Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag

[peterz]

On Thu, Aug 13, 2020 at 11:52:57AM -0700, Paul E. McKenney wrote:
> On Thu, Aug 13, 2020 at 08:26:18PM +0200, peterz@xxxxxxxxxxxxx wrote:

> > I thought the rule was:
> > 
> >  - No allocators (alloc/free) inside raw_spinlock_t, full-stop.
> > 
> > Why are we trying to craft an exception?
> 
> So that we can reduce post-grace-period cache misses by a factor of
> eight when invoking RCU callbacks.  This reduction in cache misses also
> makes it more difficult to overrun RCU with floods of either call_rcu()
> or kfree_rcu() invocations.
> 
> The idea is to allocate page-sized arrays of pointers so that the callback
> invocation can sequence through the array instead of walking a linked
> list, hence the reduction in cache misses.

I'm still not getting it, how do we end up trying to allocate memory
from under raw spinlocks if you're not allowed to use kfree_rcu() under
one ?

Can someone please spell out the actual problem?