Re: [PATCH] ring-buffer: Use a housekeeping CPU to wake up waiters

Steven Rostedt
2026-01-08T16:57:40Z

On Thu, 8 Jan 2026 09:39:32 +0100
Petr Tesarik <ptesarik@suse.com> wrote:

> > > Or we simply change it to:
> > >
> > > static inline void
> >
> > Actually, the above should be noinline, as it's in a slower path, and
> > should not be adding logic into the cache of the fast path.
>
> However, to be honest, I'm surprized this is considered slow path. My
> use case is to record a few selected trace events with "trace-cmd
> record", which spends most time polling trace_pipe_raw. Consequently,
> there is almost always a pending waiter that requires a wakeup.
>
> In short, irq_work_queue() is the hot path for me.
>
> OTOH I don't mind making it noinline, because on recent Intel and AMD
> systems, a function call (noinline) is often cheaper than an increase
> in L1 cache footprint (caused by inlining). But I'm confused. I have
> always thought most people use tracing same way as I do.

The call to rb_wakeups() is a fast path, but the wakeup itself is a slow
path. This is the case even when you have user space in a loop that is just
waiting on data.

User space tool:

ring_buffer_wait() {
wake_event_interruptible(.., rb_wait_cond(..));
}

Writer:

rb_wakeups() {
if (!full_hit())
return;
}

The full_hit() is the watermark check. If you look at the tracefs
directory, you'll see a "buffer_percent" file, which is default set to 50.
That means that full_hit() will not return true until the ring buffer is
around 50 percent full. This function is called thousands of times before
the first wakeup happens.

Let's look at even a waiter that isn't using the buffer percent. This means
it will be woken up on any event in the buffer.

rb_wakeups() {
if (buffer->irq_work.waiters_pending) {
buffer->irq_work.waiters_pending = false;
/* irq_work_queue() supplies it's own memory barriers */
irq_work_queue(&buffer->irq_work.work);

So it clears the waiters_pending flag and wakes up the waiter. Now the
waiter wakes up and starts reading the ring buffer. While the ring buffer
has content, it will continue to read and doesn't block again until the
ring buffer is empty. This means that thousands of events are being
recorded with no waiters to wake up.

See why this is a slow path?

-- Steve