    > }
    >
    > +#ifdef CONFIG_PREEMPTIRQ_FSNOTIFY
    > +
    > +static void trace_notify_workfn(struct work_struct *work)
    [snip]

    I prefer if this facility is available to other tracers as well such as
    the wakeup tracer which is similar in output (check
    Documentation/trace/ftrace.txt). I believe this should be a generic trace
    facility, and not tracer specific.

    thanks,

    - Joel

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