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Message-ID: <20250521145128.3a776466@gandalf.local.home>
Date: Wed, 21 May 2025 14:51:28 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH v2] tracing: ring_buffer: Rewind persistent ring buffer
when reboot
On Thu, 15 May 2025 20:15:56 +0900
"Masami Hiramatsu (Google)" <mhiramat@...nel.org> wrote:
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 6859008ca34d..48f5f248eb4c 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -1358,6 +1358,13 @@ static inline void rb_inc_page(struct buffer_page **bpage)
> *bpage = list_entry(p, struct buffer_page, list);
> }
>
> +static inline void rb_dec_page(struct buffer_page **bpage)
> +{
> + struct list_head *p = rb_list_head((*bpage)->list.prev);
> +
> + *bpage = list_entry(p, struct buffer_page, list);
> +}
> +
> static struct buffer_page *
> rb_set_head_page(struct ring_buffer_per_cpu *cpu_buffer)
> {
> @@ -1866,10 +1873,11 @@ static int rb_validate_buffer(struct buffer_data_page *dpage, int cpu)
> static void rb_meta_validate_events(struct ring_buffer_per_cpu *cpu_buffer)
> {
> struct ring_buffer_cpu_meta *meta = cpu_buffer->ring_meta;
> - struct buffer_page *head_page;
> + struct buffer_page *head_page, *orig_head;
> unsigned long entry_bytes = 0;
> unsigned long entries = 0;
> int ret;
> + u64 ts;
> int i;
>
> if (!meta || !meta->head_buffer)
> @@ -1885,8 +1893,93 @@ static void rb_meta_validate_events(struct ring_buffer_per_cpu *cpu_buffer)
> entry_bytes += local_read(&cpu_buffer->reader_page->page->commit);
> local_set(&cpu_buffer->reader_page->entries, ret);
>
> - head_page = cpu_buffer->head_page;
> + orig_head = head_page = cpu_buffer->head_page;
> + ts = head_page->page->time_stamp;
> +
> + /*
> + * Try to rewind the head so that we can read the pages which already
> + * read in the previous boot.
> + */
> + if (head_page == cpu_buffer->tail_page)
> + goto rewound;
Hmm, jumping to a label called "rewound" when you didn't do a rewind seems
confusing.
Perhaps call the label "skip_rewind"?
> +
> + rb_dec_page(&head_page);
> + for (i = 0; i < meta->nr_subbufs + 1; i++, rb_dec_page(&head_page)) {
> +
> + /* Rewind until tail (writer) page. */
> + if (head_page == cpu_buffer->tail_page)
> + break;
> +
> + /* Ensure the page has older data than head. */
> + if (ts < head_page->page->time_stamp)
> + break;
> +
> + ts = head_page->page->time_stamp;
> + /* Ensure the page has correct timestamp and some data. */
> + if (!ts || rb_page_commit(head_page) == 0)
> + break;
> +
> + /* Stop rewind if the page is invalid. */
> + ret = rb_validate_buffer(head_page->page, cpu_buffer->cpu);
> + if (ret < 0)
> + break;
> +
> + /* Recover the number of entries and update stats. */
> + local_set(&head_page->entries, ret);
> + if (ret)
> + local_inc(&cpu_buffer->pages_touched);
> + entries += ret;
> + entry_bytes += rb_page_commit(head_page);
> + }
> + pr_info("Rewound %d pages on cpu%d\n", i, cpu_buffer->cpu);
Should state this is coming from the ring buffer and use "[%d]" for cpu
number as the other pr_info()'s do. Also only print if it did a rewind:
if (i)
pr_info("Ring buffer [%d] rewound %d pages\n", cpu_buffer->cpu, i);
> +
> + /* The last rewound page must be skipped. */
> + if (head_page != orig_head)
> + rb_inc_page(&head_page);
>
> + /* If there are rewound pages, rewind the reader page too. */
I would change the comment to:
/*
* If the ring buffer was rewound, then inject the reader page
* into the location just before the original head page.
*/
> + if (head_page != orig_head) {
> + struct buffer_page *bpage = orig_head;
> +
> + rb_dec_page(&bpage);
> + /*
> + * Insert the reader_page before the original head page.
> + * Since the list encode RB_PAGE flags, general list
> + * operations should be avoided.
> + */
> + cpu_buffer->reader_page->list.next = &orig_head->list;
> + cpu_buffer->reader_page->list.prev = orig_head->list.prev;
> + orig_head->list.prev = &cpu_buffer->reader_page->list;
> + bpage->list.next = &cpu_buffer->reader_page->list;
> +
> + /* Make the head_page tthe new read page */
Typo "tthe" and call it "new reader page", not "read page".
> + cpu_buffer->reader_page = head_page;
> + bpage = head_page;
> + rb_inc_page(&head_page);
> + head_page->list.prev = bpage->list.prev;
> + rb_dec_page(&bpage);
> + bpage->list.next = &head_page->list;
> + rb_set_list_to_head(&bpage->list);
> +
> + cpu_buffer->head_page = head_page;
> + meta->head_buffer = (unsigned long)head_page->page;
> +
> + /* Reset all the indexes */
> + bpage = cpu_buffer->reader_page;
> + meta->buffers[0] = rb_meta_subbuf_idx(meta, bpage->page);
> + bpage->id = 0;
> +
> + for (i = 0, bpage = head_page; i < meta->nr_subbufs;
> + i++, rb_inc_page(&bpage)) {
> + meta->buffers[i + 1] = rb_meta_subbuf_idx(meta, bpage->page);
> + bpage->id = i + 1;
> + }
> +
> + /* We'll restart verifying from orig_head */
> + head_page = orig_head;
> + }
> +
> + rewound:
skip_rewind:
Also, I know other's don't like to do this, but I do add a space before
labels. It makes patch diffs easier to see which functions they are,
otherwise the patch shows the label and not the function.
-- Steve
> /* If the commit_buffer is the reader page, update the commit page */
> if (meta->commit_buffer == (unsigned long)cpu_buffer->reader_page->page) {
> cpu_buffer->commit_page = cpu_buffer->reader_page;
> @@ -5348,7 +5441,6 @@ rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer)
> */
> local_set(&cpu_buffer->reader_page->write, 0);
> local_set(&cpu_buffer->reader_page->entries, 0);
> - local_set(&cpu_buffer->reader_page->page->commit, 0);
> cpu_buffer->reader_page->real_end = 0;
>
> spin:
> @@ -6642,7 +6734,6 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
> cpu_buffer->read_bytes += rb_page_size(reader);
>
> /* swap the pages */
> - rb_init_page(bpage);
> bpage = reader->page;
> reader->page = data_page->data;
> local_set(&reader->write, 0);
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