Re: [PATCH 2/3] mm, notifier: Catch sleeping/blocking for !blockable

[Michal Hocko <mhocko@kernel.org>]

On Fri 23-11-18 13:38:38, Daniel Vetter wrote:
> On Fri, Nov 23, 2018 at 12:12:37PM +0100, Michal Hocko wrote:
> > On Thu 22-11-18 17:51:05, Daniel Vetter wrote:
> > > We need to make sure implementations don't cheat and don't have a
> > > possible schedule/blocking point deeply burried where review can't
> > > catch it.
> > > 
> > > I'm not sure whether this is the best way to make sure all the
> > > might_sleep() callsites trigger, and it's a bit ugly in the code flow.
> > > But it gets the job done.
> > 
> > Yeah, it is quite ugly. Especially because it makes DEBUG config
> > bahavior much different. So is this really worth it? Has this already
> > discovered any existing bug?
> 
> Given that we need an oom trigger to hit this we're not hitting this in CI
> (oom is just way to unpredictable to even try). I'd kinda like to also add
> some debug interface so I can provoke an oom kill of a specially prepared
> process, to make sure we can reliably exercise this path without killing
> the kernel accidentally. We do similar tricks for our shrinker already.

Create a task with oom_score_adj = 1000 and trigger the oom killer via
sysrq and you should get a predictable oom invocation and execution.

[...]
> Wrt the behavior difference: I guess we could put another counter into the
> task struct, and change might_sleep() to check it. All under
> CONFIG_DEBUG_ATOMIC_SLEEP only ofc. That would avoid the preempt-disable
> sideeffect. My worry with that is that people will spot it, and abuse it
> in creative ways that do affect semantics. See horrors like
> drm_can_sleep() (and I'm sure gfx folks are not the only ones who
> seriously lacked taste here).
> 
> Up to the experts really how to best paint this shed I think.

Actually I like a way to say non_block_{begin,end} and might_sleep
firing inside that context.
-- 
Michal Hocko
SUSE Labs