linux-kernel - Re: [PATCH v6 10/24] tracing: Introduce simple_ring

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Message-ID: <aMFJSlbHRsvzYKaF@google.com>
Date: Wed, 10 Sep 2025 10:47:54 +0100
From: Vincent Donnefort <vdonnefort@...gle.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: mhiramat@...nel.org, mathieu.desnoyers@...icios.com,
	linux-trace-kernel@...r.kernel.org, maz@...nel.org,
	oliver.upton@...ux.dev, joey.gouly@....com, suzuki.poulose@....com,
	yuzenghui@...wei.com, kvmarm@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org, jstultz@...gle.com,
	qperret@...gle.com, will@...nel.org, aneesh.kumar@...nel.org,
	kernel-team@...roid.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 10/24] tracing: Introduce simple_ring_buffer

[...]

> > +static struct ring_buffer_event *
> > +simple_rb_reserve_next(struct simple_rb_per_cpu *cpu_buffer, unsigned long length, u64 timestamp)
> > +{
> > +	unsigned long ts_ext_size = 0, event_size = rb_event_size(length);
> > +	struct simple_buffer_page *tail = cpu_buffer->tail_page;
> > +	struct ring_buffer_event *event;
> > +	u32 write, prev_write;
> > +	u64 time_delta;
> > +
> > +	time_delta = timestamp - cpu_buffer->write_stamp;
> 
> The remote buffers never get preempted do they?
> 
> That is, it doesn't need to handle different contexts like the normal
> kernel does? (normal, softirq, irq, NMI, etc).

No, luckily, we don't need to support any of that.

> 
> -- Steve
> 
> > +
> > +	if (test_time_stamp(time_delta))
> > +		ts_ext_size = 8;
> > +
> > +	prev_write = tail->write;
> > +	write = prev_write + event_size + ts_ext_size;
> > +
> > +	if (unlikely(write > (PAGE_SIZE - BUF_PAGE_HDR_SIZE)))
> > +		tail = simple_rb_move_tail(cpu_buffer);
> > +
> > +	if (!tail->entries) {
> > +		tail->page->time_stamp = timestamp;
> > +		time_delta = 0;
> > +		ts_ext_size = 0;
> > +		write = event_size;
> > +		prev_write = 0;
> > +	}
> > +
> > +	tail->write = write;
> > +	tail->entries++;
> > +
> > +	cpu_buffer->write_stamp = timestamp;
> > +
> > +	event = (struct ring_buffer_event *)(tail->page->data + prev_write);
> > +	if (ts_ext_size) {
> > +		event = rb_event_add_ts_extend(event, time_delta);
> > +		time_delta = 0;
> > +	}
> > +
> > +	event->type_len = 0;
> > +	event->time_delta = time_delta;
> > +	event->array[0] = event_size - RB_EVNT_HDR_SIZE;
> > +
> > +	return event;
> > +}
> > +