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

[Michal Hocko]

On Wed 04-08-21 10:33:41, Michal Hocko wrote:
> 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.

Does this look better?
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