docs: ftrace: Clarify the RAM impact of buffer_size_kb
Subsystems (4)
v2 cover docs: ftrace: Clarify the RAM impact of buffer_size_kb
Clarifies the RAM footprint of buffer_size_kb without getting into implementation details.
Changes in v2:
- Removes implementation description of the RAM footprint of buffer_size_kb, but still make the corresponded clarification.
- Removes a patch that was just for illustration purposes because Steven already got the issue that I was referring to.
- Adds a patch to fix other typos in the doc.
Frank A. Cancio Bello (2):
docs: ftrace: Clarify the RAM impact of buffer_size_kb
docs: ftrace: Fix typos
Documentation/trace/ftrace.rst | 9 +++++---- Documentation/trace/ring-buffer-design.txt | 2 +- 2 files changed, 6 insertions(+), 5 deletions(-)
Signature · 1 line
2.17.1
RFC v2 1/2 docs: ftrace: Clarify the RAM impact of buffer_size_kb
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.
Clarifies that a few extra pages may be allocated to accommodate buffer management meta-data.
--- Documentation/trace/ftrace.rst | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)
Documentation/trace/ftrace.rst +2 -1
@@ -185,7 +185,8 @@ of ftrace. Here is a list of some of the key files: CPU buffer and not total size of all buffers. 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 + A few extra pages may be allocated to accommodate buffer management + meta-data. 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
Signature · 1 line
2.17.1
RFC v2 2/2 docs: ftrace: Fix typos
In reply to Frank A. Cancio BelloFix minor typos in the doc.
--- Documentation/trace/ftrace.rst | 6 +++--- Documentation/trace/ring-buffer-design.txt | 2 +- 2 files changed, 4 insertions(+), 4 deletions(-)
Documentation/trace/ftrace.rst +3 -3
@@ -236,7 +236,7 @@ of ftrace. Here is a list of some of the key files: This interface also allows for commands to be used. See the "Filter commands" section for more details. - As a speed up, since processing strings can't be quite expensive + As a speed up, since processing strings can be quite expensive and requires a check of all functions registered to tracing, instead an index can be written into this file. A number (starting with "1") written will instead select the same corresponding at the line position
@@ -383,7 +383,7 @@ of ftrace. Here is a list of some of the key files: By default, 128 comms are saved (see "saved_cmdlines" above). To increase or decrease the amount of comms that are cached, echo - in a the number of comms to cache, into this file. + in the number of comms to cache, into this file. saved_tgids:
@@ -3325,7 +3325,7 @@ directories after it is created. As you can see, the new directory looks similar to the tracing directory itself. In fact, it is very similar, except that the buffer and -events are agnostic from the main director, or from any other +events are agnostic from the main directory, or from any other instances that are created. The files in the new directory work just like the files with the
Documentation/trace/ring-buffer-design.txt +1 -1
@@ -37,7 +37,7 @@ commit_page - a pointer to the page with the last finished non-nested write. cmpxchg - hardware-assisted atomic transaction that performs the following: - A = B iff previous A == C + A = B if previous A == C R = cmpxchg(A, C, B) is saying that we replace A with B if and only if current A is equal to C, and we put the old (current) A into R
Signature · 1 line
2.17.1
Steven Rostedt
maintainerRe: [RFC v2 2/2] docs: ftrace: Fix typos
Documentation/trace/ring-buffer-design.txt commit_page - a pointer to the page with the last finished non-nested write. · line 40
Quoted 8 lines · On Sun, 24 Nov 2019 23:38:41 -0500 "Frank A. Cancio Bello"…
Documentation/trace/ring-buffer-design.txt +1
-1@@ -37,7 +37,7 @@ commit_page - a pointer to the page with the last finished non-nested write. cmpxchg - hardware-assisted atomic transaction that performs the following: - A = B iff previous A == C + A = B if previous A == C
This wasn't a typo. "iff" means "if and only if" which is a standard notation. That is, this is shorthand for:
A = B if previous A == C
previous A == C if A = B
-- Steve
Quoted 4 lines
Documentation/trace/ring-buffer-design.txt +0
-0R = cmpxchg(A, C, B) is saying that we replace A with B if and only if current A is equal to C, and we put the old (current) A into R
Steven Rostedt
maintainerRe: [RFC v2 2/2] docs: ftrace: Fix typos
Quoted 9 lines · On Mon, 25 Nov 2019 12:37:09 -0500 Steven Rostedt <[email protected]> wrote:
Quoted 6 lines · On Sun, 24 Nov 2019 23:38:41 -0500 "Frank A. Cancio Bello"…
Documentation/trace/ring-buffer-design.txt +0
-0@@ -37,7 +37,7 @@ commit_page - a pointer to the page with the last finished non-nested write. cmpxchg - hardware-assisted atomic transaction that performs the following:
Quoted 8 lines
Documentation/trace/ring-buffer-design.txt commit_page - a pointer to the page with the last finished non-nested write. · line 40
Quoted 2 lines
Documentation/trace/ring-buffer-design.txt +1
-1- A = B iff previous A == C + A = B if previous A == CThis wasn't a typo. "iff" means "if and only if" which is a standard notation. That is, this is shorthand for:
A = B if previous A == C
previous A == C if A = B
Although thinking about this more, this may not be correct. If previous A == B, then A = B, thus the "iff" notation is not actually accurate.
This wouldn't then be a typo fix, but a real fix to the logic ;-)
-- Steve
Quoted 6 lines
Quoted 6 lines
Documentation/trace/ring-buffer-design.txt +0
-0R = cmpxchg(A, C, B) is saying that we replace A with B if and only if current A is equal to C, and we put the old (current) A into R
Re: [RFC v2 2/2] docs: ftrace: Fix typos
Quoted 18 lines · Steven Rostedt
Quoted 15 lines · On Mon, 25 Nov 2019 12:37:09 -0500 Steven Rostedt <[email protected]> wrote:
Documentation/trace/ring-buffer-design.txt commit_page - a pointer to the page with the last finished non-nested write. · line 40
Quoted 8 lines · On Sun, 24 Nov 2019 23:38:41 -0500 "Frank A. Cancio Bello"…
Documentation/trace/ring-buffer-design.txt +1
-1@@ -37,7 +37,7 @@ commit_page - a pointer to the page with the last finished non-nested write. cmpxchg - hardware-assisted atomic transaction that performs the following: - A = B iff previous A == C + A = B if previous A == CThis wasn't a typo. "iff" means "if and only if" which is a standard notation. That is, this is shorthand for:
Quoted 9 lines
Quoted 2 lines
A = B if previous A == C
previous A == C if A = BAlthough thinking about this more, this may not be correct. If previous A == B, then A = B, thus the "iff" notation is not actually accurate.
This wouldn't then be a typo fix, but a real fix to the logic ;-)
What are the odds ;)
I knew about the concept https://en.wikipedia.org/wiki/If_and_only_if
from school math but didn't remember the notation. For me was suspicious so, I looked in "the cmpxchg doc" and nothing referring to "iff" was there, then I decided to classified it as a typo.
Now thinking more deeply, I agreed with you that was not a typo but a "logic mistake", even not for the same reason ;)
Being not 100% sure of what I'm about to say, I will say it anyway because you have been kind enough to write the second email.
"if and only if (shortened as iff) is a biconditional logical connective between statements" and A = B can't be considered a logical statement in the context of that definition (math/logic), even it could be in C.
Thank you Steven one more time for such a quick reply! frank a.