linux-kernel - Re: [PATCH 1/3] kvfree_rcu: Allocate a page for a single argument

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Message-ID: <20210125143150.GA2282@pc638.lan>
Date:   Mon, 25 Jan 2021 15:31:50 +0100
From:   Uladzislau Rezki <urezki@...il.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     "Uladzislau Rezki (Sony)" <urezki@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Daniel Axtens <dja@...ens.net>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [PATCH 1/3] kvfree_rcu: Allocate a page for a single argument

> 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