Re: PROBLEM: kernel crashes when running xfsdump since ~6.4

[Baoquan He <bhe@redhat.com>]

On 06/25/24 at 10:05pm, Uladzislau Rezki wrote:
> > > > > > /**
> > > > > >  * cpumask_next - get the next cpu in a cpumask
> > > > > >  * @n: the cpu prior to the place to search (i.e. return will be > @n)
> > > > > >  * @srcp: the cpumask pointer
> > > > > >  *
> > > > > >  * Return: >= nr_cpu_ids if no further cpus set.
> > > > > 
> > > > > Ah, I got what you mean. In the vbq case, it may not have chance to get
> > > > > a return number as nr_cpu_ids. Becuase the hashed index limits the
> > > > > range to [0, nr_cpu_ids-1], and cpu_possible(index) will guarantee it
> > > > > won't be the highest cpu number [nr_cpu_ids-1] since CPU[nr_cpu_ids-1] must
> > > > > be possible CPU.
> > > > > 
> > > > > Do I miss some corner cases?
> > > > > 
> > > > Right. We guarantee that a highest CPU is available by doing: % nr_cpu_ids.
> > > > So we do not need to use *next_wrap() variant. You do not miss anything :)
> > > > 
> > > > Hailong Liu has proposed more simpler version:
> > > > 
> > > > <snip>
> > > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > > > index 11fe5ea208aa..e1e63ffb9c57 100644
> > > > --- a/mm/vmalloc.c
> > > > +++ b/mm/vmalloc.c
> > > > @@ -1994,8 +1994,9 @@ static struct xarray *
> > > >  addr_to_vb_xa(unsigned long addr)
> > > >  {
> > > >         int index = (addr / VMAP_BLOCK_SIZE) % num_possible_cpus();
> > > > +       int cpu = cpumask_nth(index, cpu_possible_mask);
> > > > 
> > > > -       return &per_cpu(vmap_block_queue, index).vmap_blocks;
> > > > +       return &per_cpu(vmap_block_queue, cpu).vmap_blocks;
> > > > <snip>
> > > > 
> > > > which just takes a next CPU if an index is not set in the cpu_possible_mask.
> > > > 
> > > > The only thing that can be updated in the patch is to replace num_possible_cpu()
> > > > by the nr_cpu_ids.
> > > > 
> > > > Any thoughts? I think we need to fix it by a minor change so it is
> > > > easier to back-port on stable kernels.
> > > 
> > > Yeah, sounds good since the regresson commit is merged in v6.3.
> > > Please feel free to post this and the hash array patch separately for
> > > formal reviewing.
> > > 
> > Agreed! The patch about hash array i will post later.
> > 
> > > By the way, when I am replying this mail, I check the cpumask_nth()
> > > again. I doubt it may take more checking then cpu_possible(), given most
> > > of systems don't have gaps in cpu_possible_mask. I could be dizzy at
> > > this moment.
> > > 
> > > static inline unsigned int cpumask_nth(unsigned int cpu, const struct cpumask *srcp)
> > > {
> > >         return find_nth_bit(cpumask_bits(srcp), small_cpumask_bits, cpumask_check(cpu));
> > > }
> > > 
> > Yep, i do not think it is a big problem based on your noted fact.
> > 
> Checked. There is a difference:
> 
> 1. Default
> 
> <snip>
> ...
> +   15.95%     6.05%  [kernel]        [k] __vmap_pages_range_noflush
> +   15.91%     1.74%  [kernel]        [k] addr_to_vb_xa <---------------
> +   15.13%    12.05%  [kernel]        [k] vunmap_p4d_range
> +   14.17%    13.38%  [kernel]        [k] __find_nth_bit <--------------
> +   10.62%     0.00%  [kernel]        [k] ret_from_fork_asm
> +   10.62%     0.00%  [kernel]        [k] ret_from_fork
> +   10.62%     0.00%  [kernel]        [k] kthread
> ...
> <snip>
> 
> 2. Check if cpu_possible() and then fallback to cpumask_nth() if not
> 
> <snip>
> ...
> +    6.84%     0.29%  [kernel]          [k] alloc_vmap_area
> +    6.80%     6.70%  [kernel]          [k] native_queued_spin_lock_slowpath
> +    4.24%     0.09%  [kernel]          [k] free_vmap_block
> +    2.41%     2.38%  [kernel]          [k] addr_to_vb_xa <-----------
> +    1.94%     1.91%  [kernel]          [k] xas_start
> ...
> <snip>
> 
> It is _worth_ to check if an index is in possible mask:
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 45e1506d58c3..af20f78c2cbf 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -2542,7 +2542,10 @@ static DEFINE_PER_CPU(struct vmap_block_queue, vmap_block_queue);
>  static struct xarray *
>  addr_to_vb_xa(unsigned long addr)
>  {
> -       int index = (addr / VMAP_BLOCK_SIZE) % num_possible_cpus();
> +       int index = (addr / VMAP_BLOCK_SIZE) % nr_cpu_ids;
> +
> +       if (!cpu_possible(index))
> +               index = cpumask_nth(index, cpu_possible_mask);
> 
>         return &per_cpu(vmap_block_queue, index).vmap_blocks;
>  }
> 
> cpumask_nth() is not cheap. My measurements are based on a synthetic
> tight test and it detects a difference. In a real workloads it should
> not be visible. Having gaps is not a common case plus a "slow path"
> will be mitigated by the hit against possible mask.

Ah, this is consistent with my understanding from the code, thanks
for confirming by testing.