From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756861Ab0ELWVc (ORCPT ); Wed, 12 May 2010 18:21:32 -0400 Received: from hrndva-omtalb.mail.rr.com ([71.74.56.124]:46772 "EHLO hrndva-omtalb.mail.rr.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755915Ab0ELWVa (ORCPT ); Wed, 12 May 2010 18:21:30 -0400 X-Authority-Analysis: v=1.1 cv=QBaIlYAip0cLYvdi/Wu3On4+kilSXYulM+a9j4asJ0c= c=1 sm=0 a=yiYnlo-UJTIA:10 a=ood2b7iyd8MA:10 a=7U3hwN5JcxgA:10 a=Q9fys5e9bTEA:10 a=gMqfjgEr1zLu/65IO0LwxA==:17 a=meVymXHHAAAA:8 a=a5LkSv8FAAAA:8 a=VwQbUJbxAAAA:8 a=qj-FbdcYWbN6kXrIAAoA:9 a=0QZbLZHbsW_5zIlAc_oA:7 a=HOXwQ9l5ssyKP7xXeB3Dv42OYJMA:4 a=PUjeQqilurYA:10 a=jeBq3FmKZ4MA:10 a=BBFO5nCvE_LJjRVu:21 a=xcTjOrc7BOpNr2OE:21 a=gMqfjgEr1zLu/65IO0LwxA==:117 X-Cloudmark-Score: 0 X-Originating-IP: 74.67.89.75 Subject: Re: Perf and ftrace [was Re: PyTimechart] From: Steven Rostedt Reply-To: rostedt@goodmis.org To: Mathieu Desnoyers 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 In-Reply-To: <20100512202745.GK21432@Krystal> References: <20100512164650.GH5405@nowhere> <1273683624.1626.127.camel@laptop> <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> Content-Type: text/plain; charset="ISO-8859-15" Organization: Kihon Technologies Inc. Date: Wed, 12 May 2010 18:21:26 -0400 Message-ID: <1273702886.27703.58.camel@gandalf.stny.rr.com> Mime-Version: 1.0 X-Mailer: Evolution 2.28.3 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 } > >