linux-kernel - RE: [PATCH 00/11] Introduce kernel_clone(), kill _do

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Date:   Wed, 19 Aug 2020 15:55:47 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Matthew Wilcox' <willy@...radead.org>
CC:     "'Eric W. Biederman'" <ebiederm@...ssion.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        Christoph Hewllig <hch@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        "Yoshinori Sato" <ysato@...rs.sourceforge.jp>,
        Tony Luck <tony.luck@...el.com>,
        "Fenghua Yu" <fenghua.yu@...el.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        "Ley Foon Tan" <ley.foon.tan@...el.com>,
        "David S. Miller" <davem@...emloft.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "x86@...nel.org" <x86@...nel.org>, "Arnd Bergmann" <arnd@...db.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        "Stafford Horne" <shorne@...il.com>,
        Kars de Jong <jongk@...ux-m68k.org>,
        Kees Cook <keescook@...omium.org>,
        Greentime Hu <green.hu@...il.com>,
        "Mauro Carvalho Chehab" <mchehab+huawei@...nel.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        "Tom Zanussi" <zanussi@...nel.org>,
        Xiao Yang <yangx.jy@...fujitsu.com>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "uclinux-h8-devel@...ts.sourceforge.jp" 
        <uclinux-h8-devel@...ts.sourceforge.jp>,
        "linux-ia64@...r.kernel.org" <linux-ia64@...r.kernel.org>,
        "linux-m68k@...ts.linux-m68k.org" <linux-m68k@...ts.linux-m68k.org>,
        "sparclinux@...r.kernel.org" <sparclinux@...r.kernel.org>,
        "kgdb-bugreport@...ts.sourceforge.net" 
        <kgdb-bugreport@...ts.sourceforge.net>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>
Subject: RE: [PATCH 00/11] Introduce kernel_clone(), kill _do_fork()

From: Matthew Wilcox
> Sent: 19 August 2020 16:45
> 
> On Wed, Aug 19, 2020 at 03:41:48PM +0000, David Laight wrote:
> > Does linux have an O(1) (or do I mean o(1)) pid allocator?
> > Or does it have to do a linear scan to find a gap??
> 
> O(log(n)).  It uses the IDR allocator, so 'n' in this case is the
> number of PIDs currently allocated, and it's log_64 rather than log_2
> (which makes no difference to O() but does make a bit of a difference
> to performance)

Still worse that O(1) - when that is just replacing a variable
with a value read out of an array.
Made pid lookup a trivial O(1) as well.

	David

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