linux-kernel - Re: [RFC v1 04/14] bus1: util

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Date:   Thu, 27 Oct 2016 14:56:31 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     David Herrmann <dh.herrmann@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Jiri Kosina <jikos@...nel.org>, Greg KH <greg@...ah.com>,
        Hannes Reinecke <hare@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Tom Gundersen <teg@...m.no>,
        Josh Triplett <josh@...htriplett.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC v1 04/14] bus1: util - fixed list utility library

On Thursday, October 27, 2016 2:48:46 PM CEST David Herrmann wrote:
> On Thu, Oct 27, 2016 at 2:37 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Wed, Oct 26, 2016 at 09:18:00PM +0200, David Herrmann wrote:
> >> +             e = kmalloc_array(sizeof(*e), BUS1_FLIST_BATCH + 1, gfp);
> >
> >> +#define BUS1_FLIST_BATCH (1024)
> >
> >> +struct bus1_flist {
> >> +     union {
> >> +             struct bus1_flist *next;
> >> +             void *ptr;
> >> +     };
> >> +};
> >
> > So that's an allocation of 8*(1024+1), or slightly more than 2 pages.
> >
> > kmalloc will round up to the next power of two, so you'll end up with an
> > allocation of 16*1024, wasting a whopping 8184 bytes per such allocation
> > in slack space.
> >
> > Please consider using 1023 or something for your batch size, 511 would
> > get you to exactly 1 page which would be even better.
> 
> Thanks for the hint! 511 looks like the obvious choice. Maybe even
> (PAGE_SIZE / sizeof(long) - 1). I will put a comment next to the
> definition.
> 
> 

PAGE_SIZE can be up to 64KB though, so that might lead wasting a lot
of memory.

	Arnd