From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:38226) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1TG8nK-0004Yc-S8 for qemu-devel@nongnu.org; Mon, 24 Sep 2012 09:34:04 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1TG8nF-0003P7-Rp for qemu-devel@nongnu.org; Mon, 24 Sep 2012 09:33:54 -0400 Received: from mx1.redhat.com ([209.132.183.28]:5258) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1TG8nF-0003OT-J1 for qemu-devel@nongnu.org; Mon, 24 Sep 2012 09:33:49 -0400 Date: Mon, 24 Sep 2012 10:34:33 -0300 From: Luiz Capitulino Message-ID: <20120924103433.2afcbc0b@doriath.home> In-Reply-To: <505FFD45.2050804@cn.fujitsu.com> References: <1348247243-12446-1-git-send-email-lcapitulino@redhat.com> <1348247243-12446-3-git-send-email-lcapitulino@redhat.com> <505FFD45.2050804@cn.fujitsu.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH 2/3] qmp: dump-guest-memory: don't spin if non-blocking fd would block List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Wen Congyang Cc: jan.kiszka@siemens.com, aliguori@us.ibm.com, qemu-devel@nongnu.org, d.hatayama@jp.fujitsu.com, armbru@redhat.com On Mon, 24 Sep 2012 14:27:17 +0800 Wen Congyang wrote: > At 09/22/2012 01:07 AM, Luiz Capitulino Wrote: > > fd_write_vmcore() will indefinitely spin for a non-blocking > > file-descriptor that would block. However, if the fd is non-blocking, > > how does it make sense to spin? > > > > Change this behavior to return an error instead. > > > > Note that this can only happen with an fd provided by a management > > application. The fd opened internally by dump-guest-memory is blocking. > > > > Signed-off-by: Luiz Capitulino > > --- > > dump.c | 13 +++---------- > > 1 file changed, 3 insertions(+), 10 deletions(-) > > > > diff --git a/dump.c b/dump.c > > index 2bf8d8d..5eea015 100644 > > --- a/dump.c > > +++ b/dump.c > > @@ -100,18 +100,11 @@ static void dump_error(DumpState *s, const char *reason) > > static int fd_write_vmcore(void *buf, size_t size, void *opaque) > > { > > DumpState *s = opaque; > > - int fd = s->fd; > > size_t writen_size; > > > > - /* The fd may be passed from user, and it can be non-blocked */ > > - while (size) { > > - writen_size = qemu_write_full(fd, buf, size); > > - if (writen_size != size && errno != EAGAIN) { > > Hmm, if the fd is a blocking fd, errno can't be EAGAIN. So the > function doesn't spin. What problems do you meet? The problem is with non-blocking fds, where spinning isn't correct, for two reasons: 1. If the fd is non-blocking, that means you don't want to block and spinning for a long time will have the same effects 2. Spinning consumes host resources