Re: [PATCH RFC] rcu/tree: Use GFP_MEMALLOC for alloc memory to free memory pattern

[Paul E. McKenney]

On Wed, Apr 01, 2020 at 08:16:01PM +0200, Uladzislau Rezki wrote:
> > > > 
> > > > Right. Per discussion with Paul, we discussed that it is better if we
> > > > pre-allocate N number of array blocks per-CPU and use it for the cache.
> > > > Default for N being 1 and tunable with a boot parameter. I agree with this.
> > > > 
> > > As discussed before, we can make use of memory pool API for such
> > > purpose. But i am not sure if it should be one pool per CPU or
> > > one pool per NR_CPUS, that would contain NR_CPUS * N pre-allocated
> > > blocks.
> > 
> > There are advantages and disadvantages either way.  The advantage of the
> > per-CPU pool is that you don't have to worry about something like lock
> > contention causing even more pain during an OOM event.  One potential
> > problem wtih the per-CPU pool can happen when callbacks are offloaded,
> > in which case the CPUs needing the memory might never be getting it,
> > because in the offloaded case (RCU_NOCB_CPU=y) the CPU posting callbacks
> > might never be invoking them.
> > 
> > But from what I know now, systems built with CONFIG_RCU_NOCB_CPU=y
> > either don't have heavy callback loads (HPC systems) or are carefully
> > configured (real-time systems).  Plus large systems would probably end
> > up needing something pretty close to a slab allocator to keep from dying
> > from lock contention, and it is hard to justify that level of complexity
> > at this point.
> > 
> > Or is there some way to mark a specific slab allocator instance as being
> > able to keep some amount of memory no matter what the OOM conditions are?
> > If not, the current per-CPU pre-allocated cache is a better choice in the
> > near term.
> > 
> As for mempool API:
> 
> mempool_alloc() just tries to make regular allocation taking into
> account passed gfp_t bitmask. If it fails due to memory pressure,
> it uses reserved preallocated pool that consists of number of
> desirable elements(preallocated when a pool is created).
> 
> mempoll_free() returns an element to to pool, if it detects that
> current reserved elements are lower then minimum allowed elements,
> it will add an element to reserved pool, i.e. refill it. Otherwise
> just call kfree() or whatever we define as "element-freeing function."

Unless I am missing something, mempool_alloc() acquires a per-mempool
lock on each invocation under OOM conditions.  For our purposes, this
is essentially a global lock.  This will not be at all acceptable on a
large system.

							Thanx, Paul

> > If not, the current per-CPU pre-allocated cache is a better choice in the
> > near term.
> >
> OK. I see your point.
> 
> Thank you for your comments and view :)
> 
> --
> Vlad Rezki