Re: Perf and ftrace [was Re: PyTimechart]

Steven Rostedt
2010-05-12T22:21:32Z

On Wed, 2010-05-12 at 16:27 -0400, Mathieu Desnoyers wrote:
> * Steven Rostedt (rostedt@goodmis.org) wrote:
> > On Wed, 2010-05-12 at 14:37 -0400, Mathieu Desnoyers wrote:
> >
> > > OK, I see. In LTTng, I dropped the mmap() support when I integrated splice(). In
> > > both case, I can share the pages between the "output" (mmap or splice) and the
> > > ring buffer because my ring buffer does not care about
> > > page->mapping/->index/etc, so I never have to swap them.
> >
> > I'm curious, how do you handle the overwrite mode without swapping?
>
> Explanation extracted from:
>
> http://www.lttng.org/pub/thesis/desnoyers-dissertation-2009-12.pdf
>
> 5.4 Atomic Buffering Scheme
> 5.4.3 Algorithms
>
> "This is achieved by adding a supplementary sub-buffer, owned by the reader. A
> table with pointers to the sub-buffers being used by the writer allows the
> reader to change the reference to each sub-buffer atomically. The
> ReadGetSubbuf() algorithm is responsible for atomically exchanging the reference
> to the sub-buffer about to be read with the sub-buffer currently owned by the
> reader.

AKA - swapping

As I asked, this seems to do exactly what my ring buffer does, except
you use a table where I swap out the list. But this is still swapping.

> If the CAS operation fails, the reader does not get access to the buffer
> for reading."
>
> I know your mother tongue is C, not English, so I just prepared a git repo with
> the current state of my work (please note that I'm currently in the process of
> cleaning up this code).
>
> http://git.kernel.org/?p=linux/kernel/git/compudj/linux-2.6-ringbuffer.git
>
> Interesting bits below.
>
> Thanks,
>
> Mathieu
>
> Note: The "frontend" refers to the buffer writer/reader synchronization
> algorithm. The "backend" deals with allocation of the memory buffers. This
> frontend/backend separation permits to use the same ring buffer synchronization
> code to write data to kernel pages, to video memory, to serial ports, etc etc,
> without having to deal with different synchronization schemes.

OK

>
> Where the reader grabs the sub-buffer :
>
> kernel/trace/ring_buffer_frontend.c: ring_buffer_get_subbuf()
>
> 396 ret = update_read_sb_index(&buf->backend, &chan->backend, consumed_idx);
> 397 if (ret)
> 398 return ret;
>
> and releases it:
>
> kernel/trace/ring_buffer_frontend.c: ring_buffer_put_subbuf()
>
> 415 RCHAN_SB_SET_NOREF(buf->backend.buf_rsb.pages);
>
> The writer clears the "noref" flag when it starts writing to a subbuffer, and
> clears that flag when it has fully committed a subbuffer.

Should one of the "clears" above be a set?

>
> The primitives used by the "synchronization frontend" are declared in the
> backend here:
>
> kernel/trace/ring_buffer_page_backend_internal.h:
>
> Interesting definitions and data structures for our current discussions:
>
> 17 #define RCHAN_SB_IS_NOREF(x) ((unsigned long)(x) & RCHAN_NOREF_FLAG)
> 18 #define RCHAN_SB_SET_NOREF(x) \

I really hate caps, even for macros. If it acts like a function, keep it
lowercase. Caps are for constants not functions.

Linux convention has always had lowercase for macros that act like
functions. Heck, why not just make these static inlines?

> 19 (x = (struct ring_buffer_backend_page *) \
> 20 ((unsigned long)(x) | RCHAN_NOREF_FLAG))
> 21 #define RCHAN_SB_CLEAR_NOREF(x) \
> 22 (x = (struct ring_buffer_backend_page *) \
> 23 ((unsigned long)(x) & ~RCHAN_NOREF_FLAG))
> 24
> 25 struct ring_buffer_backend_page {
> 26 void *virt; /* page virtual address (cached) */
> 27 struct page *page; /* pointer to page structure */
> 28 };
> 29
> 30 struct ring_buffer_backend_subbuffer {
> 31 /* Pointer to backend pages for subbuf */
> 32 struct ring_buffer_backend_page *pages;
> 33 };
>
> ...
>
> 41 struct ring_buffer_backend {
> 42 /* Array of chanbuf_sb for writer */
> 43 struct ring_buffer_backend_subbuffer *buf_wsb;
> 44 /* chanbuf_sb for reader */
> 45 struct ring_buffer_backend_subbuffer buf_rsb;

So this is equivalent to my reader_page?

>
> ...
>
> 97 /**
> 98 * ring_buffer_clear_noref_flag - Clear the noref subbuffer flag, for writer.
> 99 */
> 100 static __inline__
> 101 void ring_buffer_clear_noref_flag(struct ring_buffer_backend *bufb,
> 102 unsigned long idx)
> 103 {
> 104 struct ring_buffer_backend_page *sb_pages, *new_sb_pages;
> 105
> 106 sb_pages = bufb->buf_wsb[idx].pages;
> 107 for (;;) {
> 108 if (!RCHAN_SB_IS_NOREF(sb_pages))
> 109 return; /* Already writing to this buffer */
> 110 new_sb_pages = sb_pages;
> 111 RCHAN_SB_CLEAR_NOREF(new_sb_pages);
> 112 new_sb_pages = cmpxchg(&bufb->buf_wsb[idx].pages,
> 113 sb_pages, new_sb_pages);
> 114 if (likely(new_sb_pages == sb_pages))
> 115 break;
> 116 sb_pages = new_sb_pages;

The writer calls this??

> 117 }
> 118 }
> 119
> 120 /**
> 121 * ring_buffer_set_noref_flag - Set the noref subbuffer flag, for writer.
> 122 */
> 123 static __inline__
> 124 void ring_buffer_set_noref_flag(struct ring_buffer_backend *bufb,
> 125 unsigned long idx)
> 126 {
> 127 struct ring_buffer_backend_page *sb_pages, *new_sb_pages;
> 128
> 129 sb_pages = bufb->buf_wsb[idx].pages;
> 130 for (;;) {
> 131 if (RCHAN_SB_IS_NOREF(sb_pages))
> 132 return; /* Already set */
> 133 new_sb_pages = sb_pages;
> 134 RCHAN_SB_SET_NOREF(new_sb_pages);
> 135 new_sb_pages = cmpxchg(&bufb->buf_wsb[idx].pages,
> 136 sb_pages, new_sb_pages);
> 137 if (likely(new_sb_pages == sb_pages))
> 138 break;
> 139 sb_pages = new_sb_pages;

Again, the writer calls this??

> 140 }
> 141 }
> 142
> 143 /**
> 144 * update_read_sb_index - Read-side subbuffer index update.
> 145 */
> 146 static __inline__
> 147 int update_read_sb_index(struct ring_buffer_backend *bufb,
> 148 struct channel_backend *chanb,
> 149 unsigned long consumed_idx)
> 150 {
> 151 struct ring_buffer_backend_page *old_wpage, *new_wpage;
> 152
> 153 if (unlikely(chanb->extra_reader_sb)) {
> 154 /*
> 155 * Exchange the target writer subbuffer with our own unused
> 156 * subbuffer.
> 157 */
> 158 old_wpage = bufb->buf_wsb[consumed_idx].pages;
> 159 if (unlikely(!RCHAN_SB_IS_NOREF(old_wpage)))
> 160 return -EAGAIN;
> 161 WARN_ON_ONCE(!RCHAN_SB_IS_NOREF(bufb->buf_rsb.pages));
> 162 new_wpage = cmpxchg(&bufb->buf_wsb[consumed_idx].pages,
> 163 old_wpage,
> 164 bufb->buf_rsb.pages);

This looks just like the swap with reader_page that I do, except you use
a table and I use the list. How do you replenish the buf_rsb.pages if
the splice keeps the page you just received active?

-- Steve

> 165 if (unlikely(old_wpage != new_wpage))
> 166 return -EAGAIN;
> 167 bufb->buf_rsb.pages = new_wpage;
> 168 RCHAN_SB_CLEAR_NOREF(bufb->buf_rsb.pages);
> 169 } else {
> 170 /* No page exchange, use the writer page directly */
> 171 bufb->buf_rsb.pages = bufb->buf_wsb[consumed_idx].pages;
> 172 RCHAN_SB_CLEAR_NOREF(bufb->buf_rsb.pages);
> 173 }
> 174 return 0;
> 175 }
>
>