Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753751Ab0GZI4R (ORCPT ); Mon, 26 Jul 2010 04:56:17 -0400 Received: from mx1.redhat.com ([209.132.183.28]:40683 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752338Ab0GZI4Q (ORCPT ); Mon, 26 Jul 2010 04:56:16 -0400 Date: Mon, 26 Jul 2010 10:53:24 +0200 From: Oleg Nesterov To: "Zhang, Yanmin" Cc: Roland McGrath , Andrew Morton , LKML , andi.kleen@intel.com, stable@kernel.org Subject: Re: [PATCH] Don't apply for write lock on tasklist_lock if parent doesn't ptrace other processes Message-ID: <20100726085324.GA32223@redhat.com> References: <1279176663.2096.1264.camel@ymzhang.sh.intel.com> <20100721144944.5351c741.akpm@linux-foundation.org> <20100721222529.EFBAA400B6@magilla.sf.frob.com> <20100722090524.GA6647@redhat.com> <1279874705.2096.1274.camel@ymzhang.sh.intel.com> <20100723173453.GA29831@redhat.com> <1280120732.2085.3.camel@ymzhang.sh.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1280120732.2085.3.camel@ymzhang.sh.intel.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1817 Lines: 65 On 07/26, Zhang, Yanmin wrote: > > On Fri, 2010-07-23 at 19:34 +0200, Oleg Nesterov wrote: > > On 07/23, Zhang, Yanmin wrote: > > > > > > After applying my patch (although it's incorrect as there is a race with TRACEME), > > > perf shows write_lock_irq in forget_original_parent consumes less than 40% cpu time on > > > 8-socket machine. > > > > Any chance you can test the patch I sent? It should have the same effect, > > otherwise there is something interesting. > 1) with my patch, we got about 13% improvement; > 2) With your patch, we got about 11% improvement; > > Performance is very sensitive to spinlock contention on large machines. Zhang, thank you very much. But. In this case I do not trust these results or I missed something. I mean, they do not look 100% accurate. With your patch: forget_original_parent: exit_ptrace: if (list_empty(ptraced)) return; write_lock_irq(tasklist); ... do a lot more work ... With my patch: forget_original_parent: write_lock_irq(tasklist); exit_ptrace: if (list_empty(ptraced)) return; ... do a lot more work ... The only difference is that we are doing the function call + list_empty() under tasklist, just a few instructions compared to "do a lot more work" in forget_original_parent(). How this can make the 2% difference ? This looks like a noise to me, or do you think I missed something? > > Heh. We must optimize it. But it is not clear when ;) > Thanks. It's better to remove the big lock. Yes. The only problem this is very much nontrival with the current code. Oleg. -- 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/