Re: [PATCH v7 1/4] ftrace: Implement fs notification for tracing_max_latency

Joel Fernandes
2019-10-02T00:32:07Z

Some nits, but others looks good:

On Fri, Sep 20, 2019 at 05:22:16PM +0200, Viktor Rosendahl (BMW) 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().
>
> This patch creates a new workqueue. The reason for doing this is that
> I wanted to use the WQ_UNBOUND and WQ_HIGHPRI flags. My thinking was
> that WQ_UNBOUND might help with the latency in some important cases.
>
> If we use:
>
> queue_work(system_highpri_wq, &tr->fsnotify_work);
>
> then the work will (almost) always execute on the same CPU but if we are
> unlucky that CPU could be too busy while there could be another CPU in
> the system that would be able to process the work soon enough.
>
> queue_work_on() could be used to queue the work on another CPU but it
> seems difficult to select the right CPU.
>
> Signed-off-by: Viktor Rosendahl (BMW) <viktor.rosendahl@gmail.com>
> ---
> 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 947ba433865f..2d2150bf0d42 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;
> + }
> + return 0;
> +}
> +
> +late_initcall_sync(latency_fsnotify_init);
> +
> +void latency_fsnotify(struct trace_array *tr)
> +{
> + if (!fsnotify_wq)
> + return;
> + /*
> + * We cannot call queue_work(&tr->fsnotify_work) from here because it's
> + * possible that we are called from __schedule() or do_idle(), which
> + * could cause a deadlock.
> + */
> + irq_work_queue(&tr->fsnotify_irqwork);
> +}
> +
> +/*
> + * (defined(CONFIG_TRACER_MAX_TRACE) || defined(CONFIG_HWLAT_TRACER)) && \
> + * defined(CONFIG_FSNOTIFY)
> + */
> +#else
> +
> +#define trace_create_maxlat_file(tr, d_tracer) \
> + trace_create_file("tracing_max_latency", 0644, d_tracer, \
> + &tr->max_latency, &tracing_max_lat_fops)

I would forward declare tracing_max_lat_fops here as well like you are doing
in the #if section. This has 2 advantages:

1. It keeps it consistent with the #if block.
2. It prevents future breakages where trace_create_maxlat_file() might be
called before tracing_max_lat_fops is defined.

> +
> +#endif
> +
> #ifdef CONFIG_TRACER_MAX_TRACE
> /*
> * Copy the new maximum trace into the separate maximum-trace
> @@ -1518,6 +1589,7 @@ __update_max_tr(struct trace_array *tr, struct task_struct *tsk, int cpu)
>
> /* record this tasks comm */
> tracing_record_cmdline(tsk);
> + latency_fsnotify(tr);
> }
>
> /**
> @@ -8550,8 +8622,7 @@ init_tracer_tracefs(struct trace_array *tr, struct dentry *d_tracer)
> create_trace_options_dir(tr);
>
> #if defined(CONFIG_TRACER_MAX_TRACE) || defined(CONFIG_HWLAT_TRACER)
> - trace_create_file("tracing_max_latency", 0644, d_tracer,
> - &tr->max_latency, &tracing_max_lat_fops);
> + trace_create_maxlat_file(tr, d_tracer);
> #endif
>
> if (ftrace_create_function_files(tr, d_tracer))
> diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
> index 005f08629b8b..4913ed1138aa 100644
> --- a/kernel/trace/trace.h
> +++ b/kernel/trace/trace.h
> @@ -16,6 +16,8 @@
> #include <linux/trace_events.h>
> #include <linux/compiler.h>
> #include <linux/glob.h>
> +#include <linux/irq_work.h>
> +#include <linux/workqueue.h>
>
> #ifdef CONFIG_FTRACE_SYSCALLS
> #include <asm/unistd.h> /* For NR_SYSCALLS */
> @@ -264,6 +266,11 @@ struct trace_array {
> #endif
> #if defined(CONFIG_TRACER_MAX_TRACE) || defined(CONFIG_HWLAT_TRACER)
> unsigned long max_latency;
> +#ifdef CONFIG_FSNOTIFY
> + struct dentry *d_max_latency;
> + struct work_struct fsnotify_work;
> + struct irq_work fsnotify_irqwork;
> +#endif
> #endif
> struct trace_pid_list __rcu *filtered_pids;
> /*
> @@ -785,6 +792,17 @@ void update_max_tr_single(struct trace_array *tr,
> struct task_struct *tsk, int cpu);
> #endif /* CONFIG_TRACER_MAX_TRACE */
>
> +#if (defined(CONFIG_TRACER_MAX_TRACE) || defined(CONFIG_HWLAT_TRACER)) && \
> + defined(CONFIG_FSNOTIFY)
> +
> +void latency_fsnotify(struct trace_array *tr);
> +
> +#else
> +
> +#define latency_fsnotify(tr) do { } while (0)

Please define the NOOP as a function so that compiler checks the arguments.

So:
static void latency_fsnotify(struct trace_array *tr) { }

Other than these nits, the patch looks fine to me.

Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>

thanks,

- Joel