Messages in this thread | | | Subject | Re: Perf and ftrace [was Re: PyTimechart] | From | Steven Rostedt <> | Date | Thu, 13 May 2010 12:46:18 -0400 |
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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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