Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752503AbWAFTq0 (ORCPT ); Fri, 6 Jan 2006 14:46:26 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752504AbWAFTq0 (ORCPT ); Fri, 6 Jan 2006 14:46:26 -0500 Received: from ns1.siteground.net ([207.218.208.2]:30648 "EHLO serv01.siteground.net") by vger.kernel.org with ESMTP id S1752503AbWAFTqZ (ORCPT ); Fri, 6 Jan 2006 14:46:25 -0500 Date: Fri, 6 Jan 2006 11:46:23 -0800 From: Ravikiran G Thirumalai To: Christoph Lameter Cc: Oleg Nesterov , Shai Fultheim , Nippun Goel , linux-kernel@vger.kernel.org, Andrew Morton Subject: Re: [rfc][patch] Avoid taking global tasklist_lock for single threadedprocess at getrusage() Message-ID: <20060106194623.GA4078@localhost.localdomain> References: <43AD8AF6.387B357A@tv-sign.ru> <43B2874F.F41A9299@tv-sign.ru> <20051228183345.GA3755@localhost.localdomain> <20051228225752.GB3755@localhost.localdomain> <43B57515.967F53E3@tv-sign.ru> <20060104231600.GA3664@localhost.localdomain> <43BD70AD.21FC6862@tv-sign.ru> <20060106094627.GA4272@localhost.localdomain> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.4.2.1i X-AntiAbuse: This header was added to track abuse, please include it with any abuse report X-AntiAbuse: Primary Hostname - serv01.siteground.net X-AntiAbuse: Original Domain - vger.kernel.org X-AntiAbuse: Originator/Caller UID/GID - [0 0] / [47 12] X-AntiAbuse: Sender Address Domain - scalex86.org X-Source: X-Source-Args: X-Source-Dir: Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4418 Lines: 115 On Fri, Jan 06, 2006 at 09:23:30AM -0800, Christoph Lameter wrote: > On Fri, 6 Jan 2006, Ravikiran G Thirumalai wrote: > > > + need_lock = !(p == current && thread_group_empty(p)); > > Isnt > > need_lock = (p != current || !thread_group_empty(b)) > > clearer? All the same I felt, and the comments were bold and clear. Also, the above was c translation of what was said in the comment :)...I am OK either ways. So Here goes... Following patch avoids taking the global tasklist_lock when possible, if a process is single threaded during getrusage(). Any avoidance of tasklist_lock is good for NUMA boxes (and possibly for large SMPs). Thanks to Oleg Nesterov for review and suggestions. Signed-off-by: Nippun Goel Signed-off-by: Ravikiran Thirumalai Signed-off-by: Shai Fultheim Index: linux-2.6.15-delme/kernel/sys.c =================================================================== --- linux-2.6.15-delme.orig/kernel/sys.c 2006-01-05 15:40:38.000000000 -0800 +++ linux-2.6.15-delme/kernel/sys.c 2006-01-06 00:43:08.000000000 -0800 @@ -1664,9 +1664,6 @@ asmlinkage long sys_setrlimit(unsigned i * a lot simpler! (Which we're not doing right now because we're not * measuring them yet). * - * This expects to be called with tasklist_lock read-locked or better, - * and the siglock not locked. It may momentarily take the siglock. - * * When sampling multiple threads for RUSAGE_SELF, under SMP we might have * races with threads incrementing their own counters. But since word * reads are atomic, we either get new values or old values and we don't @@ -1674,6 +1671,25 @@ asmlinkage long sys_setrlimit(unsigned i * the c* fields from p->signal from races with exit.c updating those * fields when reaping, so a sample either gets all the additions of a * given child after it's reaped, or none so this sample is before reaping. + * + * tasklist_lock locking optimisation: + * If we are current and single threaded, we do not need to take the tasklist + * lock or the siglock. No one else can take our signal_struct away, + * no one else can reap the children to update signal->c* counters, and + * no one else can race with the signal-> fields. + * If we do not take the tasklist_lock, the signal-> fields could be read + * out of order while another thread was just exiting. So we place a + * read memory barrier when we avoid the lock. On the writer side, + * write memory barrier is implied in __exit_signal as __exit_signal releases + * the siglock spinlock after updating the signal-> fields. + * + * We don't really need the siglock when we access the non c* fields + * of the signal_struct (for RUSAGE_SELF) even in multithreaded + * case, since we take the tasklist lock for read and the non c* signal-> + * fields are updated only in __exit_signal, which is called with + * tasklist_lock taken for write, hence these two threads cannot execute + * concurrently. + * */ static void k_getrusage(struct task_struct *p, int who, struct rusage *r) @@ -1681,14 +1697,22 @@ static void k_getrusage(struct task_stru struct task_struct *t; unsigned long flags; cputime_t utime, stime; + int need_lock = 0; memset((char *) r, 0, sizeof *r); - - if (unlikely(!p->signal)) - return; - utime = stime = cputime_zero; + need_lock = (p != current || !thread_group_empty(p)); + if (need_lock) { + read_lock(&tasklist_lock); + if (unlikely(!p->signal)) { + read_unlock(&tasklist_lock); + return; + } + } else + /* See locking comments above */ + smp_rmb(); + switch (who) { case RUSAGE_BOTH: case RUSAGE_CHILDREN: @@ -1727,6 +1751,8 @@ static void k_getrusage(struct task_stru BUG(); } + if (need_lock) + read_unlock(&tasklist_lock); cputime_to_timeval(utime, &r->ru_utime); cputime_to_timeval(stime, &r->ru_stime); } @@ -1734,9 +1760,7 @@ static void k_getrusage(struct task_stru int getrusage(struct task_struct *p, int who, struct rusage __user *ru) { struct rusage r; - read_lock(&tasklist_lock); k_getrusage(p, who, &r); - read_unlock(&tasklist_lock); return copy_to_user(ru, &r, sizeof(r)) ? -EFAULT : 0; } - 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/