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

[Viktor Rosendahl]

On 10/2/19 5:13 PM, Steven Rostedt wrote:
> On Fri, 20 Sep 2019 17:22:16 +0200
> "Viktor Rosendahl (BMW)" <viktor.rosendahl@xxxxxxxxx> 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().
>
> Can fsnotify() be called from irq context? If so, why have the work
> queue at all? Just do the work from the irq_work handler.

fsnotify() might sleep. It calls send_to_group(), which calls 
inotify_handle_event() through a function pointer.

inotify_handle_event() calls kmalloc() without the GFP_ATOMIC flag.

There might be other reasons as well but the above is one that I have 
seen a warning for, when enabling CONFIG_DEBUG_ATOMIC_SLEEP and trying 
to call fsnotify() from an atomic context.

best regards,

Viktor