linux-kernel - Re: [PATCH] mm: compaction: some tracepoints should be defined only when CONFIG

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Date:   Tue, 12 Mar 2019 17:44:22 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Yafang Shao <laoar.shao@...il.com>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        Souptick Joarder <jrdr.linux@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, shaoyafang@...iglobal.com
Subject: Re: [PATCH] mm: compaction: some tracepoints should be defined only
 when CONFIG_COMPACTION is set

On Wed 13-03-19 00:29:57, Yafang Shao wrote:
> On Wed, Mar 13, 2019 at 12:18 AM Michal Hocko <mhocko@...nel.org> wrote:
> >
> > On Sat 02-03-19 12:38:58, Yafang Shao wrote:
> > > Only mm_compaction_isolate_{free, migrate}pages may be used when
> > > CONFIG_COMPACTION is not set.
> > > All others are used only when CONFIG_COMPACTION is set.
> >
> > Why is this an improvement?
> >
> 
> After this change, if CONFIG_COMPACTION is not set, the tracepoints
> that only work when CONFIG_COMPACTION is set will not be exposed to
> the usespace.
> Without this change, they will always be expose in debugfs no matter
> CONFIG_COMPACTION is set or not.

And this is exactly something that the changelog should mention. I
wasn't aware that we do export tracepoints even when they are not used
by any code path. This whole macro based programming is just a black
magic.
-- 
Michal Hocko
SUSE Labs