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 05:42:34PM +0100, Sebastian Andrzej Siewior wrote:
> On 2025-03-14 08:55:51 [-0700], Shakeel Butt wrote:
> > 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()?
> 
> on arm64, __this_cpu_add will "load, add, store". preemptible.
> this_cpu_add() will "disable preemption, atomic-load, add, atomic-store or
> start over with atomic-load. if succeeded enable preemption and move an"

So, this_cpu_add() on arm64 is not protected against interrupts but is
protected against preemption. We have a following comment in
include/linux/percpu-defs.h. Is this not true anymore?

/*
 * Operations with implied preemption/interrupt protection.  These
 * operations can be used without worrying about preemption or interrupt.
 */
...
#define this_cpu_add(pcp, val)          __pcpu_size_call(this_cpu_add_, pcp, val)

> 
> so no, this is not the same. On x86 it is possible to increment a memory
> value directly with one opcode so you get preempted either before or
> after that operation.
> 
> Sebastian