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:   Mon, 25 Jan 2021 15:31:50 +0100
To:     Michal Hocko <mhocko@suse.com>
Cc:     "Uladzislau Rezki (Sony)" <urezki@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>, RCU <rcu@vger.kernel.org>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Andrew Morton <akpm@linux-foundation.org>,
        Daniel Axtens <dja@axtens.net>,
        Frederic Weisbecker <frederic@kernel.org>,
        Neeraj Upadhyay <neeraju@codeaurora.org>,
        Joel Fernandes <joel@joelfernandes.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Theodore Y . Ts'o" <tytso@mit.edu>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>
Subject: Re: [PATCH 1/3] kvfree_rcu: Allocate a page for a single argument
Message-ID: <20210125143150.GA2282@pc638.lan>
References: <20210120162148.1973-1-urezki@gmail.com>
 <20210125132236.GJ827@dhcp22.suse.cz>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20210125132236.GJ827@dhcp22.suse.cz>
User-Agent: Mutt/1.10.1 (2018-07-13)
Precedence: bulk

> On Wed 20-01-21 17:21:46, Uladzislau Rezki (Sony) wrote:
> > For a single argument we can directly request a page from a caller
> > context when a "carry page block" is run out of free spots. Instead
> > of hitting a slow path we can request an extra page by demand and
> > proceed with a fast path.
> > 
> > A single-argument kvfree_rcu() must be invoked in sleepable contexts,
> > and that its fallback is the relatively high latency synchronize_rcu().
> > Single-argument kvfree_rcu() therefore uses GFP_KERNEL|__GFP_RETRY_MAYFAIL
> > to allow limited sleeping within the memory allocator.
> 
> __GFP_RETRY_MAYFAIL can be quite heavy. It is effectively the most heavy
> way to allocate without triggering the OOM killer. Is this really what
> you need/want? Is __GFP_NORETRY too weak?
> 
Hm... We agreed to proceed with limited lightwait memory direct reclaim.
Johannes Weiner proposed to go with __GFP_NORETRY flag as a starting
point: https://www.spinics.net/lists/rcu/msg02856.html

<snip>
    So I'm inclined to suggest __GFP_NORETRY as a starting point, and make
    further decisions based on instrumentation of the success rates of
    these opportunistic allocations.
<snip>

but for some reason, i can't find a tail or head of it, we introduced
__GFP_RETRY_MAYFAIL what is a heavy one from a time consuming point of view.
What we would like to avoid.

I tend to say that it was a typo.

Thank you for pointing to it!

--
Vlad Rezki