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

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Date:   Fri, 25 Sep 2020 17:17:12 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     Michal Hocko <mhocko@...e.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.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 Fri, Sep 25, 2020 at 05:31:29PM +0200, Uladzislau Rezki wrote:
> > > > > 
> > > > > All good points!
> > > > > 
> > > > > On the other hand, duplicating a portion of the allocator functionality
> > > > > within RCU increases the amount of reserved memory, and needlessly most
> > > > > of the time.
> > > > > 
> > > > 
> > > > But it's very similar to what mempools are for.
> > > > 
> > > As for dynamic caching or mempools. It requires extra logic on top of RCU
> > > to move things forward and it might be not efficient way. As a side
> > > effect, maintaining of the bulk arrays in the separate worker thread
> > > will introduce other drawbacks:
> > 
> > This is true but it is also true that it is RCU to require this special
> > logic and we can expect that we might need to fine tune this logic
> > depending on the RCU usage. We definitely do not want to tune the
> > generic page allocator for a very specific usecase, do we?
> > 
> I look at it in scope of GFP_ATOMIC/GFP_NOWAIT issues, i.e. inability
> to provide a memory service for contexts which are not allowed to
> sleep, RCU is part of them. Both flags used to provide such ability
> before but not anymore.
> 
> Do you agree with it?
> 

I was led to believe that hte problem was taking the zone lock while
holding a raw spinlock that was specific to RCU. Are you saying that
GFP_ATOMIC/GFP_NOWAIT users are also holding raw spinlocks at the same
time on RT?

-- 
Mel Gorman
SUSE Labs