linux-kernel - Re: [PATCH 2/3] mm: calculate deferred pages after skipping mirrored memory

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Date:   Thu, 26 Jul 2018 11:39:04 -0400
From:   Pavel Tatashin <pasha.tatashin@...cle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Steven Sistare <steven.sistare@...cle.com>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        kirill.shutemov@...ux.intel.com, Michal Hocko <mhocko@...e.com>,
        Linux Memory Management List <linux-mm@...ck.org>,
        dan.j.williams@...el.com, jack@...e.cz, jglisse@...hat.com,
        Souptick Joarder <jrdr.linux@...il.com>, bhe@...hat.com,
        gregkh@...uxfoundation.org, Vlastimil Babka <vbabka@...e.cz>,
        Wei Yang <richard.weiyang@...il.com>, dave.hansen@...el.com,
        rientjes@...gle.com, mingo@...nel.org,
        osalvador@...hadventures.net, abdhalee@...ux.vnet.ibm.com,
        mpe@...erman.id.au
Subject: Re: [PATCH 2/3] mm: calculate deferred pages after skipping mirrored memory

On Wed, Jul 25, 2018 at 5:30 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Tue, 24 Jul 2018 21:46:25 -0400 Pavel Tatashin <pasha.tatashin@...cle.com> wrote:
>
> > > > +static inline bool defer_init(int nid, unsigned long pfn, unsigned long end_pfn)
> > > >  {
> > > > +     static unsigned long prev_end_pfn, nr_initialised;
> > >
> > > So answer me quick, what happens with a static variable in an inlined
> > > function?  Is there one copy kernel-wide?  One copy per invocation
> > > site?  One copy per compilation unit?
> > >
> > > Well I didn't know so I wrote a little test.  One copy per compilation
> > > unit (.o file), it appears.
> > >
> > > It's OK in this case because the function is in .c (and has only one
> > > call site).  But if someone moves it into a header and uses it from a
> > > different .c file, they have problems.
> > >
> > > So it's dangerous, and poor practice.  I'll make this non-static
> > > __meminit.
> >
> > I agree, it should not be moved to header it is dangerous.
> >
> > But, on the other hand this is a hot-path. memmap_init_zone() might
> > need to go through billions of struct pages early in boot, and I did
> > not want us to waste time on function calls. With defer_init() this is
> > not a problem, because if CONFIG_DEFERRED_STRUCT_PAGE_INIT is set
> > memmap_init_zone() won't have much work to do, but for
> > overlap_memmap_init() this is a problem, especially because I expect
> > compiler to optimize the pfn dereference usage in inline function.
>
> Well.  The compiler will just go and inline defer_init() anwyay - it
> has a single callsite and is in the same __meminint section as its
> calling function.  My gcc-7.2.0 does this.  Marking it noninline
> __meminit is basically syntactic fluff designed to encourage people to
> think twice.

Makes sense. I will do the change in the next version of the patches.

>
> > >
> > > --- a/mm/page_alloc.c~mm-calculate-deferred-pages-after-skipping-mirrored-memory-fix
> > > +++ a/mm/page_alloc.c
> > > @@ -309,7 +309,8 @@ static inline bool __meminit early_page_
> > >   * Returns true when the remaining initialisation should be deferred until
> > >   * later in the boot cycle when it can be parallelised.
> > >   */
> > > -static inline bool defer_init(int nid, unsigned long pfn, unsigned long end_pfn)
> > > +static bool __meminit
> > > +defer_init(int nid, unsigned long pfn, unsigned long end_pfn)
> > >  {
> > >         static unsigned long prev_end_pfn, nr_initialised;
> > >
> > >
> > > Also, what locking protects these statics?  Our knowledge that this
> > > code is single-threaded, presumably?
> >
> > Correct, this is called only from "context == MEMMAP_EARLY", way
> > before smp_init().
>
> Might be worth a little comment to put readers minds at ease.

Will add it.

Thank you,
Pavel