From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:57787) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bwUYL-0001fV-2U for qemu-devel@nongnu.org; Tue, 18 Oct 2016 09:35:42 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1bwUYH-00062p-7j for qemu-devel@nongnu.org; Tue, 18 Oct 2016 09:35:37 -0400 Received: from mx1.redhat.com ([209.132.183.28]:55540) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1bwUYG-00062Y-Sz for qemu-devel@nongnu.org; Tue, 18 Oct 2016 09:35:33 -0400 Received: from int-mx13.intmail.prod.int.phx2.redhat.com (int-mx13.intmail.prod.int.phx2.redhat.com [10.5.11.26]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 288A2C04B312 for ; Tue, 18 Oct 2016 13:35:32 +0000 (UTC) Date: Tue, 18 Oct 2016 14:35:28 +0100 From: "Daniel P. Berrange" Message-ID: <20161018133528.GD12728@redhat.com> Reply-To: "Daniel P. Berrange" References: <20161012191502.GC16187@work-vm> <20161018100409.GH4349@redhat.com> <20161018113202.GE2190@work-vm> <20161018120121.GN4349@redhat.com> <20161018132524.GG2190@work-vm> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20161018132524.GG2190@work-vm> Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] chardev's and fd's in monitors List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Dr. David Alan Gilbert" Cc: qemu-devel@nongnu.org, armbru@redhat.com On Tue, Oct 18, 2016 at 02:25:25PM +0100, Dr. David Alan Gilbert wrote: > * Daniel P. Berrange (berrange@redhat.com) wrote: > > On Tue, Oct 18, 2016 at 12:32:02PM +0100, Dr. David Alan Gilbert wrot= e: > > > * Daniel P. Berrange (berrange@redhat.com) wrote: > > > > On Wed, Oct 12, 2016 at 08:15:02PM +0100, Dr. David Alan Gilbert = wrote: > > > > > Hi, > > > > > I had a look at a couple of readline like libraries; > > > > > editline and linenoise. A difficulty with using them is that > > > > > they both want fd's or FILE*'s; editline takes either but > > > > > from a brief look I think it's expecting to extract the fd. > > > > > That makes them tricky to integrate into qemu, where > > > > > the chardev's hide a whole bunch of non-fd things; in particula= r > > > > > tls, mux, ringbuffers etc. > > > > >=20 > > > > > If we could get away with just a FILE* then we could use fopenc= ookie, > > > > > but that's GNU only. > > > > >=20 > > > > > Is there any sane way of shepherding all chardev's into having = an > > > > > fd? > > > >=20 > > > > The entire chardev abstraction model exists precisely because we = cannot > > > > make all chardevs look like a single fd. Even those which are fd = based > > > > may have separate FDs for input and output. > > >=20 > > > Note that editline takes separate in/out streams, but it does want = those streams > > > to be FILE*'s. > > >=20 > > > > IMHO the only viable approach would be to enhance linenoise/editl= ine to > > > > not assume use of fd* or FILE * abstractions. > > >=20 > > > I think if it came to that then we'd probably end up sticking with = what we > > > had for a very long time; I'd assume it would take a long time befo= re > > > any mods we made to the libraries would come around to be generally= useful. > > >=20 > > > > BTW, what is the actual thread issue you are facing ? Chardevs at= least > > > > ought to be usable from a separate thread, as long as each distin= ct > > > > chardev object instance was only used from one thread at a time ? > > >=20 > > > Marc-Andr=C3=A9 pointed that out; I hadn't realised they were threa= d safe. > > > But what are the rules? You say 'only used from one thread at a tim= e' - > > > what happens if we have a mux and the different streams to the mux = come > > > from different threads? > >=20 > > Well there is no mutex locking on the CharDriverState objects, so the > > exact rule is "you mustn't do anything from multiple threads that wil= l > > race on contents of CharDriverState". That's too fuzzy to be useful t= o > > developers though, so I think the only sensible option right now is t= o > > say any "top level" CharDriverState should only be touch from one thr= ead > > at a time. IOW, if you have a mux, that that rule would apply to the > > mux itself and the various children it owns as if they were a single > > unnit. >=20 > OK; I think we're probably saved by the big lock at the moment, so that > all device emulation that outputs text is probably holding it and the m= onitor > is also. What about something like an error_report from a different th= read > while something is happening in the monitor? If we moved execution of monitor commands to separate thread from the thread handling monitor I/O, then we'd have to modify error_report so that it queued the text in some manner, such that it was only then fed back to the client once the command thread completed. Alternatively we'd have to introduced locking in the Monitor object, that serialized access to the underling CharDriverState I/O funcs. > > > My actual thoughts for threads came from a few sides: > > > a) Maybe I could have a shim thread that fed the editline fd from= a chardev > > > b) I'd eventually like multiple monitor threads. > >=20 > > Can you expand on what you mean by multiple monitor threads ? Presuma= bly > > you're meaning a single monitor instance, with multiple threads proce= ssing > > commands concurrently ? If so, I think that ought to be fine even wi= th > > the current thread rules around chardevs. The processing of individua= l > > monitor commands doesn't interact with the CharDriverState AFAIR, as = we > > have clean separation between parsing the incoming command, running t= he > > command, and formatting the outgoing response. IOW, for a single moni= tor > > it is still sufficient to have a single thread deal with all I/O for = the > > chardev - only the command execution needs to be delegated to other > > threads, and those wouldn't be touching the chardev at all. >=20 > Hmm, I'd thought of the other way around - multiple individual monitors= each > running one command; ie each connection for a monitor would be it's own > thread. So I guess there's two problems with the monitor handling right now wrt. - A long running command will block the event loop thread for too long - A long running command prevents a client issuing other commands while waiting for the previous command to complete. Running a thread per monitor server solves the first problem. If we make monitor command handling async though, then it solves both problems. Regards, Daniel --=20 |: http://berrange.com -o- http://www.flickr.com/photos/dberrange= / :| |: http://libvirt.org -o- http://virt-manager.or= g :| |: http://entangle-photo.org -o- http://search.cpan.org/~danberr= / :|