Re: [PATCH v2 3/3] percpu: improve allocation success rate for non-GFP_KERNEL callers

[Michal Hocko]

On Mon 27-02-17 11:32:50, Tahsin Erdogan wrote:
> >> >
> >> > Yes, this prevents adding more pcpu chunks and so cause "atomic" allocations
> >> > to fail more easily.
> >>
> >> Then I fail to see what is the problem you are trying to fix.
> >
> > To be more specific. Could you describe what more can we do in the
> > vmalloc layer for GFP_NOWAIT allocations? They certainly cannot sleep
> > and cannot perform the reclaim so you have to rely on the background
> > work.
> 
> The main problem that I am trying to fix is in percpu.c code. It
> currently doesn't even attempt to call vmalloc() for GFP_NOWAIT
> case. It solely relies on the background allocator to replenish the
> reserves. I would like percpu.c to call __vmalloc(GFP_NOWAIT) inline
> and see whether that succeeds. If that fails, it is fair to fail the
> call.

OK, that wasn't really clean from the patch to me. I guess it would be
much more easier if a preparatory patch did the gfp mask propagation and
then have patch that changes the pcpu allocator the way you need.
 
> For this to work, __vmalloc() should be ready to serve a caller
> that is holding a spinlock. The might_sleep() in alloc_vmap_area()
> basically prevents us calling vmalloc in this context.

-- 
Michal Hocko
SUSE Labs