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Subject[PATCH 3/6] ring-buffer: convert cpu buffer entries to local_t
From: Steven Rostedt <srostedt@redhat.com>

The entries counter in cpu buffer is not atomic. It can be updated by
other interrupts or from another CPU (readers).

But making entries into "atomic_t" causes an atomic operation that can
hurt performance. Instead we convert it to a local_t that will increment
a counter with a local CPU atomic operation (if the arch supports it).

Instead of fighting with readers and overwrites that decrement the counter,
I added a "read" counter. Every time a reader reads an entry it is
incremented.

We already have a overrun counter and with that, the entries counter and
the read counter, we can calculate the total number of entries in the
buffer with:

(entries - overrun) - read

As long as the total number of entries in the ring buffer is less than
the word size, this will work. But since the entries counter was previously
a long, this is no different than what we had before.

Thanks to Andrew Morton for pointing out in the first version that
atomic_t does not replace unsigned long. I switched to atomic_long_t
even though it is signed. A negative count is most likely a bug.

[ Impact: keep accurate count of cpu buffer entries ]

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
kernel/trace/ring_buffer.c | 20 +++++++++++---------
1 files changed, 11 insertions(+), 9 deletions(-)
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index dc8b2ab..f2d56e9 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -405,7 +405,8 @@ struct ring_buffer_per_cpu {
unsigned long nmi_dropped;
unsigned long commit_overrun;
unsigned long overrun;
- unsigned long entries;
+ unsigned long read;
+ local_t entries;
u64 write_stamp;
u64 read_stamp;
atomic_t record_disabled;
@@ -997,7 +998,6 @@ static void rb_update_overflow(struct ring_buffer_per_cpu *cpu_buffer)
if (event->type_len > RINGBUF_TYPE_DATA_TYPE_LEN_MAX)
continue;
cpu_buffer->overrun++;
- cpu_buffer->entries--;
}
}

@@ -1588,7 +1588,7 @@ EXPORT_SYMBOL_GPL(ring_buffer_lock_reserve);
static void rb_commit(struct ring_buffer_per_cpu *cpu_buffer,
struct ring_buffer_event *event)
{
- cpu_buffer->entries++;
+ local_inc(&cpu_buffer->entries);

/* Only process further if we own the commit */
if (!rb_is_commit(cpu_buffer, event))
@@ -1722,7 +1722,7 @@ void ring_buffer_discard_commit(struct ring_buffer *buffer,
* The commit is still visible by the reader, so we
* must increment entries.
*/
- cpu_buffer->entries++;
+ local_inc(&cpu_buffer->entries);
out:
/*
* If a write came in and pushed the tail page
@@ -1902,7 +1902,8 @@ unsigned long ring_buffer_entries_cpu(struct ring_buffer *buffer, int cpu)
return 0;

cpu_buffer = buffer->buffers[cpu];
- ret = cpu_buffer->entries;
+ ret = (local_read(&cpu_buffer->entries) - cpu_buffer->overrun)
+ - cpu_buffer->read;

return ret;
}
@@ -1985,7 +1986,7 @@ unsigned long ring_buffer_entries(struct ring_buffer *buffer)
/* if you care about this being correct, lock the buffer */
for_each_buffer_cpu(buffer, cpu) {
cpu_buffer = buffer->buffers[cpu];
- entries += cpu_buffer->entries;
+ entries += local_read(&cpu_buffer->entries);
}

return entries;
@@ -2225,7 +2226,7 @@ static void rb_advance_reader(struct ring_buffer_per_cpu *cpu_buffer)

if (event->type_len <= RINGBUF_TYPE_DATA_TYPE_LEN_MAX
|| rb_discarded_event(event))
- cpu_buffer->entries--;
+ cpu_buffer->read++;

rb_update_read_stamp(cpu_buffer, event);

@@ -2642,7 +2643,8 @@ rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer)
cpu_buffer->nmi_dropped = 0;
cpu_buffer->commit_overrun = 0;
cpu_buffer->overrun = 0;
- cpu_buffer->entries = 0;
+ cpu_buffer->read = 0;
+ local_set(&cpu_buffer->entries, 0);

cpu_buffer->write_stamp = 0;
cpu_buffer->read_stamp = 0;
@@ -2813,7 +2815,7 @@ static void rb_remove_entries(struct ring_buffer_per_cpu *cpu_buffer,
/* Only count data entries */
if (event->type_len > RINGBUF_TYPE_DATA_TYPE_LEN_MAX)
continue;
- cpu_buffer->entries--;
+ cpu_buffer->read++;
}
__raw_spin_unlock(&cpu_buffer->lock);
}
--
1.6.2.4
--


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