MIME-Version: 1.0
In-Reply-To: <20161027123726.GD3175@twins.programming.kicks-ass.net>
References: <20161026191810.12275-1-dh.herrmann@gmail.com> <20161026191810.12275-5-dh.herrmann@gmail.com>
 <20161027123726.GD3175@twins.programming.kicks-ass.net>
From: David Herrmann <dh.herrmann@gmail.com>
Date: Thu, 27 Oct 2016 14:48:46 +0200
Message-ID: <CANq1E4R_0hh=ibGr22-CctQXvT1boApfgi8Z7VDq4C+B481gKw@mail.gmail.com>
Subject: Re: [RFC v1 04/14] bus1: util - fixed list utility library
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel <linux-kernel@vger.kernel.org>,
        Andy Lutomirski <luto@amacapital.net>, Jiri Kosina <jikos@kernel.org>,
        Greg KH <greg@kroah.com>, Hannes Reinecke <hare@suse.com>,
        Steven Rostedt <rostedt@goodmis.org>, Arnd Bergmann <arnd@arndb.de>,
        Tom Gundersen <teg@jklm.no>, Josh Triplett <josh@joshtriplett.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>
Content-Type: text/plain; charset=UTF-8
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 927
Lines: 30

Hi

On Thu, Oct 27, 2016 at 2:37 PM, Peter Zijlstra <peterz@infradead.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.

Thanks
David