tracing: Allow user-space mapping of the ring-buffer

Currently, user-space extracts data from the ring-buffer via splice,
which is handy for storage or network sharing. However, due to splice
limitations, it is imposible to do real-time analysis without a copy.

A solution for that problem is to let the user-space map the ring-buffer
directly.

The mapping is exposed via the per-CPU file trace_pipe_raw. The first
element of the mapping is the meta-page. It is followed by each
subbuffer constituting the ring-buffer, ordered by their unique page ID:

  * Meta-page -- include/uapi/linux/trace_mmap.h for a description
  * Subbuf ID 0
  * Subbuf ID 1
     ...

It is therefore easy to translate a subbuf ID into an offset in the
mapping:

  reader_id = meta->reader->id;
  reader_offset = meta->meta_page_size + reader_id * meta->subbuf_size;

When new data is available, the mapper must call a newly introduced ioctl:
TRACE_MMAP_IOCTL_GET_READER. This will update the Meta-page reader ID to
point to the next reader containing unread data.

Mapping will prevent snapshot and buffer size modifications.

Link: https://lore.kernel.org/linux-trace-kernel/20240510140435.3550353-4-vdonnefort@google.com

CC: <linux-mm@kvack.org>
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
This commit is contained in:
Vincent Donnefort 2024-05-10 15:04:32 +01:00 committed by Steven Rostedt (Google)
parent 117c39200d
commit cf9f0f7c4c
3 changed files with 102 additions and 5 deletions

View File

@ -43,4 +43,6 @@ struct trace_buffer_meta {
__u64 Reserved2;
};
#define TRACE_MMAP_IOCTL_GET_READER _IO('T', 0x1)
#endif /* _TRACE_MMAP_H_ */

View File

@ -1191,6 +1191,12 @@ static void tracing_snapshot_instance_cond(struct trace_array *tr,
return;
}
if (tr->mapped) {
trace_array_puts(tr, "*** BUFFER MEMORY MAPPED ***\n");
trace_array_puts(tr, "*** Can not use snapshot (sorry) ***\n");
return;
}
local_irq_save(flags);
update_max_tr(tr, current, smp_processor_id(), cond_data);
local_irq_restore(flags);
@ -1323,7 +1329,7 @@ static int tracing_arm_snapshot_locked(struct trace_array *tr)
lockdep_assert_held(&trace_types_lock);
spin_lock(&tr->snapshot_trigger_lock);
if (tr->snapshot == UINT_MAX) {
if (tr->snapshot == UINT_MAX || tr->mapped) {
spin_unlock(&tr->snapshot_trigger_lock);
return -EBUSY;
}
@ -6068,7 +6074,7 @@ static void tracing_set_nop(struct trace_array *tr)
{
if (tr->current_trace == &nop_trace)
return;
tr->current_trace->enabled--;
if (tr->current_trace->reset)
@ -8194,15 +8200,32 @@ out:
return ret;
}
/* An ioctl call with cmd 0 to the ring buffer file will wake up all waiters */
static long tracing_buffers_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
{
struct ftrace_buffer_info *info = file->private_data;
struct trace_iterator *iter = &info->iter;
int err;
if (cmd)
return -ENOIOCTLCMD;
if (cmd == TRACE_MMAP_IOCTL_GET_READER) {
if (!(file->f_flags & O_NONBLOCK)) {
err = ring_buffer_wait(iter->array_buffer->buffer,
iter->cpu_file,
iter->tr->buffer_percent,
NULL, NULL);
if (err)
return err;
}
return ring_buffer_map_get_reader(iter->array_buffer->buffer,
iter->cpu_file);
} else if (cmd) {
return -ENOTTY;
}
/*
* An ioctl call with cmd 0 to the ring buffer file will wake up all
* waiters
*/
mutex_lock(&trace_types_lock);
/* Make sure the waiters see the new wait_index */
@ -8214,6 +8237,76 @@ static long tracing_buffers_ioctl(struct file *file, unsigned int cmd, unsigned
return 0;
}
#ifdef CONFIG_TRACER_MAX_TRACE
static int get_snapshot_map(struct trace_array *tr)
{
int err = 0;
/*
* Called with mmap_lock held. lockdep would be unhappy if we would now
* take trace_types_lock. Instead use the specific
* snapshot_trigger_lock.
*/
spin_lock(&tr->snapshot_trigger_lock);
if (tr->snapshot || tr->mapped == UINT_MAX)
err = -EBUSY;
else
tr->mapped++;
spin_unlock(&tr->snapshot_trigger_lock);
/* Wait for update_max_tr() to observe iter->tr->mapped */
if (tr->mapped == 1)
synchronize_rcu();
return err;
}
static void put_snapshot_map(struct trace_array *tr)
{
spin_lock(&tr->snapshot_trigger_lock);
if (!WARN_ON(!tr->mapped))
tr->mapped--;
spin_unlock(&tr->snapshot_trigger_lock);
}
#else
static inline int get_snapshot_map(struct trace_array *tr) { return 0; }
static inline void put_snapshot_map(struct trace_array *tr) { }
#endif
static void tracing_buffers_mmap_close(struct vm_area_struct *vma)
{
struct ftrace_buffer_info *info = vma->vm_file->private_data;
struct trace_iterator *iter = &info->iter;
WARN_ON(ring_buffer_unmap(iter->array_buffer->buffer, iter->cpu_file));
put_snapshot_map(iter->tr);
}
static const struct vm_operations_struct tracing_buffers_vmops = {
.close = tracing_buffers_mmap_close,
};
static int tracing_buffers_mmap(struct file *filp, struct vm_area_struct *vma)
{
struct ftrace_buffer_info *info = filp->private_data;
struct trace_iterator *iter = &info->iter;
int ret = 0;
ret = get_snapshot_map(iter->tr);
if (ret)
return ret;
ret = ring_buffer_map(iter->array_buffer->buffer, iter->cpu_file, vma);
if (ret)
put_snapshot_map(iter->tr);
vma->vm_ops = &tracing_buffers_vmops;
return ret;
}
static const struct file_operations tracing_buffers_fops = {
.open = tracing_buffers_open,
.read = tracing_buffers_read,
@ -8223,6 +8316,7 @@ static const struct file_operations tracing_buffers_fops = {
.splice_read = tracing_buffers_splice_read,
.unlocked_ioctl = tracing_buffers_ioctl,
.llseek = no_llseek,
.mmap = tracing_buffers_mmap,
};
static ssize_t

View File

@ -336,6 +336,7 @@ struct trace_array {
bool allocated_snapshot;
spinlock_t snapshot_trigger_lock;
unsigned int snapshot;
unsigned int mapped;
unsigned long max_latency;
#ifdef CONFIG_FSNOTIFY
struct dentry *d_max_latency;