From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754578Ab0EMNUg (ORCPT ); Thu, 13 May 2010 09:20:36 -0400 Received: from mail.openrapids.net ([64.15.138.104]:50754 "EHLO blackscsi.openrapids.net" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1754149Ab0EMNUd (ORCPT ); Thu, 13 May 2010 09:20:33 -0400 Date: Thu, 13 May 2010 09:20:29 -0400 From: Mathieu Desnoyers To: Steven Rostedt Cc: Peter Zijlstra , Frederic Weisbecker , Pierre Tardy , Ingo Molnar , Arnaldo Carvalho de Melo , Tom Zanussi , Paul Mackerras , linux-kernel@vger.kernel.org, arjan@infradead.org, ziga.mahkovec@gmail.com, davem Subject: Re: Perf and ftrace [was Re: PyTimechart] Message-ID: <20100513132029.GA22799@Krystal> References: <20100512170734.GA15953@Krystal> <1273686425.1626.142.camel@laptop> <20100512175305.GB32496@Krystal> <1273687212.1626.147.camel@laptop> <20100512180438.GE15953@Krystal> <1273687712.1626.151.camel@laptop> <20100512183704.GD21432@Krystal> <1273690012.27703.38.camel@gandalf.stny.rr.com> <20100512202745.GK21432@Krystal> <1273702886.27703.58.camel@gandalf.stny.rr.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1273702886.27703.58.camel@gandalf.stny.rr.com> X-Editor: vi X-Info: http://www.efficios.com X-Operating-System: Linux/2.6.26-2-686 (i686) X-Uptime: 08:31:21 up 110 days, 15:08, 8 users, load average: 0.35, 0.33, 0.62 User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * 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