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SubjectRe: [PATCH v6 1/4] ftrace: Implement fs notification for tracing_max_latency
On Thu, Sep 05, 2019 at 03:25:45PM +0200, Viktor Rosendahl wrote:
> This patch implements the feature that the tracing_max_latency file,
> e.g. /sys/kernel/debug/tracing/tracing_max_latency will receive
> notifications through the fsnotify framework when a new latency is
> available.
>
> One particularly interesting use of this facility is when enabling
> threshold tracing, through /sys/kernel/debug/tracing/tracing_thresh,
> together with the preempt/irqsoff tracers. 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.
>
> This facility works with the hwlat, preempt/irqsoff, and wakeup
> tracers.
>
> The tracers may call the latency_fsnotify() from places such as
> __schedule() or do_idle(); this makes it impossible to call
> queue_work() directly without risking a deadlock. The same would
> happen with a softirq, kernel thread or tasklet. For this reason we
> use the irq_work mechanism to call queue_work().
> Signed-off-by: Viktor Rosendahl <viktor.rosendahl@gmail.com>

Looks better now, one comment below:

> ---
> kernel/trace/trace.c | 75 +++++++++++++++++++++++++++++++++++++-
> kernel/trace/trace.h | 18 +++++++++
> kernel/trace/trace_hwlat.c | 4 +-
> 3 files changed, 94 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 563e80f9006a..72ac20c4aaa1 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -44,6 +44,9 @@
> #include <linux/trace.h>
> #include <linux/sched/clock.h>
> #include <linux/sched/rt.h>
> +#include <linux/fsnotify.h>
> +#include <linux/irq_work.h>
> +#include <linux/workqueue.h>
>
> #include "trace.h"
> #include "trace_output.h"
> @@ -1480,6 +1483,74 @@ static ssize_t trace_seq_to_buffer(struct trace_seq *s, void *buf, size_t cnt)
>
> unsigned long __read_mostly tracing_thresh;
>
> +#if (defined(CONFIG_TRACER_MAX_TRACE) || defined(CONFIG_HWLAT_TRACER)) && \
> + defined(CONFIG_FSNOTIFY)
> +
> +static const struct file_operations tracing_max_lat_fops;
> +static struct workqueue_struct *fsnotify_wq;
> +
> +static void latency_fsnotify_workfn(struct work_struct *work)
> +{
> + struct trace_array *tr = container_of(work, struct trace_array,
> + fsnotify_work);
> + fsnotify(tr->d_max_latency->d_inode, FS_MODIFY,
> + tr->d_max_latency->d_inode, FSNOTIFY_EVENT_INODE, NULL, 0);
> +}
> +
> +static void latency_fsnotify_workfn_irq(struct irq_work *iwork)
> +{
> + struct trace_array *tr = container_of(iwork, struct trace_array,
> + fsnotify_irqwork);
> + queue_work(fsnotify_wq, &tr->fsnotify_work);
> +}
> +
> +static void trace_create_maxlat_file(struct trace_array *tr,
> + struct dentry *d_tracer)
> +{
> + INIT_WORK(&tr->fsnotify_work, latency_fsnotify_workfn);
> + init_irq_work(&tr->fsnotify_irqwork, latency_fsnotify_workfn_irq);
> + tr->d_max_latency = trace_create_file("tracing_max_latency", 0644,
> + d_tracer, &tr->max_latency,
> + &tracing_max_lat_fops);
> +}
> +
> +__init static int latency_fsnotify_init(void)
> +{
> + fsnotify_wq = alloc_workqueue("tr_max_lat_wq",
> + WQ_UNBOUND | WQ_HIGHPRI, 0);
> + if (!fsnotify_wq) {
> + pr_err("Unable to allocate tr_max_lat_wq\n");
> + return -ENOMEM;
> + }

Why not just use the system workqueue instead of adding another workqueue?

thanks,

- Joel


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