Re: [PATCH 0/4] memcg: nmi-safe kmem charging

[Peter Zijlstra <peterz@infradead.org>]

On Tue, May 13, 2025 at 09:15:23AM +0200, Vlastimil Babka wrote:

> >> > The initial prototype tried to make memcg charging infra for kernel
> >> > memory re-entrant against irq and nmi. However upon realizing that
> >> > this_cpu_* operations are not safe on all architectures (Tejun), this
> >> 
> >> I assume it was an off-list discussion?
> >> Could we avoid this for the architectures where these are safe, which should
> >> be the major ones I hope?

IIRC Power64 has issues here, 'funnily' their local_t is NMI safe.
Perhaps we could do the same for their this_cpu_*(), but ideally someone
with actual power hardware should do this ;-)

> > Yes it was an off-list discussion. The discussion was more about the
> > this_cpu_* ops vs atomic_* ops as on x86 this_cpu_* does not have lock
> > prefix and how I should prefer this_cpu_* over atomic_* for my series on
> > objcg charging without disabling irqs. Tejun pointed out this_cpu_* are
> > not nmi safe for some archs and it would be better to handle nmi context
> > separately. So, I am not that worried about optimizing for NMI context
> 
> Well, we're introducing in_nmi() check and different execution paths to all
> charging. This could be e.g. compiled out for architectures where this_cpu*
> is NMI safe or they don't have NMIs in the first place.

Very few architectures one would care about do not have NMIs. Risc-V
seems to be the exception here ?!?

> > but your next comment on generic_atomic64_* ops is giving me headache.
> > 
> >> 
> >> > series took a different approach targeting only nmi context. Since the
> >> > number of stats that are updated in kernel memory charging path are 3,
> >> > this series added special handling of those stats in nmi context rather
> >> > than making all >100 memcg stats nmi safe.
> >> 
> >> Hmm so from patches 2 and 3 I see this relies on atomic64_add().
> >> But AFAIU lib/atomic64.c has the generic fallback implementation for
> >> architectures that don't know better, and that would be using the "void
> >> generic_atomic64_##op" macro, which AFAICS is doing:
> >> 
> >>         local_irq_save(flags);                                          \
> >>         arch_spin_lock(lock);                                           \
> >>         v->counter c_op a;                                              \
> >>         arch_spin_unlock(lock);                                         \
> >>         local_irq_restore(flags);                                       \
> >> 
> >> so in case of a nmi hitting after the spin_lock this can still deadlock?
> >> 
> >> Hm or is there some assumption that we only use these paths when already
> >> in_nmi() and then another nmi can't come in that context?
> >> 
> >> But even then, flush_nmi_stats() in patch 1 isn't done in_nmi() and uses
> >> atomic64_xchg() which in generic_atomic64_xchg() implementation also has the
> >> irq_save+spin_lock. So can't we deadlock there?
> > 
> > I was actually assuming that atomic_* ops are safe against nmis for all
> > archs.

We have HAVE_NMI_SAFE_CMPXCHG for this -- there are architectures where
this is not the case -- but again, those are typically oddball archs you
don't much care about.

But yes, *64 on 32bit archs is generally not NMI safe.

> I looked at atomic_* ops in include/asm-generic/atomic.h and it
> > is using arch_cmpxchg() for CONFIG_SMP and it seems like for archs with
> > cmpxchg should be fine against nmi. I am not sure why atomic64_* are not
> > using arch_cmpxchg() instead. I will dig more.

Not many 32bit architectures have 64bit cmpxchg. We're only now dropping
support for x86 chips without CMPXCHG8b.

> Yeah I've found https://docs.kernel.org/core-api/local_ops.html and since it
> listed Mathieu we discussed on IRC and he mentioned the same thing that
> atomic_ ops are fine, but the later added 64bit variant isn't, which PeterZ
> (who added it) acknowledged.
> 
> But there could be way out if we could somehow compile-time assert that
> either is true:
> - CONFIG_HAVE_NMI=n - we can compile out all the nmi code

Note that in_nmi() is not depending on HAVE_NMI -- nor can it be. Many
architectures treat various traps as NMI-like, even though they might
not have real NMIs.

> - this_cpu is safe on that arch - we can also compile out the nmi code

There is no config symbol for this presently.

> - (if the above leaves any 64bit arch) its 64bit atomics implementation is safe

True, only because HPPA does not in fact have NMIs.

> - (if there are any 32bit applicable arch left) 32bit atomics should be
> enough for the nmi counters even with >4GB memory as we flush them? and we
> know the 32bit ops are safe

Older ARM might qualify here.