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Date: Fri, 14 May 2010 18:26:47 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
Andrew Morton <akpm@...ux-foundation.org>,
Frederic Weisbecker <fweisbec@...il.com>,
Masami Hiramatsu <mhiramat@...hat.com>
Subject: Re: [PATCH 12/13 v3] ring-buffer: Add cached pages when freeing
reader page
* Steven Rostedt (rostedt@...dmis.org) wrote:
> From: Steven Rostedt <srostedt@...hat.com>
>
> When the pages are removed from the ring buffer for things like
> splice they are freed with ring_buffer_free_read_page().
> They are also allocated with ring_buffer_alloc_read_page().
>
> Currently the ring buffer does not take advantage of this situation.
> Every time the page is freed, the ring buffer simply frees it.
> When a new page is needed, it allocates it. This means that reading
> several pages with splice will cause a page to be freed and allocated
> several times. This is simply a waste.
>
> This patch adds a cache of the pages freed (16 max). This allows
> the pages to be reused quickly without need to go back to the memory
> pool.
Trying to understand the effect of splice() putting the page in the page
cache and how it affects this patch.
Basically, how do you know you can call free_page() from splice() in the
first place ? Answering this question will probably help us see if this
page reuse is OK.
Thanks,
Mathieu
>
> v2: Added in locking that should have been there in the first release.
>
> Reported-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
> ---
> kernel/trace/ring_buffer.c | 52 ++++++++++++++++++++++++++++++++++++++------
> 1 files changed, 45 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 7f6059c..7aded7d 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -157,6 +157,8 @@ static unsigned long ring_buffer_flags __read_mostly = RB_BUFFERS_ON;
>
> #define BUF_PAGE_HDR_SIZE offsetof(struct buffer_data_page, data)
>
> +#define RB_MAX_FREE_PAGES 16
> +
> /**
> * tracing_on - enable all tracing buffers
> *
> @@ -325,7 +327,10 @@ EXPORT_SYMBOL_GPL(ring_buffer_event_data);
> #define RB_MISSED_STORED (1 << 30)
>
> struct buffer_data_page {
> - u64 time_stamp; /* page time stamp */
> + union {
> + struct buffer_data_page *next; /* for free pages */
> + u64 time_stamp; /* page time stamp */
> + };
> local_t commit; /* write committed index */
> unsigned char data[]; /* data of buffer page */
> };
> @@ -472,6 +477,10 @@ struct ring_buffer {
> atomic_t record_disabled;
> cpumask_var_t cpumask;
>
> + struct buffer_data_page *free_pages;
> + int nr_free_pages;
> + spinlock_t free_pages_lock;
> +
> struct lock_class_key *reader_lock_key;
>
> struct mutex mutex;
> @@ -1118,6 +1127,7 @@ struct ring_buffer *__ring_buffer_alloc(unsigned long size, unsigned flags,
> buffer->flags = flags;
> buffer->clock = trace_clock_local;
> buffer->reader_lock_key = key;
> + spin_lock_init(&buffer->free_pages_lock);
>
> /* need at least two pages */
> if (buffer->pages < 2)
> @@ -1184,6 +1194,7 @@ EXPORT_SYMBOL_GPL(__ring_buffer_alloc);
> void
> ring_buffer_free(struct ring_buffer *buffer)
> {
> + struct buffer_data_page *bpage;
> int cpu;
>
> get_online_cpus();
> @@ -1200,6 +1211,11 @@ ring_buffer_free(struct ring_buffer *buffer)
> kfree(buffer->buffers);
> free_cpumask_var(buffer->cpumask);
>
> + while (buffer->free_pages) {
> + bpage = buffer->free_pages;
> + buffer->free_pages = bpage->next;
> + free_page((unsigned long)bpage);
> + };
> kfree(buffer);
> }
> EXPORT_SYMBOL_GPL(ring_buffer_free);
> @@ -3714,14 +3730,24 @@ EXPORT_SYMBOL_GPL(ring_buffer_swap_cpu);
> */
> void *ring_buffer_alloc_read_page(struct ring_buffer *buffer)
> {
> - struct buffer_data_page *bpage;
> + struct buffer_data_page *bpage = NULL;
> unsigned long addr;
>
> - addr = __get_free_page(GFP_KERNEL);
> - if (!addr)
> - return NULL;
> + spin_lock(&buffer->free_pages_lock);
> + if (buffer->free_pages) {
> + bpage = buffer->free_pages;
> + buffer->free_pages = bpage->next;
> + buffer->nr_free_pages--;
> + }
> + spin_unlock(&buffer->free_pages_lock);
>
> - bpage = (void *)addr;
> + if (!bpage) {
> + addr = __get_free_page(GFP_KERNEL);
> + if (!addr)
> + return NULL;
> +
> + bpage = (void *)addr;
> + }
>
> rb_init_page(bpage);
>
> @@ -3738,7 +3764,19 @@ EXPORT_SYMBOL_GPL(ring_buffer_alloc_read_page);
> */
> void ring_buffer_free_read_page(struct ring_buffer *buffer, void *data)
> {
> - free_page((unsigned long)data);
> + struct buffer_data_page *bpage = data;
> +
> + spin_lock(&buffer->free_pages_lock);
> + if (buffer->nr_free_pages >= RB_MAX_FREE_PAGES) {
> + spin_unlock(&buffer->free_pages_lock);
> + free_page((unsigned long)data);
> + return;
> + }
> +
> + bpage->next = buffer->free_pages;
> + buffer->free_pages = bpage;
> + buffer->nr_free_pages++;
> + spin_unlock(&buffer->free_pages_lock);
> }
> EXPORT_SYMBOL_GPL(ring_buffer_free_read_page);
>
> --
> 1.7.0
>
>
--
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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