Date:   Thu, 29 Jun 2017 08:26:20 +0200
From:   Andreas Mohr <andi@...as.de>
To:     Petr Mladek <pmladek@...e.com>
Cc:     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>,
        Andreas Mohr <andi@...as.de>,
        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 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.


For a very simple setup (which quite possibly cannot be done this easily!),
things could be:

printk_work_wakeup_one_worker()
{
  reliably_notify_only_one_available_computing_hardware_resource(); <-- kernel mechanism available, I think
}

void printk_work_dump_my_package(work_count_max)
{
  while (++work_count < work_count_max)
    printk_work_dump_element();
}


void printk_work_dump_handler(work_count_max)
{
  --> added mutex to have continuation check be done within *inner* atomic handling section (avoid race window).
  printk_work_dump_my_package(work_count_max);

  take_mutex();
  bool all_done = !have_payload_remain;
  bool need_continuation = !(all_done);
  if (need_continuation)
    printk_work_wakeup_one_worker();
  release_mutex();
}


worker thread frame function:
WORKER printk_work_worker()
{
  for (;;)
  {
    switch(select())
      case dump_requested:
        printk_work_dump_handler(printk_work_count_max_kthread);
      case termination_requested:
        return;
  }
}

/* tasked_cpus = all_cpus; */
tasked_cpus = all_cpus / 4 */
for(tasked_cpus)
{
  new_kthread(printk_work_worker());
}



printk_impl()
{
  printk_queue_element(...);
  printk_work_dump_handler(printk_work_count_max_immediate);
}

Very Q&D thoughts, but might be helpful...


(ermm, however launching #cpus workers most likely is useless,
since schedulable decision-making does *not* depend on #cpus -
if any computing resource is available, this *can* be executed, thus
quite likely only one worker is needed anyway - no, two, since
one worker needs to be able to wake up precisely *one* other
to have things precisely continue on *another* computing resource!)

Oh, and this doesn't implement
(and especially not reliably/atomically race-window-less)
the case of
having another activity trigger another printk queuing
and thus (potentially - but not necessarily!!) another package dump activity
while some worker activity already is ongoing.
I.e. we've got a race window in the (multi-)use of printk_work_dump_handler()
(which should be solvable, though).

Oh, and rather than doing some specific work_count limit comparisons,
it might actually be more elegant instead to
bundle worker work packages in *advance*,
to achieve simple/isolated/separate submission to
whichever worker one would prefer; inner handling would then simply do:

work_handler()
{
  while (!my_package_done)
    dump_package_element();
}

But splitting into separate work packages bears the risk of
illegal reordering of printk payload elements, thus
most likely one should *not* do this and instead keep doing
simple work_count checks on a *global*/*shared* printk queue payload
(insertion submission of further elements into this queue
must still be possible at any time
without (excessive) blocking, though, of course).

while (can_continue_dumping)
{
payload_mutex_begin();
element = grab_element();
payload_mutex_end();
dump_element(element);
}



payload_mutex_begin();
queue_element(element);
payload_mutex_end();

HTH,

Andreas Mohr

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