Re: [PATCH] mm/memcontrol: Disable on PREEMPT_RT

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2021-12-15 17:56:03 [+0100], Michal Hocko wrote:
> On Wed 15-12-21 17:47:54, Sebastian Andrzej Siewior wrote:
> > On 2021-12-13 11:08:26 [+0100], Michal Hocko wrote:
> > > On Fri 10-12-21 16:22:01, Sebastian Andrzej Siewior wrote:
> > > [...]
> > > I am sorry but I didn't get to read and digest the rest of the message
> > > yet. Let me just point out this
> > > 
> > > > The problematic part here is mem_cgroup_tree_per_node::lock which can
> > > > not be acquired with disabled interrupts on PREEMPT_RT.  The "locking
> > > > scope" is not always clear to me.  Also, if it is _just_ the counter,
> > > > then we might solve this differently.
> > > 
> > > I do not think you should be losing sleep over soft limit reclaim. This
> > > is certainly not something to be used for RT workloads and rather than
> > > touching that code I think it makes some sense to simply disallow soft
> > > limit with RT enabled (i.e. do not allow to set any soft limit).
> > 
> > Okay. So instead of disabling it entirely you suggest I should take
> > another stab at it? Okay. Disabling softlimit, where should I start with
> > it? Should mem_cgroup_write() for RES_SOFT_LIMIT always return an error
> > or something else?
> 
> Yeah, I would just return an error for RT configuration. If we ever need
> to implement that behavior for RT then we can look at specific fixes.

Okay. What do I gain by doing this / how do I test this? Is running
tools/testing/selftests/cgroup/test_*mem* sufficient to test all corner
cases here?

> Thanks!
Thank you ;)

Sebastian