Re: [PATCH] mm/memcg: Disable task obj_stock for PREEMPT_RT

From: Michal Hocko
Date: Wed Aug 04 2021 - 04:33:45 EST

On Wed 04-08-21 09:39:23, Vlastimil Babka wrote:
> On 8/4/21 1:21 AM, Thomas Gleixner wrote:
> > /*
> > * The only protection from memory hotplug vs. drain_stock races is
> > * that we always operate on local CPU stock here with IRQ disabled
> > */
> > - local_irq_save(flags);
> > + local_lock_irqsave(memcg_stock_lock, flags);
> > ...
> > if (use_task_obj_stock())
> > drain_obj_stock(&stock->task_obj);
> >
> > which is incomprehensible garbage.
> >
> > The comment above the existing local_irq_save() is garbage w/o any local
> > lock conversion already today (and even before the commit which
> > introduced stock::task_obj) simply because that comment does not explain
> > the why.
> Michal, this seems to be your comment from commit 72f0184c8a00 ("mm, memcg:
> remove hotplug locking from try_charge"). Was "memory hotplug" a mistake,
> because the rest of the commit is about cpu hotplug, and I don't really see a
> memory hotplug connection there?

This part of the changelog tried to explain that part IIRC
We can get rid of {get,put}_online_cpus, fortunately. We do not have to
be worried about races with memory hotplug because drain_local_stock,
which is called from both the WQ draining and the memory hotplug
contexts, is always operating on the local cpu stock with IRQs disabled.

Now I have to admit I do not remember all the details and from a quick
look the memory hotplug doesn't seem to be draining memcg pcp stock.
Maybe this has been removed since then. The only stock draining outside
of the memcg code seems to be memcg_hotplug_cpu_dead callback. That
would indicate that I really meant the cpu hotplug here indeed.

Michal Hocko