Re: Perf and ftrace [was Re: PyTimechart]

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

* Steven Rostedt (rostedt@goodmis.org) wrote:
> 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.

Yes, we could use the word swapping to explain this scheme I guess. Yes, it is
in some sense similar, with the distinction that here the ring buffer
reserve/commit (reader/writer synchronization) is all performed in the frontend,
thus independent from this page swapping.

When the buffer is in non-overwrite mode, I simply don't allocate a separate
subbuffer for the reader and don't need to perform swapping: the
producer/consumer offsets deal with reader/writer concurrency by mutually
excluding readers from the writer offset range and vice-versa.

> 
> 
> >  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?

Yes, the second "clears" in my explanation is indeed 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?

Will do. Good point!

> 
> 
> > 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?

Yes. But in this case, it is a reader "subbuffer", which is an array of pages.

> 
> > 
> > ...
> > 
> > 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??

Yes. But the common case (for each event) is simply a
"if (!RCHAN_SB_IS_NOREF(sb_pages))" test that returns. The cmpxchg() is only
performed at subbuffer boundary.

Will update the comment above to:

/**
 * ring_buffer_clear_noref - Clear the noref subbuffer flag, called by writer.
 */
static __inline__
void ring_buffer_clear_noref(struct ring_buffer_backend *bufb,
                             unsigned long idx)


> 
> > 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??

Yep.

> 
> > 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?

I don't allow other reads to proceed as long as splice is holding pages that
belong to the reader-owned subbuffer. The read semantic is basically:

ring_buffer_open_read() /* only one reader at a time can open a ring buffer */
get_subbuf_size()
while (buffer is not finalized and empty) {
  poll()
  ret = ring_buffer_get_subbuf()
  if (!ret)
    continue;
  /* The splice ops below can be performed in multiple calls, e.g. first splice
   * only a portion of a subbuffer to a pipe, then splice to the disk/network,
   * and move to the next subbuffer portion until all the subbuffer is sent.
   */
  splice one subbuffer worth of data to a pipe
  splice the data from pipe to disk/network
  ring_buffer_put_subbuf()
}
ring_buffer_close_read()

The reader code above works both with flight recorder and non-overwrite mode.

The code above assumes that upon return from the splice() to disk/network,
splice() is not using the pages anymore (I assume that splice() performs the
transfer synchronously with the call).

The VFS interface I use for get_subbuf_size(), ring_buffer_get_subbuf() and
ring_buffer_put_subbuf() are new ioctls. Note that these can be used for both
splice() and mmap() types of backend access, as they only call into the
frontend.

Thanks,

Mathieu


> 
> -- 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 }
> > 
> > 
> 
> 

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com