Re: [PATCH RFC] rcu/tree: Refactor object allocation and try harder for array allocation

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Apr 23, 2020 at 09:59:55PM +0200, Uladzislau Rezki wrote:
> b) Double argument(with rcu_head)
> This case we consider as it gets called from atomic context even though
> it can be not. Why we consider such case as atomic: we just assume that.
> The reason is to keep it simple, because it is not possible to detect whether
> a current context is attomic or not(for all type of kernels), i mean the one
> that calls kfree_rcu().
> 
> In this case we do not have synchronize_rcu() option. Instead we have an
> object with rcu_head inside. If an allocation gets failed we just make
> use of rcu_head inside the object, regular queuing.
> 
> In this case we do not need to hard in order to obtain memory. Therefore
> my question was to Johannes what is best way here. Since we decided to
> minimize reclaiming, whereas GFP_NOWAIT wakes up kswapd if no memory.
> GFP_ATOMIC also is not good, because for (b) we do not need to waste
> it.

Waking kswapd is fine, because it's a shared facility that doesn't
just reclaim on your behalf but on behalf of a central goal: to get
the freelist back to the watermarks. If they're low, somebody will
sooner or later kick kswapd anyway to do exactly that.

So unless you ask kswapd for a high order page that is unlikely to be
needed by anybody else, you're only doing the inevitable.