Re: [PATCH v4] printk: Add console owner and waiter logic to loadbalance console writes

[Tetsuo Handa]

Michal Hocko wrote:
> On Thu 09-11-17 19:22:58, Tetsuo Handa wrote:
> > Michal Hocko wrote:
> > > Hi,
> > > assuming that this passes warn stall torturing by Tetsuo, do you think
> > > we can drop http://lkml.kernel.org/r/1509017339-4802-1-git-send-email-penguin-kernel@xxxxxxxxxxxxxxxxxxx
> > > from the mmotm tree?
> > 
> > I don't think so.
> > 
> > The rule that "do not try to printk() faster than the kernel can write to
> > consoles" will remain no matter how printk() changes. Unless asynchronous
> > approach like https://lwn.net/Articles/723447/ is used, I think we can't
> > obtain useful information.
> 
> Does that mean that the patch doesn't pass your test?
> 

Test is irrelevant. See the changelog.

  Synchronous approach is prone to unexpected results (e.g. too late [1], too
  frequent [2], overlooked [3]). As far as I know, warn_alloc() never helped
  with providing information other than "something is going wrong".
  I want to consider asynchronous approach which can obtain information
  during stalls with possibly relevant threads (e.g. the owner of oom_lock
  and kswapd-like threads) and serve as a trigger for actions (e.g. turn
  on/off tracepoints, ask libvirt daemon to take a memory dump of stalling
  KVM guest for diagnostic purpose).

  [1] https://bugzilla.kernel.org/show_bug.cgi?id=192981
  [2] http://lkml.kernel.org/r/CAM_iQpWuPVGc2ky8M-9yukECtS+zKjiDasNymX7rMcBjBFyM_A@xxxxxxxxxxxxxx
  [3] commit db73ee0d46379922 ("mm, vmscan: do not loop on too_many_isolated for ever")