From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=37652 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1OFSp4-0005sj-E6 for qemu-devel@nongnu.org; Fri, 21 May 2010 10:03:35 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.69) (envelope-from ) id 1OFSot-0006Ee-Vh for qemu-devel@nongnu.org; Fri, 21 May 2010 10:03:33 -0400 Received: from mail-qy0-f173.google.com ([209.85.221.173]:50621) by eggs.gnu.org with esmtp (Exim 4.69) (envelope-from ) id 1OFSot-0006EG-Nb for qemu-devel@nongnu.org; Fri, 21 May 2010 10:03:23 -0400 Received: by qyk4 with SMTP id 4so1361452qyk.18 for ; Fri, 21 May 2010 07:03:22 -0700 (PDT) Message-ID: <4BF692A5.7080501@codemonkey.ws> Date: Fri, 21 May 2010 09:03:17 -0500 From: Anthony Liguori MIME-Version: 1.0 Subject: Re: [Qemu-devel] [PATCH 1/2] trace: Add simple tracing support References: <1274434947-2863-1-git-send-email-stefanha@linux.vnet.ibm.com> <1274434947-2863-2-git-send-email-stefanha@linux.vnet.ibm.com> <4BF67E72.5040908@codemonkey.ws> <4BF68E9B.1020405@siemens.com> In-Reply-To: <4BF68E9B.1020405@siemens.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Jan Kiszka Cc: Prerna Saxena , Stefan Hajnoczi , kvm@vger.kernel.org, qemu-devel@nongnu.org On 05/21/2010 08:46 AM, Jan Kiszka wrote: > Anthony Liguori wrote: > >> On 05/21/2010 04:42 AM, Stefan Hajnoczi wrote: >> >>> Trace events should be defined in trace.h. Events are written to >>> /tmp/trace.log and can be formatted using trace.py. Remember to add >>> events to trace.py for pretty-printing. >>> >>> Signed-off-by: Stefan Hajnoczi >>> --- >>> Makefile.objs | 2 +- >>> trace.c | 64 >>> +++++++++++++++++++++++++++++++++++++++++++++++++++++++++ >>> trace.h | 9 ++++++++ >>> trace.py | 30 ++++++++++++++++++++++++++ >>> 4 files changed, 104 insertions(+), 1 deletions(-) >>> create mode 100644 trace.c >>> create mode 100644 trace.h >>> create mode 100755 trace.py >>> >>> diff --git a/Makefile.objs b/Makefile.objs >>> index acbaf22..307e989 100644 >>> --- a/Makefile.objs >>> +++ b/Makefile.objs >>> @@ -8,7 +8,7 @@ qobject-obj-y += qerror.o >>> # block-obj-y is code used by both qemu system emulation and qemu-img >>> >>> block-obj-y = cutils.o cache-utils.o qemu-malloc.o qemu-option.o >>> module.o >>> -block-obj-y += nbd.o block.o aio.o aes.o osdep.o qemu-config.o >>> +block-obj-y += nbd.o block.o aio.o aes.o osdep.o qemu-config.o trace.o >>> block-obj-$(CONFIG_POSIX) += posix-aio-compat.o >>> block-obj-$(CONFIG_LINUX_AIO) += linux-aio.o >>> >>> diff --git a/trace.c b/trace.c >>> new file mode 100644 >>> index 0000000..2fec4d3 >>> --- /dev/null >>> +++ b/trace.c >>> @@ -0,0 +1,64 @@ >>> +#include >>> +#include >>> +#include "trace.h" >>> + >>> +typedef struct { >>> + unsigned long event; >>> + unsigned long x1; >>> + unsigned long x2; >>> + unsigned long x3; >>> + unsigned long x4; >>> + unsigned long x5; >>> +} TraceRecord; >>> + >>> +enum { >>> + TRACE_BUF_LEN = 64 * 1024 / sizeof(TraceRecord), >>> +}; >>> + >>> +static TraceRecord trace_buf[TRACE_BUF_LEN]; >>> +static unsigned int trace_idx; >>> +static FILE *trace_fp; >>> + >>> +static void trace(TraceEvent event, unsigned long x1, >>> + unsigned long x2, unsigned long x3, >>> + unsigned long x4, unsigned long x5) { >>> + TraceRecord *rec =&trace_buf[trace_idx]; >>> + rec->event = event; >>> + rec->x1 = x1; >>> + rec->x2 = x2; >>> + rec->x3 = x3; >>> + rec->x4 = x4; >>> + rec->x5 = x5; >>> + >>> + if (++trace_idx == TRACE_BUF_LEN) { >>> + trace_idx = 0; >>> + >>> + if (!trace_fp) { >>> + trace_fp = fopen("/tmp/trace.log", "w"); >>> + } >>> + if (trace_fp) { >>> + size_t result = fwrite(trace_buf, sizeof trace_buf, 1, >>> trace_fp); >>> + result = result; >>> + } >>> + } >>> +} >>> >>> >> It is probably worth while to read trace points via the monitor or >> through some other mechanism. My concern would be that writing even 64k >> out to disk would introduce enough performance overhead mainly because >> it runs lock-step with the guest's VCPU. >> >> Maybe it's worth adding a thread that syncs the ring to disk if we want >> to write to disk? >> > That's not what QEMU should worry about. If somehow possible, let's push > this into the hands of a (user space) tracing framework, ideally one > that is already designed for such requirements. E.g. there exists quite > useful work in the context of LTTng (user space RCU for application > tracing). > From what I understand, none of the current kernel approaches to userspace tracing have much momentum at the moment. > We may need simple stubs for the case that no such framework is (yet) > available. But effort should focus on a QEMU infrastructure to add > useful tracepoints to the code. Specifically when tracing over KVM, you > usually need information about kernel states as well, so you depend on > an integrated approach, not Yet Another Log File. > I think the simple code that Stefan pasted gives us 95% of what we need. Regards, Anthony Liguori > Jan > >