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

[Vlastimil Babka <vbabka@suse.cz>]

On 5/10/25 01:28, Shakeel Butt wrote:
> BPF programs can trigger memcg charged kernel allocations in nmi
> context. However memcg charging infra for kernel memory is not equipped
> to handle nmi context. This series adds support for kernel memory
> charging for nmi context.
> 
> 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?

> 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?

> 
> There will be a followup series which will make kernel memory charging
> reentrant for irq and will be able to do without disabling irqs.
> 
> We ran network intensive workload on this series and have not seen any
> significant performance differences with and without the series.
> 
> Shakeel Butt (4):
>   memcg: add infra for nmi safe memcg stats
>   memcg: add nmi-safe update for MEMCG_KMEM
>   memcg: nmi-safe slab stats updates
>   memcg: make objcg charging nmi safe
> 
>  include/linux/memcontrol.h |  6 +++
>  mm/memcontrol.c            | 87 +++++++++++++++++++++++++++++++++++---
>  2 files changed, 88 insertions(+), 5 deletions(-)
>