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

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Message-ID: <20200819154521.GE17456@casper.infradead.org>
Date:   Wed, 19 Aug 2020 16:45:21 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     David Laight <David.Laight@...lab.com>
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()

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)