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=-5.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,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 D3A37C49ED9 for ; Thu, 12 Sep 2019 09:06:25 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A6B46208C2 for ; Thu, 12 Sep 2019 09:06:25 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730622AbfILJGY (ORCPT ); Thu, 12 Sep 2019 05:06:24 -0400 Received: from mx1.redhat.com ([209.132.183.28]:40360 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730327AbfILJGY (ORCPT ); Thu, 12 Sep 2019 05:06:24 -0400 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 882FB18C4272; Thu, 12 Sep 2019 09:06:23 +0000 (UTC) Received: from llong.remote.csb (ovpn-120-238.rdu2.redhat.com [10.10.120.238]) by smtp.corp.redhat.com (Postfix) with ESMTP id B31CA600C4; Thu, 12 Sep 2019 09:06:18 +0000 (UTC) Subject: Re: [PATCH 5/5] hugetlbfs: Limit wait time when trying to share huge PMD To: Mike Kravetz , Peter Zijlstra , Ingo Molnar , Will Deacon , Alexander Viro Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, Davidlohr Bueso References: <20190911150537.19527-1-longman@redhat.com> <20190911150537.19527-6-longman@redhat.com> From: Waiman Long Organization: Red Hat Message-ID: Date: Thu, 12 Sep 2019 10:06:14 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.6.2 (mx1.redhat.com [10.5.110.62]); Thu, 12 Sep 2019 09:06:23 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 9/12/19 4:26 AM, Mike Kravetz wrote: > On 9/11/19 8:05 AM, Waiman Long wrote: >> When allocating a large amount of static hugepages (~500-1500GB) on a >> system with large number of CPUs (4, 8 or even 16 sockets), performance >> degradation (random multi-second delays) was observed when thousands >> of processes are trying to fault in the data into the huge pages. The >> likelihood of the delay increases with the number of sockets and hence >> the CPUs a system has. This only happens in the initial setup phase >> and will be gone after all the necessary data are faulted in. >> >> These random delays, however, are deemed unacceptable. The cause of >> that delay is the long wait time in acquiring the mmap_sem when trying >> to share the huge PMDs. >> >> To remove the unacceptable delays, we have to limit the amount of wait >> time on the mmap_sem. So the new down_write_timedlock() function is >> used to acquire the write lock on the mmap_sem with a timeout value of >> 10ms which should not cause a perceivable delay. If timeout happens, >> the task will abandon its effort to share the PMD and allocate its own >> copy instead. >> > >> diff --git a/mm/hugetlb.c b/mm/hugetlb.c >> index 6d7296dd11b8..445af661ae29 100644 >> --- a/mm/hugetlb.c >> +++ b/mm/hugetlb.c >> @@ -4750,6 +4750,8 @@ void adjust_range_if_pmd_sharing_possible(struct vm_area_struct *vma, >> } >> } >> >> +#define PMD_SHARE_DISABLE_THRESHOLD (1 << 8) >> + >> /* >> * Search for a shareable pmd page for hugetlb. In any case calls pmd_alloc() >> * and returns the corresponding pte. While this is not necessary for the >> @@ -4770,11 +4772,24 @@ pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud) >> pte_t *spte = NULL; >> pte_t *pte; >> spinlock_t *ptl; >> + static atomic_t timeout_cnt; >> >> - if (!vma_shareable(vma, addr)) >> - return (pte_t *)pmd_alloc(mm, pud, addr); >> + /* >> + * Don't share if it is not sharable or locking attempt timed out >> + * after 10ms. After 256 timeouts, PMD sharing will be permanently >> + * disabled as it is just too slow. >> + */ >> + if (!vma_shareable(vma, addr) || >> + (atomic_read(&timeout_cnt) >= PMD_SHARE_DISABLE_THRESHOLD)) >> + goto out_no_share; >> + >> + if (!i_mmap_timedlock_write(mapping, ms_to_ktime(10))) { >> + if (atomic_inc_return(&timeout_cnt) == >> + PMD_SHARE_DISABLE_THRESHOLD) >> + pr_info("Hugetlbfs PMD sharing disabled because of timeouts!\n"); >> + goto out_no_share; >> + } >> >> - i_mmap_lock_write(mapping); > All this got me wondering if we really need to take i_mmap_rwsem in write > mode here. We are not changing the tree, only traversing it looking for > a suitable vma. > > Unless I am missing something, the hugetlb code only ever takes the semaphore > in write mode; never read. Could this have been the result of changing the > tree semaphore to read/write? Instead of analyzing all the code, the easiest > and safest thing would have been to take all accesses in write mode. > > I can investigate more, but wanted to ask the question in case someone already > knows. > > At one time, I thought it was safe to acquire the semaphore in read mode for > huge_pmd_share, but write mode for huge_pmd_unshare. See commit b43a99900559. > This was reverted along with another patch for other reasons. > > If we change change from write to read mode, this may have significant impact > on the stalls. If we can take the rwsem in read mode, that should solve the problem AFAICS. As I don't have a full understanding of the history of that code, I didn't try to do that in my patch. Cheers, Longman