From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail203.messagelabs.com (mail203.messagelabs.com [216.82.254.243]) by kanga.kvack.org (Postfix) with ESMTP id CBF786B0092 for ; Mon, 12 Oct 2009 21:51:51 -0400 (EDT) Date: Mon, 12 Oct 2009 18:51:39 -0700 From: Andrew Morton Subject: Re: [resend][PATCH v2] mlock() doesn't wait to finish lru_add_drain_all() Message-Id: <20091012185139.75c13648.akpm@linux-foundation.org> In-Reply-To: <20091013090347.C752.A69D9226@jp.fujitsu.com> References: <20091009111709.1291.A69D9226@jp.fujitsu.com> <20091012165747.97f5bd87.akpm@linux-foundation.org> <20091013090347.C752.A69D9226@jp.fujitsu.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org To: KOSAKI Motohiro Cc: Peter Zijlstra , Mike Galbraith , Oleg Nesterov , LKML , linux-mm List-ID: On Tue, 13 Oct 2009 10:17:48 +0900 (JST) KOSAKI Motohiro wrote: > Hi > > > On Fri, 9 Oct 2009 11:21:55 +0900 (JST) > > KOSAKI Motohiro wrote: > > > > > Recently, Mike Galbraith reported mlock() makes hang-up very long time in > > > his system. Peter Zijlstra explainted the reason. > > > > > > Suppose you have 2 cpus, cpu1 is busy doing a SCHED_FIFO-99 while(1), > > > cpu0 does mlock()->lru_add_drain_all(), which does > > > schedule_on_each_cpu(), which then waits for all cpus to complete the > > > work. Except that cpu1, which is busy with the RT task, will never run > > > keventd until the RT load goes away. > > > > > > This is not so much an actual deadlock as a serious starvation case. > > > > > > His system has two partions using cpusets and RT-task partion cpu doesn't > > > have any PCP cache. thus, this result was pretty unexpected. > > > > > > The fact is, mlock() doesn't need to wait to finish lru_add_drain_all(). > > > if mlock() can't turn on PG_mlock, vmscan turn it on later. > > > > > > Thus, this patch replace it with lru_add_drain_all_async(). > > > > So why don't we just remove the lru_add_drain_all() call from sys_mlock()? > > There are small reason. the administrators and the testers (include me) > look at Mlock field in /proc/meminfo. > They natually expect Mlock field match with actual number of mlocked pages > if the system don't have any stress. Otherwise, we can't make mlock test case ;) > > > > How did you work out why the lru_add_drain_all() is present in > > sys_mlock() anyway? Neither the code nor the original changelog tell > > us. Who do I thwap for that? Nick and his reviewers. Sigh. > > [Umm, My dictionaly don't tell me the meaning of "thwap". An meaning of > an imitative word strongly depend on culture. Thus, I probably > misunderstand this paragraph.] "slap"? > I've understand the existing reason by looooooong time review. > > > > There are many callers of lru_add_drain_all() all over the place. Each > > of those is vulnerable to the same starvation issue, is it not? > > There are. > > > If so, it would be better to just fix up lru_add_drain_all(). Afaict > > all of its functions can be performed in hard IRQ context, so we can > > use smp_call_function()? > > There is a option. but it have one downside, it require lru_add_pvecs > related function call irq_disable(). I don't know what this means. ____pagevec_lru_add() (for example) can be trivially changed from spin_lock_irq() to spin_lock_irqsave(). In other cases we can perhaps split an existing foo() { spin_lock_irq(zone->lock); } into __foo() { spin_lock(zone->lock); } foo() { local_irq_disable() __foo(); } then call the new __foo(). -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org