Commit 68282dd9 authored by Steven Rostedt (Google)'s avatar Steven Rostedt (Google)
Browse files

ring-buffer: Fix resetting of shortest_full 

The "shortest_full" variable is used to keep track of the waiter that is
waiting for the smallest amount on the ring buffer before being woken up.
When a tasks waits on the ring buffer, it passes in a "full" value that is
a percentage. 0 means wake up on any data. 1-100 means wake up from 1% to
100% full buffer.

As all waiters are on the same wait queue, the wake up happens for the
waiter with the smallest percentage.

The problem is that the smallest_full on the cpu_buffer that stores the
smallest amount doesn't get reset when all the waiters are woken up. It
does get reset when the ring buffer is reset (echo > /sys/kernel/tracing/trace).

This means that tasks may be woken up more often then when they want to
be. Instead, have the shortest_full field get reset just before waking up
all the tasks. If the tasks wait again, they will update the shortest_full
before sleeping.

Also add locking around setting of shortest_full in the poll logic, and
change "work" to "rbwork" to match the variable name for rb_irq_work
structures that are used in other places.

Link: https://lore.kernel.org/linux-trace-kernel/20240308202431.948914369@goodmis.org



Cc: stable@vger.kernel.org
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linke li <lilinke99@qq.com>
Cc: Rabin Vincent <rabin@rab.in>
Fixes: 2c2b0a78 ("ring-buffer: Add percentage of ring buffer full to wake up reader")
Signed-off-by: default avatarSteven Rostedt (Google) <rostedt@goodmis.org>
parent b3594573

kernel/trace/ring_buffer.c

+23 −7
Original line number Diff line number Diff line
@@ -755,8 +755,19 @@ static void rb_wake_up_waiters(struct irq_work *work) 


	wake_up_all(&rbwork->waiters);

	if (rbwork->full_waiters_pending || rbwork->wakeup_full) {

		/* Only cpu_buffer sets the above flags */

		struct ring_buffer_per_cpu *cpu_buffer =

			container_of(rbwork, struct ring_buffer_per_cpu, irq_work);



		/* Called from interrupt context */

		raw_spin_lock(&cpu_buffer->reader_lock);

		rbwork->wakeup_full = false;

		rbwork->full_waiters_pending = false;



		/* Waking up all waiters, they will reset the shortest full */

		cpu_buffer->shortest_full = 0;

		raw_spin_unlock(&cpu_buffer->reader_lock);



		wake_up_all(&rbwork->full_waiters);

	}

}
@@ -934,28 +945,33 @@ __poll_t ring_buffer_poll_wait(struct trace_buffer *buffer, int cpu, 
			  struct file *filp, poll_table *poll_table, int full)

{

	struct ring_buffer_per_cpu *cpu_buffer;

	struct rb_irq_work *work;

	struct rb_irq_work *rbwork;



	if (cpu == RING_BUFFER_ALL_CPUS) {

		work = &buffer->irq_work;

		rbwork = &buffer->irq_work;

		full = 0;

	} else {

		if (!cpumask_test_cpu(cpu, buffer->cpumask))

			return EPOLLERR;



		cpu_buffer = buffer->buffers[cpu];

		work = &cpu_buffer->irq_work;

		rbwork = &cpu_buffer->irq_work;

	}



	if (full) {

		poll_wait(filp, &work->full_waiters, poll_table);

		work->full_waiters_pending = true;

		unsigned long flags;



		poll_wait(filp, &rbwork->full_waiters, poll_table);



		raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);

		rbwork->full_waiters_pending = true;

		if (!cpu_buffer->shortest_full ||

		    cpu_buffer->shortest_full > full)

			cpu_buffer->shortest_full = full;

		raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);

	} else {

		poll_wait(filp, &work->waiters, poll_table);

		work->waiters_pending = true;

		poll_wait(filp, &rbwork->waiters, poll_table);

		rbwork->waiters_pending = true;

	}



	/*