From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751358AbeEEOKZ (ORCPT ); Sat, 5 May 2018 10:10:25 -0400 Received: from mail-pg0-f41.google.com ([74.125.83.41]:37173 "EHLO mail-pg0-f41.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750967AbeEEOKY (ORCPT ); Sat, 5 May 2018 10:10:24 -0400 X-Google-Smtp-Source: AB8JxZrOuT/QmOc/nZntSRBGDsroMhp1R6cuM/d07txTLW7emvcswZnxjeUEoGuXSQIiS0v111uoig== Subject: Re: [PATCH] percpu_ida: Use _irqsave() instead of local_irq_save() + spin_lock To: Matthew Wilcox , Andrew Morton Cc: Sebastian Andrzej Siewior , linux-kernel@vger.kernel.org, tglx@linutronix.de, Nicholas Bellinger , Shaohua Li , Kent Overstreet References: <20180504153218.7301-1-bigeasy@linutronix.de> <20180504162216.ae91654b68eddafe38df7d7f@linux-foundation.org> <20180505035154.GB20495@bombadil.infradead.org> From: Jens Axboe Message-ID: <60a88d5f-95eb-ba45-e59c-5a822a3d370b@kernel.dk> Date: Sat, 5 May 2018 08:10:20 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.0 MIME-Version: 1.0 In-Reply-To: <20180505035154.GB20495@bombadil.infradead.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 5/4/18 9:51 PM, Matthew Wilcox wrote: > On Fri, May 04, 2018 at 04:22:16PM -0700, Andrew Morton wrote: >> I'm feeling a bit hostile toward lib/percpu_ida.c in general ;) It has >> very few users and seems rather complicated (what's with that >> schedule() in percpu_ida_alloc?). I'm suspecting and hoping that if >> someone can figure out what the requirements were, this could all be >> zapped and reimplemented using something else which we already have. > > Note that I have no code in percpu_ida ... it's quite different from > the regular IDA. But I have noticed the stunning similarity between the > percpu_ida and the code in lib/sbitmap.c. I have no idea which one is > better, but they're essentially doing the same thing. Not sure where you see that "stunning similarity"? The sbitmap code is basically the blk-mq tagging sparse bitmaps, abstracted into a generally usable form. The percpu_ida design works fine for lower utilization, but it fell apart for the tagging use case where we can easily run at full utilization. percpu_ida has percpu caches, sbitmap gets away with just percpu hints. These caches are why it doesn't work well for > 50% utilization. sbitmap also supports shallow operations, and online resizing. Outside of the sharing the same basic functionality of "give me some free ID", I really don't see a lot of similarities. In terms of functionality, yes, I don't think it would be hard to get rid of percpu_ida and just replace it with sbitmap. Probably a worthwhile pursuit. -- Jens Axboe