Date: Fri, 17 Feb 2012 09:58:41 +0100 From: Pawel Jakub Dawidek <pjd@FreeBSD.org> To: Marcel Moolenaar <marcel@xcllnt.net> Cc: marcel@freebsd.org, Eitan Adler <eadler@freebsd.org>, svn-src-all@freebsd.org, "Kenneth D. Merry" <ken@freebsd.org>, Andriy Gapon <avg@freebsd.org>, src-committers@freebsd.org, Bruce Evans <brde@optusnet.com.au>, svn-src-head@freebsd.org, Julian Elischer <julian@freebsd.org> Subject: Re: svn commit: r231814 - in head/sys: kern sys Message-ID: <20120217085840.GC1358@garage.freebsd.pl> In-Reply-To: <9CB7ECE8-FF10-43BE-9EBD-16953BE3B193@xcllnt.net> References: <201202160511.q1G5BZNk099785@svn.freebsd.org> <20120216181210.K1423@besplex.bde.org> <4F3CC40D.4000307@freebsd.org> <4F3CC5C4.7020501@FreeBSD.org> <4F3CC8A5.3030107@FreeBSD.org> <20120216174758.GA64180@nargothrond.kdm.org> <20120217053341.R1256@besplex.bde.org> <20120217000846.GA7641@nargothrond.kdm.org> <4F3D9D03.6020507@FreeBSD.org> <9CB7ECE8-FF10-43BE-9EBD-16953BE3B193@xcllnt.net>
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On Thu, Feb 16, 2012 at 08:49:05PM -0800, Marcel Moolenaar wrote:
>=20
> On Feb 16, 2012, at 4:19 PM, Andriy Gapon wrote:
>=20
> > on 17/02/2012 02:08 Kenneth D. Merry said the following:
> > [snip]
> >>>> On Thu, Feb 16, 2012 at 11:13:09 +0200, Andriy Gapon wrote:
> > [snip]
> >>>>> For me personally the immediate benefits in the common situations=
=20
> >>>>> outweighed the
> >>>>> problems in the edge cases, although I still believe that we can ge=
t the=20
> >>>>> former
> >>>>> without sacrifices in the latter.
> > [snip]
> >> It sounds fine, but I don't have sufficient time to spend on this right
> >> now. So I can either back out the changes I mentioned above (assuming=
we
> >> get agreement from avg), or leave things as is.
> >=20
> > I stick to what I wrote above and so chose the status quo.
> > The backout would make sense if it is immediately followed by commit of=
a better
> > solution. Unfortunately, a lack of time here too.
>=20
> I think we should lift above the immediate problem and allow for
> single- and multi-line messages that are atomically appended to
> the message buffer. Console output and propagation of messages
> outside of the kernel should all come out of the message buffer
> and preserving the atomicity of the messages.
>=20
> The message buffer does not have to be a chunk of memory that
> we circularly scribble to. It can be a per-cpu linked list of
> messages even.
>=20
> The advantage of thinking along these lines is that:
> 1. Console output can be made optional very easily, allowing
> us to implement quiet boots without loosing the ability
> to look at messages collected during boot.
> 2. Atomicity allows us to parse the messages reliably, which
> works very well in the embedded space where monitoring of
> kernel messages is common.
> 3. You can decouple writing into the message buffer from
> extracting messages out of the message buffer, allowing
> the low-level console to become just another channel to
> send messages to, rather than be fundamental for printf.
> 4. A linked list (for example) eliminates the problem of
> scribbling over old messages and possibly leaving partial
> output that gets misinterpreted.
> 5. A per-cpu message buffer eliminates serialization to
> guarantee atomcity and with timestamping can very easily
> be turned into a sequential log.
> 6. We haven't introduced complications (e.g. locking) to
> solve these problems and that make using printf in low-
> level code impossible. Thank trap handlers or interrupt
> handlers.
I agree with everything except for per-CPU buffers. I understand the
need for using printf in low-level code and it indeed complicates things.
The reason I don't like the idea of per-CPU buffers is that locking
would allow me to implement atomicity across multiple printfs.
For example I often use macros like this:
#define G_MIRROR_DEBUG(lvl, ...) do { \
if (g_mirror_debug >=3D (lvl)) { \
printf("GEOM_MIRROR"); \
if (g_mirror_debug > 0) \
printf("[%u]", lvl); \
printf(": "); \
printf(__VA_ARGS__); \
printf("\n"); \
} \
} while (0)
And I'd like all the printfs to be committed as one message without
using some additional buffer first and then single printf.
With some kind of printf-lock we could use recursive locking to achieve
this. In your proposal I may run each printf on different CPU.
I could eventually use sched_pin() around all printfs, I guess.
This still doesn't cover the case when I'm preempted between my printfs,
so maybe I need critical section there? I don't expect printf should be
fast, so it might be ok.
--=20
Pawel Jakub Dawidek http://www.wheelsystems.com
FreeBSD committer http://www.FreeBSD.org
Am I Evil? Yes, I Am! http://tupytaj.pl
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