From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-it0-f71.google.com (mail-it0-f71.google.com [209.85.214.71]) by kanga.kvack.org (Postfix) with ESMTP id 1DC0E6B0005 for ; Tue, 21 Jun 2016 20:56:05 -0400 (EDT) Received: by mail-it0-f71.google.com with SMTP id 13so88158072itl.0 for ; Tue, 21 Jun 2016 17:56:05 -0700 (PDT) Received: from na01-by2-obe.outbound.protection.outlook.com (mail-by2on0080.outbound.protection.outlook.com. [207.46.100.80]) by mx.google.com with ESMTPS id q83si1177336oia.161.2016.06.21.17.56.04 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Tue, 21 Jun 2016 17:56:04 -0700 (PDT) Date: Wed, 22 Jun 2016 03:51:35 +0300 From: Yury Norov Subject: Re: [PATCH] mm: slab.h: use ilog2() in kmalloc_index() Message-ID: <20160622005135.GA342@yury-N73SV> References: <1466465586-22096-1-git-send-email-yury.norov@gmail.com> <20160621145237.dae264ea5fe6b3b7f2d2d4e6@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: <20160621145237.dae264ea5fe6b3b7f2d2d4e6@linux-foundation.org> Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton Cc: Yury Norov , masmart@yandex.ru, linux-mm@kvack.org, linux-kernel@vger.kernel.org, cl@linux.com, enberg@kernel.org, rientjes@google.com, iamjoonsoo.kim@lge.com, linux@rasmusvillemoes.dk, Alexey Klimov On Tue, Jun 21, 2016 at 02:52:37PM -0700, Andrew Morton wrote: > On Tue, 21 Jun 2016 02:33:06 +0300 Yury Norov wrote: > > > kmalloc_index() uses simple straightforward way to calculate > > bit position of nearest or equal upper power of 2. > > This effectively results in generation of 24 episodes of > > compare-branch instructions in assembler. > > > > There is shorter way to calculate this: fls(size - 1). > > > > The patch removes hard-coded calculation of kmalloc slab and > > uses ilog2() instead that works on top of fls(). ilog2 is used > > with intention that compiler also might optimize constant case > > during compile time if it detects that. > > > > BUG() is moved to the beginning of function. We left it here to > > provide identical behaviour to previous version. It may be removed > > if there's no requirement in it anymore. > > > > While we're at this, fix comment that describes return value. > > kmalloc_index() is always called with a constant-valued `size' (see > __builtin_constant_p() tests) It might change one day. This function is public to any slab user. If you really want to allow call kmalloc_index() for constants only, you'd place __builtin_constant_p() tests inside kmalloc_index(). > so the compiler will evaluate the switch > statement at compile-time. This will be more efficient than calling > fls() at runtime. There will be no fls() for constant at runtime because ilog2() calculates constant values at compile-time as well. From this point of view, this patch removes code duplication, as we already have compile-time log() calculation in kernel, and should re-use it whenever possible.\ Yury. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org