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

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Date:   Tue, 5 May 2020 14:17:43 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        linux-kernel@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        rcu@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH RFC] rcu/tree: Refactor object allocation and try harder
 for array allocation

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.