From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pa0-f54.google.com (mail-pa0-f54.google.com [209.85.220.54]) by kanga.kvack.org (Postfix) with ESMTP id 1D8A06B0158 for ; Tue, 26 May 2015 08:10:18 -0400 (EDT) Received: by pacwv17 with SMTP id wv17so91442616pac.2 for ; Tue, 26 May 2015 05:10:17 -0700 (PDT) Received: from mail-pa0-x234.google.com (mail-pa0-x234.google.com. [2607:f8b0:400e:c03::234]) by mx.google.com with ESMTPS id x3si17553918pbw.198.2015.05.26.05.10.16 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 26 May 2015 05:10:17 -0700 (PDT) Received: by paza2 with SMTP id a2so82303224paz.3 for ; Tue, 26 May 2015 05:10:16 -0700 (PDT) Subject: Re: [RFC PATCH 2/2] arm64: Implement vmalloc based thread_info allocator Mime-Version: 1.0 (Apple Message framework v1283) Content-Type: text/plain; charset=us-ascii From: Jungseok Lee In-Reply-To: <20150525145857.GF14922@blaptop> Date: Tue, 26 May 2015 21:10:11 +0900 Content-Transfer-Encoding: quoted-printable Message-Id: References: <1432483340-23157-1-git-send-email-jungseoklee85@gmail.com> <5992243.NYDGjLH37z@wuerfel> <20150525145857.GF14922@blaptop> Sender: owner-linux-mm@kvack.org List-ID: To: Minchan Kim Cc: Arnd Bergmann , linux-arm-kernel@lists.infradead.org, Catalin Marinas , barami97@gmail.com, Will Deacon , linux-kernel@vger.kernel.org, linux-mm@kvack.org On May 25, 2015, at 11:58 PM, Minchan Kim wrote: > On Mon, May 25, 2015 at 07:01:33PM +0900, Jungseok Lee wrote: >> On May 25, 2015, at 2:49 AM, Arnd Bergmann wrote: >>> On Monday 25 May 2015 01:02:20 Jungseok Lee wrote: >>>> Fork-routine sometimes fails to get a physically contiguous region = for >>>> thread_info on 4KB page system although free memory is enough. That = is, >>>> a physically contiguous region, which is currently 16KB, is not = available >>>> since system memory is fragmented. >>>>=20 >>>> This patch tries to solve the problem as allocating thread_info = memory >>>> from vmalloc space, not 1:1 mapping one. The downside is one = additional >>>> page allocation in case of vmalloc. However, vmalloc space is large = enough, >>>> around 240GB, under a combination of 39-bit VA and 4KB page. Thus, = it is >>>> not a big tradeoff for fork-routine service. >>>=20 >>> vmalloc has a rather large runtime cost. I'd argue that failing to = allocate >>> thread_info structures means something has gone very wrong. >>=20 >> That is why the feature is marked "N" by default. >> I focused on fork-routine stability rather than performance. >=20 > If VM has trouble with order-2 allocation, your system would be > trouble soon although fork at the moment manages to be successful > because such small high-order(ex, order <=3D PAGE_ALLOC_COSTLY_ORDER) > allocation is common in the kernel so VM should handle it smoothly. > If VM didn't, it means we should fix VM itself, not a specific > allocation site. Fork is one of victim by that. A problem I observed is an user space, not a kernel side. As user = applications fail to create threads in order to distribute their jobs properly, they = are getting in trouble slowly and then gone. Yes, fork is one of victim, but damages user applications seriously. At this snapshot, free memory is enough. >> Could you give me an idea how to evaluate performance degradation? >> Running some benchmarks would be helpful, but I would like to try to >> gather data based on meaningful methodology. >>=20 >>> Can you describe the scenario that leads to fragmentation this bad? >>=20 >> Android, but I could not describe an exact reproduction procedure = step >> by step since it's behaved and reproduced randomly. As reading the = following >> thread from mm mailing list, a similar symptom is observed on other = systems.=20 >>=20 >> https://lkml.org/lkml/2015/4/28/59 >>=20 >> Although I do not know the details of a system mentioned in the = thread, >> even order-2 page allocation is not smoothly operated due to = fragmentation on >> low memory system. >=20 > What Joonsoo have tackle is generic fragmentation problem, not *a* = fork fail, > which is more right approach to handle small high-order allocation = problem. I totally agree with that point. One of the best ways is to figure out a = generic anti-fragmentation with VM system improvement. Reducing the stack size = to 8KB is also a really great approach. My intention is not to overlook them or figure = out a workaround. IMHO, vmalloc would be a different option in case of ARM64 on low memory = systems since *fork failure from fragmentation* is a nontrivial issue. Do you think the patch set doesn't need to be considered? Best Regards Jungseok Lee= -- 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