From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752479AbdLMD1g (ORCPT ); Tue, 12 Dec 2017 22:27:36 -0500 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:36228 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752288AbdLMD1c (ORCPT ); Tue, 12 Dec 2017 22:27:32 -0500 Date: Tue, 12 Dec 2017 19:27:25 -0800 From: "Paul E. McKenney" To: "Huang, Ying" Cc: Andrew Morton , Minchan Kim , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Hugh Dickins , Johannes Weiner , Tim Chen , Shaohua Li , Mel Gorman , =?utf-8?B?Su+/vXLvv71tZQ==?= Glisse , Michal Hocko , Andrea Arcangeli , David Rientjes , Rik van Riel , Jan Kara , Dave Jiang , Aaron Lu Subject: Re: [PATCH -mm] mm, swap: Fix race between swapoff and some swap operations Reply-To: paulmck@linux.vnet.ibm.com References: <20171208014346.GA8915@bbox> <87po7pg4jt.fsf@yhuang-dev.intel.com> <20171208082644.GA14361@bbox> <87k1xxbohp.fsf@yhuang-dev.intel.com> <20171208140909.4e31ba4f1235b638ae68fd5c@linux-foundation.org> <87609dvnl0.fsf@yhuang-dev.intel.com> <20171211170449.GS7829@linux.vnet.ibm.com> <87374grbpn.fsf@yhuang-dev.intel.com> <20171212171133.GC7829@linux.vnet.ibm.com> <87indbnzga.fsf@yhuang-dev.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87indbnzga.fsf@yhuang-dev.intel.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17121303-0036-0000-0000-0000029A595D X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00008196; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000244; SDB=6.00959421; UDB=6.00485214; IPR=6.00739443; BA=6.00005738; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00018513; XFM=3.00000015; UTC=2017-12-13 03:27:30 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17121303-0037-0000-0000-000042A6AADB Message-Id: <20171213032725.GJ7829@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-12-13_01:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 impostorscore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1709140000 definitions=main-1712130045 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Dec 13, 2017 at 10:17:41AM +0800, Huang, Ying wrote: > "Paul E. McKenney" writes: > > > On Tue, Dec 12, 2017 at 09:12:20AM +0800, Huang, Ying wrote: > >> Hi, Pual, > >> > >> "Paul E. McKenney" writes: > >> > >> > On Mon, Dec 11, 2017 at 01:30:03PM +0800, Huang, Ying wrote: > >> >> Andrew Morton writes: > >> >> > >> >> > On Fri, 08 Dec 2017 16:41:38 +0800 "Huang\, Ying" wrote: > >> >> > > >> >> >> > Why do we need srcu here? Is it enough with rcu like below? > >> >> >> > > >> >> >> > It might have a bug/room to be optimized about performance/naming. > >> >> >> > I just wanted to show my intention. > >> >> >> > >> >> >> Yes. rcu should work too. But if we use rcu, it may need to be called > >> >> >> several times to make sure the swap device under us doesn't go away, for > >> >> >> example, when checking si->max in __swp_swapcount() and > >> >> >> add_swap_count_continuation(). And I found we need rcu to protect swap > >> >> >> cache radix tree array too. So I think it may be better to use one > >> >> >> calling to srcu_read_lock/unlock() instead of multiple callings to > >> >> >> rcu_read_lock/unlock(). > >> >> > > >> >> > Or use stop_machine() ;) It's very crude but it sure is simple. Does > >> >> > anyone have a swapoff-intensive workload? > >> >> > >> >> Sorry, I don't know how to solve the problem with stop_machine(). > >> >> > >> >> The problem we try to resolved is that, we have a swap entry, but that > >> >> swap entry can become invalid because of swappoff between we check it > >> >> and we use it. So we need to prevent swapoff to be run between checking > >> >> and using. > >> >> > >> >> I don't know how to use stop_machine() in swapoff to wait for all users > >> >> of swap entry to finish. Anyone can help me on this? > >> > > >> > You can think of stop_machine() as being sort of like a reader-writer > >> > lock. The readers can be any section of code with preemption disabled, > >> > and the writer is the function passed to stop_machine(). > >> > > >> > Users running real-time applications on Linux don't tend to like > >> > stop_machine() much, but perhaps it is nevertheless the right tool > >> > for this particular job. > >> > >> Thanks a lot for explanation! Now I understand this. > >> > >> Another question, for this specific problem, I think both stop_machine() > >> based solution and rcu_read_lock/unlock() + synchronize_rcu() based > >> solution work. If so, what is the difference between them? I guess rcu > >> based solution will be a little better for real-time applications? So > >> what is the advantage of stop_machine() based solution? > > > > The stop_machine() solution places similar restrictions on readers as > > does rcu_read_lock/unlock() + synchronize_rcu(), if that is what you > > are asking. > > > > More precisely, the stop_machine() solution places exactly the > > same restrictions on readers as does preempt_disable/enable() and > > synchronize_sched(). > > > > I would expect stop_machine() to be faster than either synchronize_rcu() > > synchronize_sched(), or synchronize_srcu(), but stop_machine() operates > > by making each CPU spin with interrupts until all the other CPUs arrive. > > This normally does not make real-time people happy. > > > > An compromise position is available in the form of > > synchronize_rcu_expedited() and synchronize_sched_expedited(). These > > are faster than their non-expedited counterparts, and only momentarily > > disturb each CPU, rather than spinning with interrupts disabled. However, > > stop_machine() is probably a bit faster. > > > > Finally, syncrhonize_srcu_expedited() is reasonably fast, but > > avoids disturbing other CPUs. Last I checked, not quite as fast as > > synchronize_rcu_expedited() and synchronize_sched_expedited(), though. > > > > You asked! ;-) > > Thanks a lot Paul! That exceeds my expectation! > > The performance of swapoff() isn't very important, probably it's not > necessary to accelerate it at the cost of realtime. I think it is > better to use a rcu or srcu based solution. I think the cost at reader > side should be almost same between rcu and srcu? To use srcu, we need > to select CONFIG_SRCU when CONFIG_SWAP is enabled in Kconfig. I think > that should be OK? The thing to do is to try SRCU and see if you can see significant performance degradation. Given that there is swapping involved, I would be surprised if the added read-side overhead of SRCU was even measurable, but then again I have been surprised before. And yes, just select CONFIG_SRCU when you need it. Thanx, Paul