From: Steven Rostedt 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 --- kernel/trace/ring_buffer.c | 21 ++++++++++++--------- 1 files changed, 12 insertions(+), 9 deletions(-) diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index 26e1359..c792ea8 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 +1986,8 @@ 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) - + cpu_buffer->overrun) - cpu_buffer->read; } return entries; @@ -2225,7 +2227,7 @@ 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 +2644,8 @@ 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 +2816,7 @@ 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 -- -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/