Commit f0d2c681 authored by Steven Rostedt's avatar Steven Rostedt Committed by Steven Rostedt
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ring-buffer: add counters for commit overrun and nmi dropped entries 



The WARN_ON in the ring buffer when a commit is preempted and the
buffer is filled by preceding writes can happen in normal operations.
The WARN_ON makes it look like a bug, not to mention, because
it does not stop tracing and calls printk which can also recurse, this
is prone to deadlock (the WARN_ON is not in a position to recurse).

This patch removes the WARN_ON and replaces it with a counter that
can be retrieved by a tracer. This counter is called commit_overrun.

While at it, I added a nmi_dropped counter to count any time an NMI entry
is dropped because the NMI could not take the spinlock.

[ Impact: prevent deadlock by printing normal case warning ]

Signed-off-by: default avatarSteven Rostedt <rostedt@goodmis.org>
parent d6ce96da

include/linux/ring_buffer.h

+2 −0
Original line number Diff line number Diff line
@@ -153,6 +153,8 @@ unsigned long ring_buffer_entries(struct ring_buffer *buffer); 
unsigned long ring_buffer_overruns(struct ring_buffer *buffer);

unsigned long ring_buffer_entries_cpu(struct ring_buffer *buffer, int cpu);

unsigned long ring_buffer_overrun_cpu(struct ring_buffer *buffer, int cpu);

unsigned long ring_buffer_commit_overrun_cpu(struct ring_buffer *buffer, int cpu);

unsigned long ring_buffer_nmi_dropped_cpu(struct ring_buffer *buffer, int cpu);



u64 ring_buffer_time_stamp(struct ring_buffer *buffer, int cpu);

void ring_buffer_normalize_time_stamp(struct ring_buffer *buffer,

kernel/trace/ring_buffer.c

+49 −3
Original line number Diff line number Diff line
@@ -402,6 +402,8 @@ struct ring_buffer_per_cpu { 
	struct buffer_page		*tail_page;	/* write to tail */

	struct buffer_page		*commit_page;	/* committed pages */

	struct buffer_page		*reader_page;

	unsigned long			nmi_dropped;

	unsigned long			commit_overrun;

	unsigned long			overrun;

	unsigned long			entries;

	u64				write_stamp;
@@ -1216,8 +1218,10 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer, 
		 * simply fail.

		 */

		if (unlikely(in_nmi())) {

			if (!__raw_spin_trylock(&cpu_buffer->lock))

			if (!__raw_spin_trylock(&cpu_buffer->lock)) {

				cpu_buffer->nmi_dropped++;

				goto out_reset;

			}

		} else

			__raw_spin_lock(&cpu_buffer->lock);
@@ -1238,8 +1242,7 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer, 
		 * about it.

		 */

		if (unlikely(next_page == commit_page)) {

			/* This can easily happen on small ring buffers */

			WARN_ON_ONCE(buffer->pages > 2);

			cpu_buffer->commit_overrun++;

			goto out_reset;

		}
@@ -1925,6 +1928,47 @@ unsigned long ring_buffer_overrun_cpu(struct ring_buffer *buffer, int cpu) 
}

EXPORT_SYMBOL_GPL(ring_buffer_overrun_cpu);



/**

 * ring_buffer_nmi_dropped_cpu - get the number of nmis that were dropped

 * @buffer: The ring buffer

 * @cpu: The per CPU buffer to get the number of overruns from

 */

unsigned long ring_buffer_nmi_dropped_cpu(struct ring_buffer *buffer, int cpu)

{

	struct ring_buffer_per_cpu *cpu_buffer;

	unsigned long ret;



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

		return 0;



	cpu_buffer = buffer->buffers[cpu];

	ret = cpu_buffer->nmi_dropped;



	return ret;

}

EXPORT_SYMBOL_GPL(ring_buffer_nmi_dropped_cpu);



/**

 * ring_buffer_commit_overrun_cpu - get the number of overruns caused by commits

 * @buffer: The ring buffer

 * @cpu: The per CPU buffer to get the number of overruns from

 */

unsigned long

ring_buffer_commit_overrun_cpu(struct ring_buffer *buffer, int cpu)

{

	struct ring_buffer_per_cpu *cpu_buffer;

	unsigned long ret;



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

		return 0;



	cpu_buffer = buffer->buffers[cpu];

	ret = cpu_buffer->commit_overrun;



	return ret;

}

EXPORT_SYMBOL_GPL(ring_buffer_commit_overrun_cpu);



/**

 * ring_buffer_entries - get the number of entries in a buffer

 * @buffer: The ring buffer
@@ -2595,6 +2639,8 @@ rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer) 
	local_set(&cpu_buffer->reader_page->page->commit, 0);

	cpu_buffer->reader_page->read = 0;



	cpu_buffer->nmi_dropped = 0;

	cpu_buffer->commit_overrun = 0;

	cpu_buffer->overrun = 0;

	cpu_buffer->entries = 0;

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