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    SubjectRe: Perf and ftrace [was Re: PyTimechart]
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    On Thu, 2010-05-13 at 12:31 -0400, Mathieu Desnoyers wrote:
    > * Steven Rostedt (rostedt@goodmis.org) wrote:
    > > On Thu, 2010-05-13 at 09:20 -0400, Mathieu Desnoyers wrote:
    > [...]
    > > > > >
    > > > > > ...
    > > > > >
    > > > > > 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.
    > >
    > > Is the cmpxchg only contending with other writers?
    >
    > No. Would have this been the case, I would have used a cmpxchg_local(). This
    > cmpxchg used to deal with subbuffer swap is touching the subbuffer "pages"
    > pointer, which can be updated concurrently by other writers as well as readers.
    >
    > The writer clears the noref flags when starting to write in a subbuffers, and
    > sets it when delivering the subbuffer (when it is fully committed).
    >
    > The reader can only ever swap the subbuffer with the one it owns if the noref
    > flag is set. The reader uses a cmpxchg() too to perform the swap.
    >

    So the writer can loop because of a reader? And theoretically, a
    aggressive reader that is constantly swapping pages can halt a writer?

    I thought you said you did not have loops in the writers?

    > [...]
    >
    > > > >
    > > > > 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.
    > >
    > > Hmm, so basically you lose pages until they are returned. I guess I can
    > > trivially add the same thing now to the current ring buffer.
    >
    > Yep. Having the ability to keep an array of pages (rather that just a single
    > page at a time) allows splice() to move many pages at once efficiently, while
    > permitting this "pages owned by the readers, lend to splice() until it returns"
    > simplification. I also never have to allocate pages while tracing: all the pages
    > I need are allocated when the buffer is created (and at the special case of cpu
    > hotplug, but this is expected for per-cpu buffers).

    Yep, I just wrote the changes for my buffer. It was rather trivial ;-)
    Although, I just set a hard coded 16 pages max of cached pages.

    -- Steve

    >
    > In addition, this would play well with mmap() too: we can simply add a
    > ring_buffer_get_mmap_offset() method to the backend (exported through another
    > ioctl) that would let user-space know the start of the mmap'd buffer range
    > currently owned by the reader. So we can inform user-space of the currently
    > owned page range without even changing the underlying memory map.
    >
    > Thanks,
    >
    > Mathieu
    >
    >




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