Re: [PATCH bpf 2/2] bpf/memalloc: Schedule highprio wq for non-atomic alloc when atomic fails

[YiFei Zhu <zhuyifei@google.com>]

On Thu, Jul 20, 2023 at 7:24 PM Hou Tao <houtao@huaweicloud.com> wrote:
> Hi,
>
> On 7/21/2023 4:44 AM, YiFei Zhu wrote:
> > Atomic refill can fail, such as when all percpu chunks are full,
> > and when that happens there's no guarantee when more space will be
> > available for atomic allocations.
> >
> > Instead of having the caller wait for memory to be available by
> > retrying until the related BPF API no longer gives -ENOMEM, we can
> > kick off a non-atomic GFP_KERNEL allocation with highprio workqueue.
> > This should make it much less likely for those APIs to return
> > -ENOMEM.
> >
> > Because alloc_bulk can now be called from the workqueue,
> > non-atomic calls now also calls local_irq_save/restore to reduce
> > the chance of races.
> >
> > Fixes: 7c8199e24fa0 ("bpf: Introduce any context BPF specific memory allocator.")
> > Signed-off-by: YiFei Zhu <zhuyifei@google.com>
> > ---
> >  kernel/bpf/memalloc.c | 47 ++++++++++++++++++++++++++++++-------------
> >  1 file changed, 33 insertions(+), 14 deletions(-)
> >
> > diff --git a/kernel/bpf/memalloc.c b/kernel/bpf/memalloc.c
> > index 016249672b43..2915639a5e16 100644
> > --- a/kernel/bpf/memalloc.c
> > +++ b/kernel/bpf/memalloc.c
> > @@ -84,14 +84,15 @@ struct bpf_mem_cache {
> >       struct llist_head free_llist;
> >       local_t active;
> >
> > -     /* Operations on the free_list from unit_alloc/unit_free/bpf_mem_refill
> > +     /* Operations on the free_list from unit_alloc/unit_free/bpf_mem_refill_*
> >        * are sequenced by per-cpu 'active' counter. But unit_free() cannot
> >        * fail. When 'active' is busy the unit_free() will add an object to
> >        * free_llist_extra.
> >        */
> >       struct llist_head free_llist_extra;
> >
> > -     struct irq_work refill_work;
> > +     struct irq_work refill_work_irq;
> > +     struct work_struct refill_work_wq;
> >       struct obj_cgroup *objcg;
> >       int unit_size;
> >       /* count of objects in free_llist */
> > @@ -153,7 +154,7 @@ static struct mem_cgroup *get_memcg(const struct bpf_mem_cache *c)
> >  #endif
> >  }
> >
> > -/* Mostly runs from irq_work except __init phase. */
> > +/* Mostly runs from irq_work except workqueue and __init phase. */
> >  static void alloc_bulk(struct bpf_mem_cache *c, int cnt, int node, bool atomic)
> >  {
> >       struct mem_cgroup *memcg = NULL, *old_memcg;
> > @@ -188,10 +189,18 @@ static void alloc_bulk(struct bpf_mem_cache *c, int cnt, int node, bool atomic)
> >                        * want here.
> >                        */
> >                       obj = __alloc(c, node, gfp);
> > -                     if (!obj)
> > +                     if (!obj) {
> > +                             /* We might have exhausted the percpu chunks, schedule
> > +                              * non-atomic allocation so hopefully caller can get
> > +                              * a free unit upon next invocation.
> > +                              */
> > +                             if (atomic)
> > +                                     queue_work_on(smp_processor_id(),
> > +                                                   system_highpri_wq, &c->refill_work_wq);
>
> I am not a MM expert. But according to the code in
> pcpu_balance_workfn(), it will try to do pcpu_create_chunk() when
> pcpu_atomic_alloc_failed is true, so the reason for introducing
> refill_work_wq is that pcpu_balance_workfn is too slow to fulfill the
> allocation request from bpf memory allocator ?

Oh I missed that part of the code. In one of my tests I had the
previous patch applied, and I had a lot of assertions around the code
(for debugging-by-kdumping), and I was able to get some crashes that
suggested I needed to add more, so I wrote this. However I wasn't able
to reproduce that again. Though, after giving it another thought, this
sequence of events could still happen:

  initial condition: free_cnt = 1, low_watermark = 1
  unit_alloc()
    sets free_cnt = 0
    free_cnt < low_watermark
      irq_work_raise()
  irq work: bpf_mem_refill()
    alloc_bulk()
      __alloc()
        __alloc_percpu_gfp()
          fails
          pcpu_schedule_balance_work()
          return NULL
  pcpu_balance_workfn()
    succeeds, next __alloc_percpu_gfp will succeed
  unit_alloc()
    free_cnt is still 0
    return NULL

The thing here is that, even if pcpu_balance_workfn is fast enough to
run before the next unit_alloc, unit_alloc will still return NULL. I'm
not sure if this is desired, but this should be a very rare condition
requiring 8k unit_size. I'm not exactly sure what happened in that
dump. And since I'm unable to reproduce this again, and if we are okay
with the rare case above, I'm happy to drop this patch until I have a
better idea of what happened (or it was just my bad assertions, which
could very well be what happened).

> >                               break;
> > +                     }
> >               }
> > -             if (IS_ENABLED(CONFIG_PREEMPT_RT))
> > +             if (IS_ENABLED(CONFIG_PREEMPT_RT) || !atomic)
> >                       /* In RT irq_work runs in per-cpu kthread, so disable
> >                        * interrupts to avoid preemption and interrupts and
> >                        * reduce the chance of bpf prog executing on this cpu
> > @@ -208,7 +217,7 @@ static void alloc_bulk(struct bpf_mem_cache *c, int cnt, int node, bool atomic)
> >               __llist_add(obj, &c->free_llist);
> >               c->free_cnt++;
> >               local_dec(&c->active);
> > -             if (IS_ENABLED(CONFIG_PREEMPT_RT))
> > +             if (IS_ENABLED(CONFIG_PREEMPT_RT) || !atomic)
> >                       local_irq_restore(flags);
> >       }
> >       set_active_memcg(old_memcg);
> > @@ -314,9 +323,9 @@ static void free_bulk(struct bpf_mem_cache *c)
> >       do_call_rcu(c);
> >  }
> >
> > -static void bpf_mem_refill(struct irq_work *work)
> > +static void bpf_mem_refill_irq(struct irq_work *work)
> >  {
> > -     struct bpf_mem_cache *c = container_of(work, struct bpf_mem_cache, refill_work);
> > +     struct bpf_mem_cache *c = container_of(work, struct bpf_mem_cache, refill_work_irq);
> >       int cnt;
> >
> >       /* Racy access to free_cnt. It doesn't need to be 100% accurate */
> > @@ -332,7 +341,14 @@ static void bpf_mem_refill(struct irq_work *work)
> >
> >  static void notrace irq_work_raise(struct bpf_mem_cache *c)
> >  {
> > -     irq_work_queue(&c->refill_work);
> > +     irq_work_queue(&c->refill_work_irq);
> > +}
> > +
> > +static void bpf_mem_refill_wq(struct work_struct *work)
> > +{
> > +     struct bpf_mem_cache *c = container_of(work, struct bpf_mem_cache, refill_work_wq);
> > +
> > +     alloc_bulk(c, c->batch, NUMA_NO_NODE, false);
>
> Considering that the kworker may be interrupted by irq work, so there
> will be concurrent __llist_del_first() operations on free_by_rcu, andI
> think it is not safe to call alloc_bulk directly here. Maybe we can just
> skip __llist_del_first() for !atomic context.

Ack.

> >  }
> >
> >  /* For typical bpf map case that uses bpf_mem_cache_alloc and single bucket
> > @@ -352,7 +368,8 @@ static void notrace irq_work_raise(struct bpf_mem_cache *c)
> >
> >  static void prefill_mem_cache(struct bpf_mem_cache *c, int cpu)
> >  {
> > -     init_irq_work(&c->refill_work, bpf_mem_refill);
> > +     init_irq_work(&c->refill_work_irq, bpf_mem_refill_irq);
> > +     INIT_WORK(&c->refill_work_wq, bpf_mem_refill_wq);
> >       if (c->unit_size <= 256) {
> >               c->low_watermark = 32;
> >               c->high_watermark = 96;
> > @@ -529,7 +546,7 @@ void bpf_mem_alloc_destroy(struct bpf_mem_alloc *ma)
> >               for_each_possible_cpu(cpu) {
> >                       c = per_cpu_ptr(ma->cache, cpu);
> >                       /*
> > -                      * refill_work may be unfinished for PREEMPT_RT kernel
> > +                      * refill_work_irq may be unfinished for PREEMPT_RT kernel
> >                        * in which irq work is invoked in a per-CPU RT thread.
> >                        * It is also possible for kernel with
> >                        * arch_irq_work_has_interrupt() being false and irq
> > @@ -537,7 +554,8 @@ void bpf_mem_alloc_destroy(struct bpf_mem_alloc *ma)
> >                        * the completion of irq work to ease the handling of
> >                        * concurrency.
> >                        */
> > -                     irq_work_sync(&c->refill_work);
> > +                     irq_work_sync(&c->refill_work_irq);
> > +                     cancel_work_sync(&c->refill_work_wq);
>
> cancel_work_sync() may be time-consuming. We may need to move it to
> free_mem_alloc_deferred() to prevent blocking the destroy of bpf memory
> allocator.

Ack.

> >                       drain_mem_cache(c);
> >                       rcu_in_progress += atomic_read(&c->call_rcu_in_progress);
> >               }
> > @@ -552,7 +570,8 @@ void bpf_mem_alloc_destroy(struct bpf_mem_alloc *ma)
> >                       cc = per_cpu_ptr(ma->caches, cpu);
> >                       for (i = 0; i < NUM_CACHES; i++) {
> >                               c = &cc->cache[i];
> > -                             irq_work_sync(&c->refill_work);
> > +                             irq_work_sync(&c->refill_work_irq);
> > +                             cancel_work_sync(&c->refill_work_wq);
> >                               drain_mem_cache(c);
> >                               rcu_in_progress += atomic_read(&c->call_rcu_in_progress);
> >                       }
> > @@ -580,7 +599,7 @@ static void notrace *unit_alloc(struct bpf_mem_cache *c)
> >        *
> >        * but prog_B could be a perf_event NMI prog.
> >        * Use per-cpu 'active' counter to order free_list access between
> > -      * unit_alloc/unit_free/bpf_mem_refill.
> > +      * unit_alloc/unit_free/bpf_mem_refill_*.
> >        */
> >       local_irq_save(flags);
> >       if (local_inc_return(&c->active) == 1) {
>