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

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Message-ID: <20201002101952.GE3227@techsingularity.net>
Date:   Fri, 2 Oct 2020 11:19:52 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Michal Hocko <mhocko@...e.com>,
        Uladzislau Rezki <urezki@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        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>
Subject: Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func.

On Fri, Oct 02, 2020 at 11:58:58AM +0200, Peter Zijlstra wrote:
> > It's enabled by default by enough distros that adding too many checks
> > is potentially painful. Granted it would be missed by most benchmarking
> > which tend to control allocations from userspace but a lot of performance
> > problems I see are the "death by a thousand cuts" variety.
> 
> Oh quite agreed, aka death by accounting. But if people are enabling
> DEBUG options in production kernels, there's something wrong, no?
> 

You'd think but historically I believe DEBUG_VM was enabled for some
distributions because it made certain classes of problems easier to debug
early. There is also a recent trend for enabling various DEBUG options for
"hardening" even when they protect very specific corner cases or are for
intended for kernel development. I've pushed back where I have an opinion
that matters but it's generally corrosive.

> Should we now go add CONFIG_REALLY_DEBUG_STAY_AWAY_ALREADY options?

It's heading in that direction :(

-- 
Mel Gorman
SUSE Labs