On Wed 29-03-23 19:20:59, Shakeel Butt wrote: > On Wed, Mar 29, 2023 at 11:41:39AM -0700, Yosry Ahmed wrote: > > On Wed, Mar 29, 2023 at 4:22 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > > > On Tue 28-03-23 22:16:39, Yosry Ahmed wrote: > > > > rstat flushing is too expensive to perform in irq context. > > > > The previous patch removed the only context that may invoke an rstat > > > > flush from irq context, add a WARN_ON_ONCE() to detect future > > > > violations, or those that we are not aware of. > > > > > > > > Ideally, we wouldn't flush with irqs disabled either, but we have one > > > > context today that does so in mem_cgroup_usage(). Forbid callers from > > > > irq context for now, and hopefully we can also forbid callers with irqs > > > > disabled in the future when we can get rid of this callsite. > > > > > > I am sorry to be late to the discussion. I wanted to follow up on > > > Johannes reply in the previous version but you are too fast ;) > > > > > > I do agree that this looks rather arbitrary. You do not explain how the > > > warning actually helps. Is the intention to be really verbose to the > > > kernel log when somebody uses this interface from the IRQ context and > > > get bug reports? What about configurations with panic on warn? Do we > > > really want to crash their systems for something like that? > > > > Thanks for taking a look, Michal! > > > > The ultimate goal is not to flush in irq context or with irqs > > disabled, as in some cases it causes irqs to be disabled for a long > > time, as flushing is an expensive operation. The previous patch in the > > series should have removed the only context that flushes in irq > > context, and the purpose of the WARN_ON_ONCE() is to catch future uses > > or uses that we might have missed. > > > > There is still one code path that flushes with irqs disabled (also > > mem_cgroup_usage()), and we cannot remove this just yet; we need to > > deprecate usage threshold events for root to do that. So we cannot > > enforce not flushing with irqs disabled yet. > > > > So basically the patch is trying to enforce what we have now, not > > flushing in irq context, and hopefully at some point we will also be > > able to enforce not flushing with irqs disabled. > > > > If WARN_ON_ONCE() is the wrong tool for this, please let me know. > > > > If I understand Michal's concern, the question is should be start with > pr_warn_once() instead of WARN_ON_ONCE() and I think yes we should start > with pr_warn_once(). Yes, I do not really like the WARN_ON here. It is an overkill. pr_warn would much less intrusive but potentially incomplete because you won't know who that offender is. So if you really care about those then you would need to call dump_stack as well. So the real question is. Do we really care so deeply? After all somebody might be calling this from within a spin lock or irq disabled section resulting in a similar situation without noticing. -- Michal Hocko SUSE Labs
