linux-kernel - Re: [RFC][PATCHv3 2/5] printk: introduce printing kernel thread

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Date:   Thu, 29 Jun 2017 09:00:13 +0200
From:   Andreas Mohr <andi@...as.de>
To:     Andreas Mohr <andi@...as.de>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>, Jan Kara <jack@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Eric Biederman <ebiederm@...ssion.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, Pavel Machek <pavel@....cz>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCHv3 2/5] printk: introduce printing kernel thread

On Thu, Jun 29, 2017 at 08:26:20AM +0200, Andreas Mohr wrote:
> On Wed, Jun 28, 2017 at 02:19:25PM +0200, Petr Mladek wrote:
> > On Wed 2017-05-31 16:22:33, Sergey Senozhatsky wrote:
> > > so I try to minimize the negative impact of RT prio here. printk_kthread
> > > is not special any more. it's an auxiliary kthread that we sometimes
> > > wake_up. the thing is that printk_kthread also must offload at some
> > > point, basically the same `atomic_print_limit' limit applies to it as
> > > well.
> > 
> > You might call cond_resched() outside console_unlock(). But you have
> > to keep printk_kthread in runnable state as long as there are pending
> > messages. Then scheduler will always prefer this RT task over non-RT
> > tasks. Or am I wrong?
> 
> Not sure whether I mentioned this advice before, but:
> I believe we should strive to achieve a design where
> cond_resched() etc. is *not* needed -
> cond_resched() / sleep() etc. likely are signs of extended code smell:
> one should strive to achieve handling which has
> a properly *precisely*/*strictly* handshaked
> request/response (producer/consumer) communication protocol.
> I.e., IPC mechanism objects (mutex etc.).
> That way, one avoids
> the polling-type, imprecise-type "are we there yet? is it our job now?" handling
> and instead uses
> properly precise (thus, *not* needlessly inefficient!)
> scheduler wakeup mechanisms.
> 
> Thus, it's "merely" (hah!) a matter of
> designing handling where responsibilities / transitions are clearly spelt out,
> thus end up as
> properly precisely implemented notifications via IPC mechanisms.

Big picture:

IMHO (I might be missing something, or even more)
this still is a simple producer/consumer type scenario:
- you've got any producer inserting payload elements into a *shared* (*global*) store
  shared store --> store mutex needed
- reordering is very undesirable --> keep as one *shared*/*global* data store
- producer side(!): in case of hitting a max limit, queuing of elements
  is discarded completely (emergency situation), BUT with a printk marker element LIMIT EXCEEDED
- consumer side:
  - you've got one central handler which
    simply knows nothing other (SEPARATION OF CONCERNS!) than
    how to reliably dequeue and dump (to console) elements,
    in *any* processing context
- producer side (as a special shortcut handling) will *also* subsequently turn to some consumption stuff
  "dequeue elements until a limit is reached"
  (here: with its special direct-context limit count)
- consumption side is specially constructed
  to have properly atomic inner handling of
  decision-making who is the one to dequeue the store
  at a particular moment in time


Thus, you want:
- reliable insertion at-any-moment
- one consistent/global/shared store
- properly precise scheduler decision-making on how the consumer side is done
- and of course keep concerns (printk user-side handling, printk payload
  element store handling, *thread-specific* worker context handling) properly
implementation-separate

I currently don't see any other complications to this.

HTH,

Andreas Mohr