From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail191.messagelabs.com (mail191.messagelabs.com [216.82.242.19]) by kanga.kvack.org (Postfix) with ESMTP id B54186B006A for ; Mon, 12 Oct 2009 19:58:28 -0400 (EDT) Date: Mon, 12 Oct 2009 16:57:47 -0700 From: Andrew Morton Subject: Re: [resend][PATCH v2] mlock() doesn't wait to finish lru_add_drain_all() Message-Id: <20091012165747.97f5bd87.akpm@linux-foundation.org> In-Reply-To: <20091009111709.1291.A69D9226@jp.fujitsu.com> References: <20091009111709.1291.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 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()? 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. 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? 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()? -- 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