Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
From:   Uladzislau Rezki <urezki@gmail.com>
Date:   Tue, 5 May 2020 20:33:17 +0200
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Uladzislau Rezki <urezki@gmail.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Joel Fernandes <joel@joelfernandes.org>,
        linux-kernel@vger.kernel.org,
        Josh Triplett <josh@joshtriplett.org>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        rcu@vger.kernel.org, Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH RFC] rcu/tree: Refactor object allocation and try harder
 for array allocation
Message-ID: <20200505183317.GA28175@pc636>
References: <20200416131745.GA90777@google.com>
 <20200416180100.GT17661@paulmck-ThinkPad-P72>
 <20200422145752.GB362484@cmpxchg.org>
 <20200422153503.GQ17661@paulmck-ThinkPad-P72>
 <20200423174831.GB389168@cmpxchg.org>
 <20200423180249.GT17661@paulmck-ThinkPad-P72>
 <20200423182750.GA32451@pc636>
 <20200423192115.GV17661@paulmck-ThinkPad-P72>
 <20200423195955.GA476@pc636>
 <20200505181743.GA109369@cmpxchg.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200505181743.GA109369@cmpxchg.org>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

> > 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.
>
Johannes, thank you for the clarification!

--
Vlad Rezki