On Mon, Sep 21, 2026 at 07:30:45PM +0800, Xiang Gao wrote: > Report the memory consumed by the tracing ring buffers, rather than the > usable data capacity exposed by buffer_size_kb. Android low-memory > diagnostics need this to attribute the memory used by tracing when > calculating lost RAM. > > The buffers can be spread across the global trace array, dynamically > created instances, and snapshot buffers. Userspace currently has to > discover and sum every instance, and snapshot memory is not exposed by > the per-instance totals. > > Add a trace_stats directory with memory_usage_kb reporting: > > buffers: > snapshot_buffers: > > covering the global trace array, all instances, and the bootstrapping > temp_buffer across all CPUs. > > The values account for the full pages backing the data sub-buffers and > reader page, plus the cached read page and mmap metadata page when > present. Slab-allocated ring-buffer metadata is not included, as it is > already reported through Slab and would be double-counted when > subtracting tracing memory from lost RAM. Remote buffers, whose pages > are externally owned, report zero.
For the next version, it is good practice to __not__ in-reply-to with previous version. > > Signed-off-by: Xiang Gao <[email protected]> > --- > Documentation/trace/ftrace.rst | 12 +++++ > include/linux/ring_buffer.h | 1 + > kernel/trace/ring_buffer.c | 41 +++++++++++++++ > kernel/trace/trace.c | 95 ++++++++++++++++++++++++++++++++++ > 4 files changed, 149 insertions(+) > > diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst > index 7261f25f8b4b..99ddfe26b7cd 100644 > --- a/Documentation/trace/ftrace.rst > +++ b/Documentation/trace/ftrace.rst > @@ -218,6 +218,18 @@ of ftrace. Here is a list of some of the key files: > > This displays the total combined size of all the trace buffers. > > + trace_stats/memory_usage_kb: > + > + This reports the memory consumed by the ring buffers, as opposed to > + the usable data capacity shown by buffer_size_kb. The value covers the > + main and snapshot buffers of the global trace array and all tracing > + instances. It does not include slab-allocated ring-buffer metadata. > + > + Output:: > + > + buffers: ... > + snapshot_buffers: ... > + > buffer_subbuf_size_kb: > > This sets or displays the sub buffer size. The ring buffer is broken up > diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h > index eac3e9080c3c..96b99e6757d4 100644 > --- a/include/linux/ring_buffer.h > +++ b/include/linux/ring_buffer.h > @@ -167,6 +167,7 @@ int ring_buffer_iter_empty(struct ring_buffer_iter *iter); > bool ring_buffer_iter_dropped(struct ring_buffer_iter *iter); > > unsigned long ring_buffer_size(struct trace_buffer *buffer, int cpu); > +unsigned long ring_buffer_memory_size(struct trace_buffer *buffer, int cpu); > unsigned long ring_buffer_max_event_size(struct trace_buffer *buffer); > > void ring_buffer_reset_cpu(struct trace_buffer *buffer, int cpu); > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c > index 04bb94c29f58..efb88bf8970c 100644 > --- a/kernel/trace/ring_buffer.c > +++ b/kernel/trace/ring_buffer.c > @@ -6559,6 +6559,47 @@ unsigned long ring_buffer_size(struct trace_buffer > *buffer, int cpu) > } > EXPORT_SYMBOL_GPL(ring_buffer_size); > > +/** > + * ring_buffer_memory_size - return the memory used by the buffer (in bytes) > + * @buffer: The ring buffer. > + * @cpu: The CPU to get ring buffer memory from. > + * > + * Returns the page-allocator memory consumed by @cpu, including the data > + * sub-buffers, the reader page, the cached read page, and the mmap > + * metadata page. Unlike ring_buffer_size(), which reports the usable data > + * capacity, this accounts for the full pages allocated to the buffer. > + * Remote buffers do not own page-allocator memory and report zero. > + */ > +unsigned long ring_buffer_memory_size(struct trace_buffer *buffer, int cpu) > +{ > + struct ring_buffer_per_cpu *cpu_buffer; > + unsigned long subbuf_size; > + unsigned long size; > + > + if (!cpumask_test_cpu(cpu, buffer->cpumask)) > + return 0; > + > + /* Remote buffers use externally owned memory. */ > + if (buffer->remote) > + return 0; This is a generic interface. If you want to call this function on a remote buffer, you should be able to. Moreover, remote buffer in-production current use is for Android... So not only ring_buffer_memory_size() should support them, but they should probably be actively reported somewhere... > + > + cpu_buffer = buffer->buffers[cpu]; > + subbuf_size = PAGE_SIZE << READ_ONCE(buffer->subbuf_order); > + > + /* Data sub-buffers plus the reader page. */ > + size = (READ_ONCE(cpu_buffer->nr_pages) + 1) * subbuf_size; > + > + /* The cached read page, if present, is a full sub-buffer page. */ > + if (READ_ONCE(cpu_buffer->free_page.data)) > + size += subbuf_size; > + > + /* The mmap metadata page is a single system page. */ > + if (READ_ONCE(cpu_buffer->meta_page)) > + size += PAGE_SIZE; > + > + return size; > +} > + > /** > * ring_buffer_max_event_size - return the max data size of an event > * @buffer: The ring buffer. > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c > index e4a490d3d08c..d4a913ff8a69 100644 > --- a/kernel/trace/trace.c > +++ b/kernel/trace/trace.c > @@ -5771,6 +5771,79 @@ tracing_total_entries_read(struct file *filp, char > __user *ubuf, > return simple_read_from_buffer(ubuf, cnt, ppos, buf, r); > } > > +struct trace_mem_stats { > + unsigned long buffers; > + unsigned long snapshot; > +}; > + > +static void > +trace_array_buffer_memory(struct trace_array *tr, int cpu, > + unsigned long *buffers, unsigned long *snapshot) > +{ > + if (tr->array_buffer.buffer) > + *buffers += ring_buffer_memory_size(tr->array_buffer.buffer, > cpu); > + > +#ifdef CONFIG_TRACER_SNAPSHOT > + if (tr->snapshot_buffer.buffer) > + *snapshot += > ring_buffer_memory_size(tr->snapshot_buffer.buffer, cpu); > +#endif > +} > + > +static struct trace_mem_stats trace_buffers_memory(void) > +{ > + struct trace_mem_stats stats = {}; > + struct trace_array *tr; > + int cpu; > + > + guard(mutex)(&trace_types_lock); > + > + list_for_each_entry(tr, &ftrace_trace_arrays, list) { > + for_each_tracing_cpu(cpu) > + trace_array_buffer_memory(tr, cpu, &stats.buffers, > + &stats.snapshot); > + } > + > + /* > + * temp_buffer is allocated in tracer_alloc_buffers() and is never > + * attached to a trace array. It temporarily holds event data for > + * triggers when tracing is off. Account for its pages too. > + */ > + if (temp_buffer) { > + for_each_tracing_cpu(cpu) > + stats.buffers += ring_buffer_memory_size(temp_buffer, > cpu); > + } > + > + return stats; > +} > + > +static int trace_mem_show(struct seq_file *m, void *v) > +{ > + struct trace_mem_stats stats = trace_buffers_memory(); > + > + seq_printf(m, "buffers: %lu\n", stats.buffers >> 10); > + seq_printf(m, "snapshot_buffers: %lu\n", stats.snapshot >> 10); > + > + return 0; > +} > + > +static int trace_mem_open(struct inode *inode, struct file *file) > +{ > + int ret; > + > + ret = tracing_check_open_get_tr(NULL); > + if (ret) > + return ret; > + > + return single_open(file, trace_mem_show, inode->i_private); > +} > + > +static const struct file_operations trace_mem_fops = { > + .open = trace_mem_open, > + .read = seq_read, > + .llseek = seq_lseek, > + .release = single_release, > +}; > + > #define LAST_BOOT_HEADER ((void *)1) > > static void *l_next(struct seq_file *m, void *v, loff_t *pos) > @@ -9285,6 +9358,26 @@ static struct notifier_block trace_module_nb = { > }; > #endif /* CONFIG_MODULES */ > > +static __init void init_trace_stats_tracefs(void) > +{ > + struct dentry *stats_dir; > + > + /* > + * tracer_alloc_buffers() frees tracing_buffer_mask and temp_buffer > + * on failure without NULLing them, so do not iterate tracing CPUs > + * here when tracing failed to initialize. > + */ > + if (tracing_disabled) > + return; > + > + stats_dir = tracefs_create_dir("trace_stats", NULL); > + if (!stats_dir) > + return; > + > + trace_create_file("memory_usage_kb", TRACE_MODE_READ, stats_dir, > + NULL, &trace_mem_fops); > +} > + > static __init void tracer_init_tracefs_work_func(struct work_struct *work) > { > > @@ -9293,6 +9386,8 @@ static __init void tracer_init_tracefs_work_func(struct > work_struct *work) > init_tracer_tracefs(&global_trace, NULL); > ftrace_init_tracefs_toplevel(&global_trace, NULL); > > + init_trace_stats_tracefs(); > + > trace_create_file("tracing_thresh", TRACE_MODE_WRITE, NULL, > &global_trace, &tracing_thresh_fops); > > -- > 2.34.1 > -- Vincent
