Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755130AbbB0PBp (ORCPT ); Fri, 27 Feb 2015 10:01:45 -0500 Received: from mail.bmw-carit.de ([62.245.222.98]:59934 "EHLO mail.bmw-carit.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755104AbbB0PBl (ORCPT ); Fri, 27 Feb 2015 10:01:41 -0500 Message-ID: <54F086CA.5060606@bmw-carit.de> Date: Fri, 27 Feb 2015 16:01:30 +0100 From: Daniel Wagner User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.4.0 MIME-Version: 1.0 To: Jeff Layton CC: Andi Kleen , , , John Kacur , Alexander Viro , "J. Bruce Fields" Subject: Re: [RFC v1 0/5] fs/locks: Use plain percpu spinlocks instead of lglock to protect file_lock References: <1424443195-18676-1-git-send-email-daniel.wagner@bmw-carit.de> <87vbiwwotb.fsf@tassilo.jf.intel.com> <54EC9FA2.1030000@bmw-carit.de> <20150224160643.1d3366d5@tlielax.poochiereds.net> In-Reply-To: <20150224160643.1d3366d5@tlielax.poochiereds.net> Content-Type: text/plain; charset="windows-1252" Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2821 Lines: 68 Sorry for the late response. Got dragged away. On 02/24/2015 10:06 PM, Jeff Layton wrote: > On Tue, 24 Feb 2015 16:58:26 +0100 > Daniel Wagner wrote: > >> On 02/20/2015 05:05 PM, Andi Kleen wrote: >>> Daniel Wagner writes: >>>> >>>> I am looking at how to get rid of lglock. Reason being -rt is not too >>>> happy with that lock, especially that it uses arch_spinlock_t and >>> >>> AFAIK it could just use normal spinlock. Have you tried that? >> >> I have tried it. At least fs/locks.c didn't blow up. The benchmark >> results (lockperf) indicated that using normal spinlocks is even >> slightly faster. Simply converting felt like cheating. It might be >> necessary for the other user (kernel/stop_machine.c). Currently it looks >> like there is some additional benefit getting lglock away in fs/locks.c. >> > > What would that benefit be? > > lglocks are basically percpu spinlocks. Fixing some underlying > infrastructure that provides that seems like it might be a better > approach than declaring them "manually" and avoiding them altogether. > > Note that you can still do basically what you're proposing here with > lglocks as well. Avoid using lg_global_* and just lock each one in > turn. Yes, that was I was referring to as benefit. My main point is that there are only lg_local_* calls we could as well use normal spinlocks. No need to fancy. > That said, now that I've thought about this, I'm not sure that's really > something we want to do when accessing /proc/locks. If you lock each > one in turn, then you aren't freezing the state of the file_lock_list > percpu lists. Won't that mean that you aren't necessarily getting a > consistent view of the locks on those lists when you cat /proc/locks? Maybe I am overlooking something here but I don't see a consistency problem. We list a blocker and all its waiter in a go since only the blocker is added to flock_lock_list and the waiters are added blocker's fl_block list. > I think having a consistent view there might trump any benefit to > performance. Reading /proc/locks is a *very* rare activity in the big > scheme of things. I agree, but I hope that I got it right with my consistency argument than there shouldn't be a problem. > I do however like the idea of moving more to be protected by the > lglocks, and minimizing usage of the blocked_lock_lock. Good to hear. I am trying to write a new test (a variation of the dinning philosophers 'problem') case which benchmarks blocked_lock_lock after the re-factoring. cheers, daniel -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/