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

[Uladzislau Rezki <urezki@gmail.com>]

On Wed, Apr 01, 2020 at 11:54:39AM -0700, Paul E. McKenney wrote:
> On Wed, Apr 01, 2020 at 08:37:45PM +0200, Uladzislau Rezki wrote:
> > On Wed, Apr 01, 2020 at 11:26:15AM -0700, Paul E. McKenney wrote:
> > > 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.
> > > 
> > It uses pool->lock to access to reserved objects, so if we have one memory
> > pool per one CPU then it would be serialized.
> 
> I am having difficulty parsing your sentence.  It looks like your thought
> is to invoke mempool_create() for each CPU, so that the locking would be
> on a per-CPU basis, as in 128 invocations of mempool_init() on a system
> having 128 hardware threads.  Is that your intent?
> 
In order to serialize it, you need to have it per CPU. So if you have 128
cpus, it means:

<snip>
for_each_possible_cpu(...)
    cpu_pool = mempool_create();
<snip>

but please keep in mind that it is not my intention, but i had a though
about mempool API. Because it has pre-reserve logic inside.

--
Vlad Rezki