Re: [RFC PATCH 10/10] memcg: no more irq disabling for stock locks

[Shakeel Butt <shakeel.butt@linux.dev>]

On Fri, Mar 14, 2025 at 12:58:02PM +0100, Sebastian Andrzej Siewior wrote:
> On 2025-03-14 11:54:34 [+0100], Vlastimil Babka wrote:
> > On 3/14/25 07:15, Shakeel Butt wrote:
> > > Let's switch all memcg_stock locks acquire and release places to not
> > > disable and enable irqs. There are two still functions (i.e.
> > > mod_objcg_state() and drain_obj_stock) which needs to disable irqs to
> > > update the stats on non-RT kernels. For now add a simple wrapper for
> > > that functionality.
> > 
> > BTW, which part of __mod_objcg_mlstate() really needs disabled irqs and not
> > just preemption? I see it does rcu_read_lock() anyway, which disables
> > preemption. Then in __mod_memcg_lruvec_state() we do some __this_cpu_add()
> > updates. I think these also are fine with just disabled preemption as they
> > are atomic vs irqs (but don't need LOCK prefix to be atomic vs other cpus
> > updates).
> 
> __this_cpu_add() is not safe if also used in interrupt context. Some
> architectures (not x86) implemented as read, add, write.
> this_cpu_add()() does the same but disables interrupts during the
> operation.
> So __this_cpu_add() should not be used if interrupts are not disabled
> and a modification can happen from interrupt context.

So, if I use this_cpu_add() instead of __this_cpu_add() in
__mod_memcg_state(), __mod_memcg_lruvec_state(), __count_memcg_events()
then I can call these functions without disabling interrupts. Also
this_cpu_add() does not disable interrupts for x86 and arm64, correct?
For x86 and arm64, can I assume that the cost of this_cpu_add() is the
same as __this_cpu_add()?

> 
> > Is it just memcg_rstat_updated() which does READ_ONCE/WRITE_ONCE? Could we
> > perhaps just change it to operations where disabled preemption is enough?

Yes, I will look into it.

> > 
> > > Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
> …
> > > @@ -2757,6 +2745,28 @@ static void replace_stock_objcg(struct memcg_stock_pcp *stock,
> > >  	WRITE_ONCE(stock->cached_objcg, objcg);
> > >  }
> > >  
> > > +static unsigned long rt_lock(void)
> > > +{
> 
> No, we don't name it rt_lock(). We have local_lock() for this exact
> reason. And migrate_disable() does not protect vs re-enter of the
> function on the CPU while local_irq_save() does.

Thanks for clarification. Here do nothing for RT kernel and disable
interrupts for non-RT kernels. (Let'e see how the other conversation
goes, maybe we can remove the interrupt disabling requirement)