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

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Message-ID: <20200924111907.GE2628@hirez.programming.kicks-ass.net>
Date:   Thu, 24 Sep 2020 13:19:07 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     Mel Gorman <mgorman@...hsingularity.net>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        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 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.