From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0B9CBC433E1 for ; Tue, 7 Jul 2020 03:24:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DC146206E6 for ; Tue, 7 Jul 2020 03:24:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728103AbgGGDYq (ORCPT ); Mon, 6 Jul 2020 23:24:46 -0400 Received: from mga18.intel.com ([134.134.136.126]:18678 "EHLO mga18.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726869AbgGGDYq (ORCPT ); Mon, 6 Jul 2020 23:24:46 -0400 IronPort-SDR: aIBXuSXs3M95yXAdMx3dvFv3CsAzQf6QUrBHZgUgHoJHxqePsjUSPwjE7iptyq9nBCWlU3Xw9a lp0D0spvZW6Q== X-IronPort-AV: E=McAfee;i="6000,8403,9674"; a="134998742" X-IronPort-AV: E=Sophos;i="5.75,321,1589266800"; d="scan'208";a="134998742" X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga005.jf.intel.com ([10.7.209.41]) by orsmga106.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 06 Jul 2020 20:24:45 -0700 IronPort-SDR: zSVTX1Nq7QXFBpZ1GVH8tusaG2I8RYXGEBq1xPESk6/tU3IyaQsoZmIhFXW8u7SBI6JpJEjI+g imJHTpyPnxtQ== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.75,321,1589266800"; d="scan'208";a="456955198" Received: from shbuild999.sh.intel.com (HELO localhost) ([10.239.146.107]) by orsmga005.jf.intel.com with ESMTP; 06 Jul 2020 20:24:40 -0700 Date: Tue, 7 Jul 2020 11:24:39 +0800 From: Feng Tang To: Dennis Zhou Cc: Qian Cai , Andrew Morton , Tejun Heo , Christoph Lameter , kernel test robot , Michal Hocko , Johannes Weiner , Matthew Wilcox , Mel Gorman , Kees Cook , Luis Chamberlain , Iurii Zaikin , andi.kleen@intel.com, tim.c.chen@intel.com, dave.hansen@intel.com, ying.huang@intel.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, lkp@lists.01.org Subject: Re: [mm] 4e2c82a409: ltp.overcommit_memory01.fail Message-ID: <20200707032439.GA21741@shbuild999.sh.intel.com> References: <20200705044454.GA90533@shbuild999.sh.intel.com> <20200705125854.GA66252@shbuild999.sh.intel.com> <20200705155232.GA608@lca.pw> <20200706014313.GB66252@shbuild999.sh.intel.com> <20200706023614.GA1231@lca.pw> <20200706132443.GA34488@shbuild999.sh.intel.com> <20200707010651.GA2384124@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200707010651.GA2384124@google.com> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jul 07, 2020 at 01:06:51AM +0000, Dennis Zhou wrote: > On Mon, Jul 06, 2020 at 09:24:43PM +0800, Feng Tang wrote: > > Hi All, > > > > Please help to review this fix patch, thanks! > > > > It is against today's linux-mm tree. For easy review, I put the fix > > into one patch, and I could split it to 2 parts for percpu-counter > > and mm/util.c if it's preferred. > > > > From 593f9dc139181a7c3bb1705aacd1f625f400e458 Mon Sep 17 00:00:00 2001 > > From: Feng Tang > > Date: Mon, 6 Jul 2020 14:48:29 +0800 > > Subject: [PATCH] mm/util.c: sync vm_committed_as when changing memory policy > > to OVERCOMMIT_NEVER > > > > With the patch to improve scalability of vm_committed_as [1], 0day reported > > the ltp overcommit_memory test case could fail (fail rate is about 5/50) [2]. > > The root cause is when system is running with loose memory overcommit policy > > like OVERCOMMIT_GUESS/ALWAYS, the deviation of vm_committed_as could be big, > > and once the policy is runtime changed to OVERCOMMIT_NEVER, vm_committed_as's > > batch is decreased to 1/64 of original one, but the deviation is not > > compensated accordingly, and following __vm_enough_memory() check for vm > > overcommit could be wrong due to this deviation, which breaks the ltp > > overcommit_memory case. > > > > Fix it by forcing a sync for percpu counter vm_committed_as when overcommit > > policy is changed to OVERCOMMIT_NEVER (sysctl -w vm.overcommit_memory=2). > > The sync itself is not a fast operation, and is toleratable given user is > > not expected to frequently changing policy to OVERCOMMIT_NEVER. > > > > [1] https://lore.kernel.org/lkml/1592725000-73486-1-git-send-email-feng.tang@intel.com/ > > [2] https://marc.info/?l=linux-mm&m=159367156428286 (can't find a link in lore.kernel.org) > > > > Reported-by: kernel test robot > > Signed-off-by: Feng Tang > > --- > > include/linux/percpu_counter.h | 4 ++++ > > lib/percpu_counter.c | 14 ++++++++++++++ > > mm/util.c | 11 ++++++++++- > > 3 files changed, 28 insertions(+), 1 deletion(-) > > > > diff --git a/include/linux/percpu_counter.h b/include/linux/percpu_counter.h > > index 0a4f54d..01861ee 100644 > > --- a/include/linux/percpu_counter.h > > +++ b/include/linux/percpu_counter.h > > @@ -44,6 +44,7 @@ void percpu_counter_add_batch(struct percpu_counter *fbc, s64 amount, > > s32 batch); > > s64 __percpu_counter_sum(struct percpu_counter *fbc); > > int __percpu_counter_compare(struct percpu_counter *fbc, s64 rhs, s32 batch); > > +void percpu_counter_sync(struct percpu_counter *fbc); > > > > static inline int percpu_counter_compare(struct percpu_counter *fbc, s64 rhs) > > { > > @@ -172,6 +173,9 @@ static inline bool percpu_counter_initialized(struct percpu_counter *fbc) > > return true; > > } > > > > +static inline void percpu_counter_sync(struct percpu_counter *fbc) > > +{ > > +} > > #endif /* CONFIG_SMP */ > > > > static inline void percpu_counter_inc(struct percpu_counter *fbc) > > diff --git a/lib/percpu_counter.c b/lib/percpu_counter.c > > index a66595b..02d87fc 100644 > > --- a/lib/percpu_counter.c > > +++ b/lib/percpu_counter.c > > @@ -98,6 +98,20 @@ void percpu_counter_add_batch(struct percpu_counter *fbc, s64 amount, s32 batch) > > } > > EXPORT_SYMBOL(percpu_counter_add_batch); > > > > +void percpu_counter_sync(struct percpu_counter *fbc) > > +{ > > + unsigned long flags; > > + s64 count; > > + > > + raw_spin_lock_irqsave(&fbc->lock, flags); > > + count = __this_cpu_read(*fbc->counters); > > + fbc->count += count; > > + __this_cpu_sub(*fbc->counters, count); > > + raw_spin_unlock_irqrestore(&fbc->lock, flags); > > +} > > +EXPORT_SYMBOL(percpu_counter_sync); > > + > > + > > /* > > * Add up all the per-cpu counts, return the result. This is a more accurate > > * but much slower version of percpu_counter_read_positive() > > diff --git a/mm/util.c b/mm/util.c > > index 52ed9c1..5fb62c0 100644 > > --- a/mm/util.c > > +++ b/mm/util.c > > @@ -746,14 +746,23 @@ int overcommit_ratio_handler(struct ctl_table *table, int write, void *buffer, > > return ret; > > } > > > > +static void sync_overcommit_as(struct work_struct *dummy) > > +{ > > + percpu_counter_sync(&vm_committed_as); > > +} > > + > > This seems like a rather niche use case as it's currently coupled with a > schedule_on_each_cpu(). I can't imagine a use case where you'd want to > do this without being called by schedule_on_each_cpu(). Yes! > > Would it be better to modify or introduce something akin to > percpu_counter_sum() which sums and folds in the counter state? I'd be > curious to see what the cost of always folding would be as this is > already considered the cold path and would help with the next batch too. Initially, I also thought about doing the sync just like percpu_counter_sum(): raw_spin_lock_irqsave for_each_online_cpu(cpu) } do-the-sync raw_spin_unlock_irqrestore One problem is the per_cpu_ptr(fbc->counters, cpu) could still be updated on other CPUs as the fast path update is not protected by fbc->lock. As for cost, it is about about 800 nanoseconds on a 2C/4T platform and 2~3 microseconds on a 2S/36C/72T Skylake server in normal case, and in worst case where vm_committed_as's spinlock is under severe contention, it costs 30~40 microseconds for the 2S/36C/72T Skylake sever. Thanks, Feng > > int overcommit_policy_handler(struct ctl_table *table, int write, void *buffer, > > size_t *lenp, loff_t *ppos) > > { > > int ret; > > > > ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos); > > - if (ret == 0 && write) > > + if (ret == 0 && write) { > > + if (sysctl_overcommit_memory == OVERCOMMIT_NEVER) > > + schedule_on_each_cpu(sync_overcommit_as); > > + > > mm_compute_batch(); > > + } > > > > return ret; > > } > > -- > > 2.7.4