From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751465AbbFKNli (ORCPT ); Thu, 11 Jun 2015 09:41:38 -0400 Received: from mail-wg0-f44.google.com ([74.125.82.44]:35058 "EHLO mail-wg0-f44.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750718AbbFKNla (ORCPT ); Thu, 11 Jun 2015 09:41:30 -0400 Date: Thu, 11 Jun 2015 15:41:24 +0200 From: Ingo Molnar To: Waiman Long Cc: Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , x86@kernel.org, linux-kernel@vger.kernel.org, Scott J Norton , Douglas Hatch , Linus Torvalds , Andrew Morton , Peter Zijlstra , Borislav Petkov Subject: Re: [PATCH] x86/mm/fault: Put pgd_lock in its own cacheline Message-ID: <20150611134124.GA23728@gmail.com> References: <1433941708-40686-1-git-send-email-Waiman.Long@hp.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1433941708-40686-1-git-send-email-Waiman.Long@hp.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Waiman Long wrote: > On a large system with many cores, massive creation or destruction > of processes/threads can sometime cause a fair amount of spinlock > contention in the pgd_lock used by the pgd_alloc() and pgd_free() > functions. This patch tries to reduce false cacheline sharing by > putting the pgd_lock in its own cacheline which help to reduce > contention on the lock. > > Signed-off-by: Waiman Long > --- > arch/x86/mm/fault.c | 2 +- > 1 files changed, 1 insertions(+), 1 deletions(-) > > diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c > index 181c53b..fae48df 100644 > --- a/arch/x86/mm/fault.c > +++ b/arch/x86/mm/fault.c > @@ -185,7 +185,7 @@ force_sig_info_fault(int si_signo, int si_code, unsigned long address, > force_sig_info(si_signo, &info, tsk); > } > > -DEFINE_SPINLOCK(pgd_lock); > +__cacheline_aligned_in_smp DEFINE_SPINLOCK(pgd_lock); > LIST_HEAD(pgd_list); Yeah, so if you are seeing pgd_lock contention we could do a lot more to improve the performance of pgd allocation than just putting it on a separate cacheline: we can actually eliminate pgd_list and turn all pgd_list users into RCU walkers of the tasklist lock, and make pgd_alloc()/pgd_free() lockless on x86. I've done a series that implements this, I'll post it soon, please stay tuned! Thanks, Ingo