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Date: Wed, 1 Dec 2021 17:30:54 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: "Tzvetomir Stoyanov (VMware)" <tz.stoyanov@...il.com>
Cc: linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 5/5] [RFC] tracing: Read and write to ring buffers
with custom sub buffer size
On Thu, 25 Nov 2021 19:52:53 +0200
"Tzvetomir Stoyanov (VMware)" <tz.stoyanov@...il.com> wrote:
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 867a220b4ef2..5fcf5e9cba76 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -2740,9 +2740,11 @@ trace_event_buffer_lock_reserve(struct trace_buffer **current_rb,
> {
> struct ring_buffer_event *entry;
> struct trace_array *tr = trace_file->tr;
> + int page_size;
> int val;
>
> *current_rb = tr->array_buffer.buffer;
> + page_size = ring_buffer_subbuf_size_get(*current_rb);
>
> if (!tr->no_filter_buffering_ref &&
> (trace_file->flags & (EVENT_FILE_FL_SOFT_DISABLED | EVENT_FILE_FL_FILTERED)) &&
> @@ -2764,7 +2766,7 @@ trace_event_buffer_lock_reserve(struct trace_buffer **current_rb,
> * is still quicker than no copy on match, but having
> * to discard out of the ring buffer on a failed match.
> */
> - int max_len = PAGE_SIZE - struct_size(entry, array, 1);
> + int max_len = page_size - struct_size(entry, array, 1);
OK, so this is not part of the ring buffer, and is always going to be
PAGE_SIZE.
The temp buffer is allocated from trace_buffered_event_enable() which does:
for_each_tracing_cpu(cpu) {
page = alloc_pages_node(cpu_to_node(cpu),
GFP_KERNEL | __GFP_NORETRY, 0);
if (!page)
goto failed;
event = page_address(page);
memset(event, 0, sizeof(*event));
per_cpu(trace_buffered_event, cpu) = event;
preempt_disable();
if (cpu == smp_processor_id() &&
__this_cpu_read(trace_buffered_event) !=
per_cpu(trace_buffered_event, cpu))
WARN_ON_ONCE(1);
preempt_enable();
}
That allocates one page per CPU to store as a temp buffer.
This must always be PAGE_SIZE.
-- Steve
>
> val = this_cpu_inc_return(trace_buffered_event_cnt);
>
> @@ -8004,6 +8006,8 @@ tracing_buffers_read(struct file *filp, char __user *ubuf,
> {
> struct ftrace_buffer_info *info = filp->private_data;
> struct trace_iterator *iter = &info->iter;
> + void *trace_data;
> + int page_size;
> ssize_t ret = 0;
> ssize_t size;
>
> @@ -8015,6 +8019,8 @@ tracing_buffers_read(struct file *filp, char __user *ubuf,
> return -EBUSY;
> #endif
>
> + page_size = ring_buffer_subbuf_size_get(iter->array_buffer->buffer);
> +
> if (!info->spare) {
> info->spare = ring_buffer_alloc_read_page(iter->array_buffer->buffer,
> iter->cpu_file);
> @@ -8029,13 +8035,13 @@ tracing_buffers_read(struct file *filp, char __user *ubuf,
> return ret;
>
> /* Do we have previous read data to read? */
> - if (info->read < PAGE_SIZE)
> + if (info->read < page_size)
> goto read;
>
> again:
> trace_access_lock(iter->cpu_file);
> ret = ring_buffer_read_page(iter->array_buffer->buffer,
> - &info->spare,
> + info->spare,
> count,
> iter->cpu_file, 0);
> trace_access_unlock(iter->cpu_file);
> @@ -8056,11 +8062,11 @@ tracing_buffers_read(struct file *filp, char __user *ubuf,
>
> info->read = 0;
> read:
> - size = PAGE_SIZE - info->read;
> + size = page_size - info->read;
> if (size > count)
> size = count;
> -
> - ret = copy_to_user(ubuf, info->spare + info->read, size);
> + trace_data = ring_buffer_read_page_data(info->spare);
> + ret = copy_to_user(ubuf, trace_data + info->read, size);
> if (ret == size)
> return -EFAULT;
>
> @@ -8165,6 +8171,7 @@ tracing_buffers_splice_read(struct file *file, loff_t *ppos,
> .spd_release = buffer_spd_release,
> };
> struct buffer_ref *ref;
> + int page_size;
> int entries, i;
> ssize_t ret = 0;
>
> @@ -8173,13 +8180,14 @@ tracing_buffers_splice_read(struct file *file, loff_t *ppos,
> return -EBUSY;
> #endif
>
> - if (*ppos & (PAGE_SIZE - 1))
> + page_size = ring_buffer_subbuf_size_get(iter->array_buffer->buffer);
> + if (*ppos & (page_size - 1))
> return -EINVAL;
>
> - if (len & (PAGE_SIZE - 1)) {
> - if (len < PAGE_SIZE)
> + if (len & (page_size - 1)) {
> + if (len < page_size)
> return -EINVAL;
> - len &= PAGE_MASK;
> + len &= (~(page_size - 1));
> }
>
> if (splice_grow_spd(pipe, &spd))
> @@ -8189,7 +8197,7 @@ tracing_buffers_splice_read(struct file *file, loff_t *ppos,
> trace_access_lock(iter->cpu_file);
> entries = ring_buffer_entries_cpu(iter->array_buffer->buffer, iter->cpu_file);
>
> - for (i = 0; i < spd.nr_pages_max && len && entries; i++, len -= PAGE_SIZE) {
> + for (i = 0; i < spd.nr_pages_max && len && entries; i++, len -= page_size) {
> struct page *page;
> int r;
>
> @@ -8210,7 +8218,7 @@ tracing_buffers_splice_read(struct file *file, loff_t *ppos,
> }
> ref->cpu = iter->cpu_file;
>
> - r = ring_buffer_read_page(ref->buffer, &ref->page,
> + r = ring_buffer_read_page(ref->buffer, ref->page,
> len, iter->cpu_file, 1);
> if (r < 0) {
> ring_buffer_free_read_page(ref->buffer, ref->cpu,
> @@ -8219,14 +8227,14 @@ tracing_buffers_splice_read(struct file *file, loff_t *ppos,
> break;
> }
>
> - page = virt_to_page(ref->page);
> + page = virt_to_page(ring_buffer_read_page_data(ref->page));
>
> spd.pages[i] = page;
> - spd.partial[i].len = PAGE_SIZE;
> + spd.partial[i].len = page_size;
> spd.partial[i].offset = 0;
> spd.partial[i].private = (unsigned long)ref;
> spd.nr_pages++;
> - *ppos += PAGE_SIZE;
> + *ppos += page_size;
>
> entries = ring_buffer_entries_cpu(iter->array_buffer->buffer, iter->cpu_file);
> }
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