[RFC 0/2] docs: ftrace: Clarify the RAM impact of buffer_size_kb

[RFC 0/2] docs: ftrace: Clarify the RAM impact of buffer_size_kb

[Frank A. Cancio Bello]

Improves the documentation of buffer_size_kb by clarifying how is
calculated the number of allocated pages for the ring buffer.

** Do not apply the second patch **. It's just for illustration
purposes.

Frank A. Cancio Bello (2):
  docs: ftrace: Clarify the RAM impact of buffer_size_kb
  ** do not apply this patch ** Just for illustration purposes

 Documentation/trace/ftrace.rst | 13 +++++++++++--
 kernel/trace/ring_buffer.c     |  2 ++
 2 files changed, 13 insertions(+), 2 deletions(-)

-- 
2.17.1

[RFC 1/2] docs: ftrace: Clarify the RAM impact of buffer_size_kb

[Frank A. Cancio Bello]

The current text could mislead the user into believing that the number
of pages allocated by each CPU ring buffer is calculated by the round
up of the division: buffer_size_kb / PAGE_SIZE.

Clarify that the number of pages allocated is the round up of the
division: buffer_size_kb / (PAGE_SIZE - BUF_PAGE_HDR_SIZE). Add an
example that shows how the number of pages allocated could be off by
5 pages more compared with how the current text suggests it should be.

Suggested-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Frank A. Cancio Bello <frank@generalsoftwareinc.com>
---
 Documentation/trace/ftrace.rst | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst
index e3060eedb22d..ec2c4eff95a6 100644
--- a/Documentation/trace/ftrace.rst
+++ b/Documentation/trace/ftrace.rst
@@ -188,8 +188,17 @@ of ftrace. Here is a list of some of the key files:
 	If the last page allocated has room for more bytes
 	than requested, the rest of the page will be used,
 	making the actual allocation bigger than requested or shown.
-	( Note, the size may not be a multiple of the page size
-	due to buffer management meta-data. )
+
+        The number of pages allocated for each CPU buffer may not
+        be the same than the round up of the division:
+        buffer_size_kb / PAGE_SIZE. This is because part of each page is
+        used to store a page header with metadata. E.g. with
+        buffer_size_kb=4096 (kilobytes), a PAGE_SIZE=4096 bytes and a
+        BUF_PAGE_HDR_SIZE=16 bytes (BUF_PAGE_HDR_SIZE is the size of the
+        page header with metadata) the number of pages allocated for each
+        CPU buffer is 1029, not 1024. The formula for calculating the
+        number of pages allocated for each CPU buffer is the round up of:
+        buffer_size_kb / (PAGE_SIZE - BUF_PAGE_HDR_SIZE).
 
 	Buffer sizes for individual CPUs may vary
 	(see "per_cpu/cpu0/buffer_size_kb" below), and if they do
-- 
2.17.1

[RFC 2/2] ** do not apply this patch ** Just for illustration purposes

[Frank A. Cancio Bello]

Prints a message that will allow us to evaluate the number of pages
allocated by each CPU buffer as well the main values that participate
in its calculation.

 $ echo 0 > /sys/kernel/debug/tracing/tracing_on
 $ echo 4096 > /sys/kernel/debug/tracing/buffer_size_kb
   .... e.g. of output:
   PAGE_SIZE: 4096, BUF_PAGE_HDR_SIZE: 16, size: 4194304, nr_pages: 1029

Signed-off-by: Frank A. Cancio Bello <frank@generalsoftwareinc.com>
---
 kernel/trace/ring_buffer.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 66358d66c933..c10b6bcb29b9 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -1730,6 +1730,8 @@ int ring_buffer_resize(struct ring_buffer *buffer, unsigned long size,
 		return size;
 
 	nr_pages = DIV_ROUND_UP(size, BUF_PAGE_SIZE);
+	printk(KERN_ALERT "PAGE_SIZE: %lu, BUF_PAGE_HDR_SIZE: %lu, size: %lu, nr_pages: %ld",
+	       PAGE_SIZE, BUF_PAGE_HDR_SIZE, size, nr_pages);
 
 	/* we need a minimum of two pages */
 	if (nr_pages < 2)
-- 
2.17.1

Re: [RFC 1/2] docs: ftrace: Clarify the RAM impact of buffer_size_kb

[Steven Rostedt]

On Wed, 13 Nov 2019 11:32:36 -0500
"Frank A. Cancio Bello" <frank@generalsoftwareinc.com> wrote:

> The current text could mislead the user into believing that the number
> of pages allocated by each CPU ring buffer is calculated by the round
> up of the division: buffer_size_kb / PAGE_SIZE.
> 
> Clarify that the number of pages allocated is the round up of the
> division: buffer_size_kb / (PAGE_SIZE - BUF_PAGE_HDR_SIZE). Add an
> example that shows how the number of pages allocated could be off by
> 5 pages more compared with how the current text suggests it should be.
> 
> Suggested-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> Signed-off-by: Frank A. Cancio Bello <frank@generalsoftwareinc.com>
> ---
>  Documentation/trace/ftrace.rst | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst
> index e3060eedb22d..ec2c4eff95a6 100644
> --- a/Documentation/trace/ftrace.rst
> +++ b/Documentation/trace/ftrace.rst
> @@ -188,8 +188,17 @@ of ftrace. Here is a list of some of the key files:
>  	If the last page allocated has room for more bytes
>  	than requested, the rest of the page will be used,
>  	making the actual allocation bigger than requested or shown.
> -	( Note, the size may not be a multiple of the page size
> -	due to buffer management meta-data. )

The above is not untrue ;-)

> +
> +        The number of pages allocated for each CPU buffer may not
> +        be the same than the round up of the division:
> +        buffer_size_kb / PAGE_SIZE. This is because part of each page is
> +        used to store a page header with metadata. E.g. with
> +        buffer_size_kb=4096 (kilobytes), a PAGE_SIZE=4096 bytes and a
> +        BUF_PAGE_HDR_SIZE=16 bytes (BUF_PAGE_HDR_SIZE is the size of the
> +        page header with metadata) the number of pages allocated for each
> +        CPU buffer is 1029, not 1024. The formula for calculating the
> +        number of pages allocated for each CPU buffer is the round up of:
> +        buffer_size_kb / (PAGE_SIZE - BUF_PAGE_HDR_SIZE).

I have no problem with this patch, but the concern of documenting the
implementation here, which will most likely not be updated if the
implementation is ever changed, which is why I was vague to begin with.

But it may never be changed as that code has been like that for a
decade now.

-- Steve


>  
>  	Buffer sizes for individual CPUs may vary
>  	(see "per_cpu/cpu0/buffer_size_kb" below), and if they do

Re: [RFC 2/2] ** do not apply this patch ** Just for illustration purposes

[kbuild test robot]

[-- Attachment #1: Type: text/plain, Size: 9198 bytes --]

Hi "Frank,

[FYI, it's a private test report for your RFC patch.]
[auto build test WARNING on tip/perf/core]
[cannot apply to v5.4-rc7 next-20191113]
[if your patch is applied to the wrong git tree, please drop us a note to help
improve the system. BTW, we also suggest to use '--base' option to specify the
base tree in git format-patch, please see https://stackoverflow.com/a/37406982]

url:    https://github.com/0day-ci/linux/commits/Frank-A-Cancio-Bello/docs-ftrace-Clarify-the-RAM-impact-of-buffer_size_kb/20191114-040608
base:   https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git 295c52ee1485e4dee660fc1a0e6ceed6c803c9d3
config: i386-defconfig (attached as .config)
compiler: gcc-7 (Debian 7.4.0-14) 7.4.0
reproduce:
        # save the attached .config to linux build tree
        make ARCH=i386 

If you fix the issue, kindly add following tag
Reported-by: kbuild test robot <lkp@intel.com>

All warnings (new ones prefixed by >>):

   In file included from include/linux/printk.h:7:0,
                    from include/linux/kernel.h:15,
                    from include/asm-generic/bug.h:19,
                    from arch/x86/include/asm/bug.h:83,
                    from include/linux/bug.h:5,
                    from include/linux/mmdebug.h:5,
                    from include/linux/mm.h:9,
                    from include/linux/ring_buffer.h:5,
                    from include/linux/trace_events.h:6,
                    from kernel/trace/ring_buffer.c:7:
   kernel/trace/ring_buffer.c: In function 'ring_buffer_resize':
   include/linux/kern_levels.h:5:18: warning: format '%lu' expects argument of type 'long unsigned int', but argument 3 has type 'unsigned int' [-Wformat=]
    #define KERN_SOH "\001"  /* ASCII Start Of Header */
                     ^
   include/linux/kern_levels.h:9:20: note: in expansion of macro 'KERN_SOH'
    #define KERN_ALERT KERN_SOH "1" /* action must be taken immediately */
                       ^~~~~~~~
>> kernel/trace/ring_buffer.c:1733:9: note: in expansion of macro 'KERN_ALERT'
     printk(KERN_ALERT "PAGE_SIZE: %lu, BUF_PAGE_HDR_SIZE: %lu, size: %lu, nr_pages: %ld",
            ^~~~~~~~~~
   kernel/trace/ring_buffer.c:1733:58: note: format string is defined here
     printk(KERN_ALERT "PAGE_SIZE: %lu, BUF_PAGE_HDR_SIZE: %lu, size: %lu, nr_pages: %ld",
                                                           ~~^
                                                           %u

vim +/KERN_ALERT +1733 kernel/trace/ring_buffer.c

  1703	
  1704	/**
  1705	 * ring_buffer_resize - resize the ring buffer
  1706	 * @buffer: the buffer to resize.
  1707	 * @size: the new size.
  1708	 * @cpu_id: the cpu buffer to resize
  1709	 *
  1710	 * Minimum size is 2 * BUF_PAGE_SIZE.
  1711	 *
  1712	 * Returns 0 on success and < 0 on failure.
  1713	 */
  1714	int ring_buffer_resize(struct ring_buffer *buffer, unsigned long size,
  1715				int cpu_id)
  1716	{
  1717		struct ring_buffer_per_cpu *cpu_buffer;
  1718		unsigned long nr_pages;
  1719		int cpu, err = 0;
  1720	
  1721		/*
  1722		 * Always succeed@resizing a non-existent buffer:
  1723		 */
  1724		if (!buffer)
  1725			return size;
  1726	
  1727		/* Make sure the requested buffer exists */
  1728		if (cpu_id != RING_BUFFER_ALL_CPUS &&
  1729		    !cpumask_test_cpu(cpu_id, buffer->cpumask))
  1730			return size;
  1731	
  1732		nr_pages = DIV_ROUND_UP(size, BUF_PAGE_SIZE);
> 1733		printk(KERN_ALERT "PAGE_SIZE: %lu, BUF_PAGE_HDR_SIZE: %lu, size: %lu, nr_pages: %ld",
  1734		       PAGE_SIZE, BUF_PAGE_HDR_SIZE, size, nr_pages);
  1735	
  1736		/* we need a minimum of two pages */
  1737		if (nr_pages < 2)
  1738			nr_pages = 2;
  1739	
  1740		size = nr_pages * BUF_PAGE_SIZE;
  1741	
  1742		/*
  1743		 * Don't succeed if resizing is disabled, as a reader might be
  1744		 * manipulating the ring buffer and is expecting a sane state while
  1745		 * this is true.
  1746		 */
  1747		if (atomic_read(&buffer->resize_disabled))
  1748			return -EBUSY;
  1749	
  1750		/* prevent another thread from changing buffer sizes */
  1751		mutex_lock(&buffer->mutex);
  1752	
  1753		if (cpu_id == RING_BUFFER_ALL_CPUS) {
  1754			/* calculate the pages to update */
  1755			for_each_buffer_cpu(buffer, cpu) {
  1756				cpu_buffer = buffer->buffers[cpu];
  1757	
  1758				cpu_buffer->nr_pages_to_update = nr_pages -
  1759								cpu_buffer->nr_pages;
  1760				/*
  1761				 * nothing more to do for removing pages or no update
  1762				 */
  1763				if (cpu_buffer->nr_pages_to_update <= 0)
  1764					continue;
  1765				/*
  1766				 * to add pages, make sure all new pages can be
  1767				 * allocated without receiving ENOMEM
  1768				 */
  1769				INIT_LIST_HEAD(&cpu_buffer->new_pages);
  1770				if (__rb_allocate_pages(cpu_buffer->nr_pages_to_update,
  1771							&cpu_buffer->new_pages, cpu)) {
  1772					/* not enough memory for new pages */
  1773					err = -ENOMEM;
  1774					goto out_err;
  1775				}
  1776			}
  1777	
  1778			get_online_cpus();
  1779			/*
  1780			 * Fire off all the required work handlers
  1781			 * We can't schedule on offline CPUs, but it's not necessary
  1782			 * since we can change their buffer sizes without any race.
  1783			 */
  1784			for_each_buffer_cpu(buffer, cpu) {
  1785				cpu_buffer = buffer->buffers[cpu];
  1786				if (!cpu_buffer->nr_pages_to_update)
  1787					continue;
  1788	
  1789				/* Can't run something on an offline CPU. */
  1790				if (!cpu_online(cpu)) {
  1791					rb_update_pages(cpu_buffer);
  1792					cpu_buffer->nr_pages_to_update = 0;
  1793				} else {
  1794					schedule_work_on(cpu,
  1795							&cpu_buffer->update_pages_work);
  1796				}
  1797			}
  1798	
  1799			/* wait for all the updates to complete */
  1800			for_each_buffer_cpu(buffer, cpu) {
  1801				cpu_buffer = buffer->buffers[cpu];
  1802				if (!cpu_buffer->nr_pages_to_update)
  1803					continue;
  1804	
  1805				if (cpu_online(cpu))
  1806					wait_for_completion(&cpu_buffer->update_done);
  1807				cpu_buffer->nr_pages_to_update = 0;
  1808			}
  1809	
  1810			put_online_cpus();
  1811		} else {
  1812			/* Make sure this CPU has been initialized */
  1813			if (!cpumask_test_cpu(cpu_id, buffer->cpumask))
  1814				goto out;
  1815	
  1816			cpu_buffer = buffer->buffers[cpu_id];
  1817	
  1818			if (nr_pages == cpu_buffer->nr_pages)
  1819				goto out;
  1820	
  1821			cpu_buffer->nr_pages_to_update = nr_pages -
  1822							cpu_buffer->nr_pages;
  1823	
  1824			INIT_LIST_HEAD(&cpu_buffer->new_pages);
  1825			if (cpu_buffer->nr_pages_to_update > 0 &&
  1826				__rb_allocate_pages(cpu_buffer->nr_pages_to_update,
  1827						    &cpu_buffer->new_pages, cpu_id)) {
  1828				err = -ENOMEM;
  1829				goto out_err;
  1830			}
  1831	
  1832			get_online_cpus();
  1833	
  1834			/* Can't run something on an offline CPU. */
  1835			if (!cpu_online(cpu_id))
  1836				rb_update_pages(cpu_buffer);
  1837			else {
  1838				schedule_work_on(cpu_id,
  1839						 &cpu_buffer->update_pages_work);
  1840				wait_for_completion(&cpu_buffer->update_done);
  1841			}
  1842	
  1843			cpu_buffer->nr_pages_to_update = 0;
  1844			put_online_cpus();
  1845		}
  1846	
  1847	 out:
  1848		/*
  1849		 * The ring buffer resize can happen with the ring buffer
  1850		 * enabled, so that the update disturbs the tracing as little
  1851		 * as possible. But if the buffer is disabled, we do not need
  1852		 * to worry about that, and we can take the time to verify
  1853		 * that the buffer is not corrupt.
  1854		 */
  1855		if (atomic_read(&buffer->record_disabled)) {
  1856			atomic_inc(&buffer->record_disabled);
  1857			/*
  1858			 * Even though the buffer was disabled, we must make sure
  1859			 * that it is truly disabled before calling rb_check_pages.
  1860			 * There could have been a race between checking
  1861			 * record_disable and incrementing it.
  1862			 */
  1863			synchronize_rcu();
  1864			for_each_buffer_cpu(buffer, cpu) {
  1865				cpu_buffer = buffer->buffers[cpu];
  1866				rb_check_pages(cpu_buffer);
  1867			}
  1868			atomic_dec(&buffer->record_disabled);
  1869		}
  1870	
  1871		mutex_unlock(&buffer->mutex);
  1872		return size;
  1873	
  1874	 out_err:
  1875		for_each_buffer_cpu(buffer, cpu) {
  1876			struct buffer_page *bpage, *tmp;
  1877	
  1878			cpu_buffer = buffer->buffers[cpu];
  1879			cpu_buffer->nr_pages_to_update = 0;
  1880	
  1881			if (list_empty(&cpu_buffer->new_pages))
  1882				continue;
  1883	
  1884			list_for_each_entry_safe(bpage, tmp, &cpu_buffer->new_pages,
  1885						list) {
  1886				list_del_init(&bpage->list);
  1887				free_buffer_page(bpage);
  1888			}
  1889		}
  1890		mutex_unlock(&buffer->mutex);
  1891		return err;
  1892	}
  1893	EXPORT_SYMBOL_GPL(ring_buffer_resize);
  1894	

---
0-DAY kernel test infrastructure                 Open Source Technology Center
https://lists.01.org/hyperkitty/list/kbuild-all(a)lists.01.org Intel Corporation

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Re: [RFC 1/2] docs: ftrace: Clarify the RAM impact of buffer_size_kb

[Joel Fernandes]

On Wed, Nov 13, 2019 at 11:37:30AM -0500, Steven Rostedt wrote:
> On Wed, 13 Nov 2019 11:32:36 -0500
> "Frank A. Cancio Bello" <frank@generalsoftwareinc.com> wrote:
[snip]
> > +
> > +        The number of pages allocated for each CPU buffer may not
> > +        be the same than the round up of the division:
> > +        buffer_size_kb / PAGE_SIZE. This is because part of each page is
> > +        used to store a page header with metadata. E.g. with
> > +        buffer_size_kb=4096 (kilobytes), a PAGE_SIZE=4096 bytes and a
> > +        BUF_PAGE_HDR_SIZE=16 bytes (BUF_PAGE_HDR_SIZE is the size of the
> > +        page header with metadata) the number of pages allocated for each
> > +        CPU buffer is 1029, not 1024. The formula for calculating the
> > +        number of pages allocated for each CPU buffer is the round up of:
> > +        buffer_size_kb / (PAGE_SIZE - BUF_PAGE_HDR_SIZE).
> 
> I have no problem with this patch, but the concern of documenting the
> implementation here, which will most likely not be updated if the
> implementation is ever changed, which is why I was vague to begin with.
> 
> But it may never be changed as that code has been like that for a
> decade now.

Agreed. To give some context, Frank is an outreachy intern I am working with and
one of his starter tasks was to understand the ring buffer's basics.  I asked
him to send a patch since I thought he mentioned there was an error in the
documnentation. It looks like all that was missing is some explanation which
the deleted text in brackets above should already cover.

Steve, your call if you want this patch. Looks like Frank understands the
page header taking up some space, so one of the goals of the exercise is
accomplished ;-)

thanks,

 - Joel

Re: [RFC 1/2] docs: ftrace: Clarify the RAM impact of buffer_size_kb

[Steven Rostedt]

On Thu, 14 Nov 2019 15:20:59 -0500
Joel Fernandes <joel@joelfernandes.org> wrote:

> On Wed, Nov 13, 2019 at 11:37:30AM -0500, Steven Rostedt wrote:
> > On Wed, 13 Nov 2019 11:32:36 -0500
> > "Frank A. Cancio Bello" <frank@generalsoftwareinc.com> wrote:  
> [snip]
> > > +
> > > +        The number of pages allocated for each CPU buffer may not
> > > +        be the same than the round up of the division:
> > > +        buffer_size_kb / PAGE_SIZE. This is because part of each page is
> > > +        used to store a page header with metadata. E.g. with
> > > +        buffer_size_kb=4096 (kilobytes), a PAGE_SIZE=4096 bytes and a
> > > +        BUF_PAGE_HDR_SIZE=16 bytes (BUF_PAGE_HDR_SIZE is the size of the
> > > +        page header with metadata) the number of pages allocated for each
> > > +        CPU buffer is 1029, not 1024. The formula for calculating the
> > > +        number of pages allocated for each CPU buffer is the round up of:
> > > +        buffer_size_kb / (PAGE_SIZE - BUF_PAGE_HDR_SIZE).  
> > 
> > I have no problem with this patch, but the concern of documenting the
> > implementation here, which will most likely not be updated if the
> > implementation is ever changed, which is why I was vague to begin with.
> > 
> > But it may never be changed as that code has been like that for a
> > decade now.  
> 
> Agreed. To give some context, Frank is an outreachy intern I am working with and
> one of his starter tasks was to understand the ring buffer's basics.  I asked
> him to send a patch since I thought he mentioned there was an error in the
> documnentation. It looks like all that was missing is some explanation which
> the deleted text in brackets above should already cover.
> 
> Steve, your call if you want this patch. Looks like Frank understands the
> page header taking up some space, so one of the goals of the exercise is
> accomplished ;-)

Yes agreed, what was written was not wrong (thus understood). But the
more I think about this, the less I like the implementation details in
the documentation directory. Now I am looking forward for some other
patches from Frank, and perhaps he could add some comments in
ring_buffer.c about this. ;-)

-- Steve

Re: [RFC 1/2] docs: ftrace: Clarify the RAM impact of buffer_size_kb

[Frank A. Cancio Bello]

On Thu, Nov 14, 2019 at 04:36:39PM -0500, Steven Rostedt wrote:
> On Thu, 14 Nov 2019 15:20:59 -0500
> Joel Fernandes <joel@joelfernandes.org> wrote:
> 
> > On Wed, Nov 13, 2019 at 11:37:30AM -0500, Steven Rostedt wrote:
> > > On Wed, 13 Nov 2019 11:32:36 -0500
> > > "Frank A. Cancio Bello" <frank@generalsoftwareinc.com> wrote:  
> > [snip]
> > > > +
> > > > +        The number of pages allocated for each CPU buffer may not
> > > > +        be the same than the round up of the division:
> > > > +        buffer_size_kb / PAGE_SIZE. This is because part of each page is
> > > > +        used to store a page header with metadata. E.g. with
> > > > +        buffer_size_kb=4096 (kilobytes), a PAGE_SIZE=4096 bytes and a
> > > > +        BUF_PAGE_HDR_SIZE=16 bytes (BUF_PAGE_HDR_SIZE is the size of the
> > > > +        page header with metadata) the number of pages allocated for each
> > > > +        CPU buffer is 1029, not 1024. The formula for calculating the
> > > > +        number of pages allocated for each CPU buffer is the round up of:
> > > > +        buffer_size_kb / (PAGE_SIZE - BUF_PAGE_HDR_SIZE).  
> > > 
> > > I have no problem with this patch, but the concern of documenting the
> > > implementation here, which will most likely not be updated if the
> > > implementation is ever changed, which is why I was vague to begin with.
> > > 
> > > But it may never be changed as that code has been like that for a
> > > decade now.  
> > 
> > Agreed. To give some context, Frank is an outreachy intern I am working with and
> > one of his starter tasks was to understand the ring buffer's basics.  I asked
> > him to send a patch since I thought he mentioned there was an error in the
> > documnentation. It looks like all that was missing is some explanation which
> > the deleted text in brackets above should already cover.
> > 

Not exactly in my opinion ;) The deleted text was not the problem. I
just deleted it because with the added text it turns to be redundant.

The issue that I found with the documentation (maybe just to my
newbie's eyes) is in this part:

"The trace buffers are allocated in pages (blocks of memory that the
kernel uses for allocation, usually 4 KB in size). If the last page
allocated has room for more bytes than requested, the rest of the
page will be used, making the actual allocation bigger than requested
or shown."

For me that "suggests" the interpretation that the number of pages
allocated in the current implementation correspond with the round
integer division of buffer_size_kb / PAGE_SIZE, which is inaccurate
(for 5 pages in the example that I mentioned).


> > Steve, your call if you want this patch. Looks like Frank understands the
> > page header taking up some space, so one of the goals of the exercise is
> > accomplished ;-)
> 
> Yes agreed, what was written was not wrong (thus understood). But the
> more I think about this, the less I like the implementation details in
> the documentation directory.

Understood and agreed. It is funny that what I spotted as "a problem"
was precisely an incomplete description of the implementation (the
sentences that I quoted above). What do you think about removing
those two sentences?

> Now I am looking forward for some other
> patches from Frank, and perhaps he could add some comments in
> ring_buffer.c about this. ;-)
>

You can count on it. I'm just starting to learn. I'm very grateful
that Joel took me under his wing ;) and with the time I hope to be
able to give back more to the community with the help of experts like
you Steve.

Thank you, Steve and Joel, for such quick feedback!
frank a.

Re: [RFC 1/2] docs: ftrace: Clarify the RAM impact of buffer_size_kb

[Steven Rostedt]

On Thu, 14 Nov 2019 23:24:28 -0500
"Frank A. Cancio Bello" <frank@generalsoftwareinc.com> wrote:

> On Thu, Nov 14, 2019 at 04:36:39PM -0500, Steven Rostedt wrote:
> > On Thu, 14 Nov 2019 15:20:59 -0500
> > Joel Fernandes <joel@joelfernandes.org> wrote:
> >   
> > > On Wed, Nov 13, 2019 at 11:37:30AM -0500, Steven Rostedt wrote:  
> > > > On Wed, 13 Nov 2019 11:32:36 -0500
> > > > "Frank A. Cancio Bello" <frank@generalsoftwareinc.com> wrote:    
> > > [snip]  
> > > > > +
> > > > > +        The number of pages allocated for each CPU buffer may not
> > > > > +        be the same than the round up of the division:
> > > > > +        buffer_size_kb / PAGE_SIZE. This is because part of each page is
> > > > > +        used to store a page header with metadata. E.g. with
> > > > > +        buffer_size_kb=4096 (kilobytes), a PAGE_SIZE=4096 bytes and a
> > > > > +        BUF_PAGE_HDR_SIZE=16 bytes (BUF_PAGE_HDR_SIZE is the size of the
> > > > > +        page header with metadata) the number of pages allocated for each
> > > > > +        CPU buffer is 1029, not 1024. The formula for calculating the
> > > > > +        number of pages allocated for each CPU buffer is the round up of:
> > > > > +        buffer_size_kb / (PAGE_SIZE - BUF_PAGE_HDR_SIZE).    
> > > > 
> > > > I have no problem with this patch, but the concern of documenting the
> > > > implementation here, which will most likely not be updated if the
> > > > implementation is ever changed, which is why I was vague to begin with.
> > > > 
> > > > But it may never be changed as that code has been like that for a
> > > > decade now.    
> > > 
> > > Agreed. To give some context, Frank is an outreachy intern I am working with and
> > > one of his starter tasks was to understand the ring buffer's basics.  I asked
> > > him to send a patch since I thought he mentioned there was an error in the
> > > documnentation. It looks like all that was missing is some explanation which
> > > the deleted text in brackets above should already cover.
> > >   
> 
> Not exactly in my opinion ;) The deleted text was not the problem. I
> just deleted it because with the added text it turns to be redundant.
> 
> The issue that I found with the documentation (maybe just to my
> newbie's eyes) is in this part:
> 
> "The trace buffers are allocated in pages (blocks of memory that the
> kernel uses for allocation, usually 4 KB in size). If the last page
> allocated has room for more bytes than requested, the rest of the
> page will be used, making the actual allocation bigger than requested
> or shown."
> 
> For me that "suggests" the interpretation that the number of pages
> allocated in the current implementation correspond with the round
> integer division of buffer_size_kb / PAGE_SIZE, which is inaccurate
> (for 5 pages in the example that I mentioned).

If you would like, you could reword that to something more accurate,
but still not detailing the implementation.

> Understood and agreed. It is funny that what I spotted as "a problem"
> was precisely an incomplete description of the implementation (the
> sentences that I quoted above). What do you think about removing
> those two sentences?

I wouldn't remove them, just reword them to something you find more
accurate.

-- Steve

Re: [RFC 1/2] docs: ftrace: Clarify the RAM impact of buffer_size_kb

[Frank A. Cancio Bello]

On Fri, Nov 15, 2019 at 08:30:00AM -0500, Steven Rostedt wrote:
> On Thu, 14 Nov 2019 23:24:28 -0500
> "Frank A. Cancio Bello" <frank@generalsoftwareinc.com> wrote:
> 
> > On Thu, Nov 14, 2019 at 04:36:39PM -0500, Steven Rostedt wrote:
> > > On Thu, 14 Nov 2019 15:20:59 -0500
> > > Joel Fernandes <joel@joelfernandes.org> wrote:
> > >   
> > > > On Wed, Nov 13, 2019 at 11:37:30AM -0500, Steven Rostedt wrote:  
> > > > > On Wed, 13 Nov 2019 11:32:36 -0500
> > > > > "Frank A. Cancio Bello" <frank@generalsoftwareinc.com> wrote:    
> > > > [snip]  
> > > > > > +
> > > > > > +        The number of pages allocated for each CPU buffer may not
> > > > > > +        be the same than the round up of the division:
> > > > > > +        buffer_size_kb / PAGE_SIZE. This is because part of each page is
> > > > > > +        used to store a page header with metadata. E.g. with
> > > > > > +        buffer_size_kb=4096 (kilobytes), a PAGE_SIZE=4096 bytes and a
> > > > > > +        BUF_PAGE_HDR_SIZE=16 bytes (BUF_PAGE_HDR_SIZE is the size of the
> > > > > > +        page header with metadata) the number of pages allocated for each
> > > > > > +        CPU buffer is 1029, not 1024. The formula for calculating the
> > > > > > +        number of pages allocated for each CPU buffer is the round up of:
> > > > > > +        buffer_size_kb / (PAGE_SIZE - BUF_PAGE_HDR_SIZE).    
> > > > > 
> > > > > I have no problem with this patch, but the concern of documenting the
> > > > > implementation here, which will most likely not be updated if the
> > > > > implementation is ever changed, which is why I was vague to begin with.
> > > > > 
> > > > > But it may never be changed as that code has been like that for a
> > > > > decade now.    
> > > > 
> > > > Agreed. To give some context, Frank is an outreachy intern I am working with and
> > > > one of his starter tasks was to understand the ring buffer's basics.  I asked
> > > > him to send a patch since I thought he mentioned there was an error in the
> > > > documnentation. It looks like all that was missing is some explanation which
> > > > the deleted text in brackets above should already cover.
> > > >   
> > 
> > Not exactly in my opinion ;) The deleted text was not the problem. I
> > just deleted it because with the added text it turns to be redundant.
> > 
> > The issue that I found with the documentation (maybe just to my
> > newbie's eyes) is in this part:
> > 
> > "The trace buffers are allocated in pages (blocks of memory that the
> > kernel uses for allocation, usually 4 KB in size). If the last page
> > allocated has room for more bytes than requested, the rest of the
> > page will be used, making the actual allocation bigger than requested
> > or shown."
> > 
> > For me that "suggests" the interpretation that the number of pages
> > allocated in the current implementation correspond with the round
> > integer division of buffer_size_kb / PAGE_SIZE, which is inaccurate
> > (for 5 pages in the example that I mentioned).
> 
> If you would like, you could reword that to something more accurate,
> but still not detailing the implementation.
> 
> > Understood and agreed. It is funny that what I spotted as "a problem"
> > was precisely an incomplete description of the implementation (the
> > sentences that I quoted above). What do you think about removing
> > those two sentences?
> 
> I wouldn't remove them, just reword them to something you find more
> accurate.
> 

I feel that adding:

"A few extra pages may be allocated to accommodate buffer management
meta-data."

between the two sentences that I quoted will address the issue. If
that is OK with you I will proceed to package this change in a new
patchset along with a few fixes of typos that I spotted in other
parts of the doc.

thanks one more time for your quick response.
frank a.

Re: [RFC 1/2] docs: ftrace: Clarify the RAM impact of buffer_size_kb

[Steven Rostedt]

On Fri, 15 Nov 2019 10:59:55 -0500
"Frank A. Cancio Bello" <frank@generalsoftwareinc.com> wrote:

> I feel that adding:
> 
> "A few extra pages may be allocated to accommodate buffer management
> meta-data."
> 
> between the two sentences that I quoted will address the issue. If
> that is OK with you I will proceed to package this change in a new
> patchset along with a few fixes of typos that I spotted in other
> parts of the doc.

Yep, I'm OK with that.

Thanks!

-- Steve

Re: [RFC 2/2] ** do not apply this patch ** Just for illustration purposes

[kbuild test robot]

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Hi "Frank,

[FYI, it's a private test report for your RFC patch.]
[auto build test WARNING on tip/perf/core]
[cannot apply to v5.4-rc7 next-20191114]
[if your patch is applied to the wrong git tree, please drop us a note to help
improve the system. BTW, we also suggest to use '--base' option to specify the
base tree in git format-patch, please see https://stackoverflow.com/a/37406982]

url:    https://github.com/0day-ci/linux/commits/Frank-A-Cancio-Bello/docs-ftrace-Clarify-the-RAM-impact-of-buffer_size_kb/20191114-040608
base:   https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git 295c52ee1485e4dee660fc1a0e6ceed6c803c9d3
config: i386-randconfig-a002-20191115 (attached as .config)
compiler: gcc-4.9 (Debian 4.9.2-10+deb8u1) 4.9.2
reproduce:
        # save the attached .config to linux build tree
        make ARCH=i386 

If you fix the issue, kindly add following tag
Reported-by: kbuild test robot <lkp@intel.com>

All warnings (new ones prefixed by >>):

   In file included from <command-line>:0:0:
   kernel/trace/ring_buffer.c: In function 'ring_buffer_resize':
>> kernel/trace/ring_buffer.c:125:43: warning: format '%lu' expects argument of type 'long unsigned int', but argument 3 has type 'unsigned int' [-Wformat=]
    #define BUF_PAGE_HDR_SIZE offsetof(struct buffer_data_page, data)
                                              ^
   include/linux/compiler_types.h:129:54: note: in definition of macro '__compiler_offsetof'
    #define __compiler_offsetof(a, b) __builtin_offsetof(a, b)
                                                         ^
>> kernel/trace/ring_buffer.c:125:27: note: in expansion of macro 'offsetof'
    #define BUF_PAGE_HDR_SIZE offsetof(struct buffer_data_page, data)
                              ^
>> kernel/trace/ring_buffer.c:1734:20: note: in expansion of macro 'BUF_PAGE_HDR_SIZE'
            PAGE_SIZE, BUF_PAGE_HDR_SIZE, size, nr_pages);
                       ^

vim +125 kernel/trace/ring_buffer.c

a358324466b171 Steven Rostedt 2008-11-11  124  
499e547057f5bb Steven Rostedt 2012-02-22 @125  #define BUF_PAGE_HDR_SIZE offsetof(struct buffer_data_page, data)
033601a32b2012 Steven Rostedt 2008-11-21  126  

:::::: The code at line 125 was first introduced by commit
:::::: 499e547057f5bba5cd6f87ebe59b05d0c59da905 tracing/ring-buffer: Only have tracing_on disable tracing buffers

:::::: TO: Steven Rostedt <srostedt@redhat.com>
:::::: CC: Steven Rostedt <rostedt@goodmis.org>

---
0-DAY kernel test infrastructure                 Open Source Technology Center
https://lists.01.org/hyperkitty/list/kbuild-all(a)lists.01.org Intel Corporation

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