linux-kernel - Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page

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Date:   Thu, 24 Sep 2020 08:38:34 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Uladzislau Rezki <urezki@...il.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Michal Hocko <mhocko@...e.com>,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Joel Fernandes <joel@...lfernandes.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>,
        Mel Gorman <mgorman@...e.de>
Subject: Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func.

On Thu, Sep 24, 2020 at 01:19:07PM +0200, Peter Zijlstra wrote:
> On Thu, Sep 24, 2020 at 10:16:14AM +0200, Uladzislau Rezki wrote:
> > The key point is "enough". We need pages to make a) fast progress b) support
> > single argument of kvfree_rcu(one_arg). Not vice versa. That "enough" depends
> > on scheduler latency and vague pre-allocated number of pages, it might
> > be not enough what would require to refill it more and more or we can overshoot
> > that would lead to memory overhead. So we have here timing issues and
> > not accurate model. IMHO.
> 
> I'm firmly opposed to the single argument kvfree_rcu() idea, that's
> requiring memory to free memory.

Not quite.

First, there is a fallback when memory allocation fails.  Second,
in heavy-use situations, there is only one allocation per about
500 kvfree_rcu() calls on 64-bit systems.  Third, there are other
long-standing situations that require allocating memory in order to
free memory.

So I agree that it is a good general rule of thumb to avoid allocating
on free paths, but there are exceptions.  This is one of them.

						Thanx, Paul