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=-3.5 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS autolearn=no 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 27FDFC43461 for ; Wed, 16 Sep 2020 08:45:37 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E9275206B5 for ; Wed, 16 Sep 2020 08:45:36 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="iJxcOZA4" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726473AbgIPIp3 (ORCPT ); Wed, 16 Sep 2020 04:45:29 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:35210 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726149AbgIPIp0 (ORCPT ); Wed, 16 Sep 2020 04:45:26 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 8ABD7C06174A; Wed, 16 Sep 2020 01:45:25 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=RwS3rW8B+ViP4YJxJK0d4LsKrqUL+lbB0Q2Eor68chU=; b=iJxcOZA45Nc6D2m+4Ydi8g6iJE EIzSywwIr1olZ0MonhuBswR4j28Emdvcj3jqhxn4trt4bZiPuz6hRsxDQp+fF7u4Vad9mYlIrmHUy 9C/pTn05QR6nzpYSwGfgoXkM7jKEaxnMo7nTmVAqT1m+8uiBBgURlasPg0kQvKSJZC7F9NAmZ7+xC ZRw7rcabO84VdXXNwGH8BS+iwALMldc9Bviz0KNZe146/g6y81I3eldK7E1C9Rwr9CeozfvuEh1We mIYRhjxWmfLoboJQrS7XaqwDFCpHSVjEqo4e34iVo4XUVW+KAwXAtewUPHgnak02iqj3iRtJgU0Nu rqrQfD3A==; Received: from hch by casper.infradead.org with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1kIT3u-000883-2t; Wed, 16 Sep 2020 08:45:10 +0000 Date: Wed, 16 Sep 2020 09:45:10 +0100 From: Christoph Hellwig To: "Darrick J. Wong" Cc: Brian Foster , Dave Chinner , Christoph Hellwig , Ritesh Harjani , Anju T Sudhakar , linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, willy@infradead.org, minlei@redhat.com Subject: Re: [PATCH] iomap: Fix the write_count in iomap_add_to_ioend(). Message-ID: <20200916084510.GA30815@infradead.org> References: <20200820231140.GE7941@dread.disaster.area> <20200821044533.BBFD1A405F@d06av23.portsmouth.uk.ibm.com> <20200821215358.GG7941@dread.disaster.area> <20200822131312.GA17997@infradead.org> <20200824142823.GA295033@bfoster> <20200824150417.GA12258@infradead.org> <20200824154841.GB295033@bfoster> <20200825004203.GJ12131@dread.disaster.area> <20200825144917.GA321765@bfoster> <20200916001242.GE7955@magnolia> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200916001242.GE7955@magnolia> X-SRS-Rewrite: SMTP reverse-path rewritten from by casper.infradead.org. See http://www.infradead.org/rpr.html Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Tue, Sep 15, 2020 at 05:12:42PM -0700, Darrick J. Wong wrote: > On Tue, Aug 25, 2020 at 10:49:17AM -0400, Brian Foster wrote: > > cc Ming > > > > On Tue, Aug 25, 2020 at 10:42:03AM +1000, Dave Chinner wrote: > > > On Mon, Aug 24, 2020 at 11:48:41AM -0400, Brian Foster wrote: > > > > On Mon, Aug 24, 2020 at 04:04:17PM +0100, Christoph Hellwig wrote: > > > > > On Mon, Aug 24, 2020 at 10:28:23AM -0400, Brian Foster wrote: > > > > > > Do I understand the current code (__bio_try_merge_page() -> > > > > > > page_is_mergeable()) correctly in that we're checking for physical page > > > > > > contiguity and not necessarily requiring a new bio_vec per physical > > > > > > page? > > > > > > > > > > > > > > > Yes. > > > > > > > > > > > > > Ok. I also realize now that this occurs on a kernel without commit > > > > 07173c3ec276 ("block: enable multipage bvecs"). That is probably a > > > > contributing factor, but it's not clear to me whether it's feasible to > > > > backport whatever supporting infrastructure is required for that > > > > mechanism to work (I suspect not). > > > > > > > > > > With regard to Dave's earlier point around seeing excessively sized bio > > > > > > chains.. If I set up a large memory box with high dirty mem ratios and > > > > > > do contiguous buffered overwrites over a 32GB range followed by fsync, I > > > > > > can see upwards of 1GB per bio and thus chains on the order of 32+ bios > > > > > > for the entire write. If I play games with how the buffered overwrite is > > > > > > submitted (i.e., in reverse) however, then I can occasionally reproduce > > > > > > a ~32GB chain of ~32k bios, which I think is what leads to problems in > > > > > > I/O completion on some systems. Granted, I don't reproduce soft lockup > > > > > > issues on my system with that behavior, so perhaps there's more to that > > > > > > particular issue. > > > > > > > > > > > > Regardless, it seems reasonable to me to at least have a conservative > > > > > > limit on the length of an ioend bio chain. Would anybody object to > > > > > > iomap_ioend growing a chain counter and perhaps forcing into a new ioend > > > > > > if we chain something like more than 1k bios at once? > > > > > > > > > > So what exactly is the problem of processing a long chain in the > > > > > workqueue vs multiple small chains? Maybe we need a cond_resched() > > > > > here and there, but I don't see how we'd substantially change behavior. > > > > > > > > > > > > > The immediate problem is a watchdog lockup detection in bio completion: > > > > > > > > NMI watchdog: Watchdog detected hard LOCKUP on cpu 25 > > > > > > > > This effectively lands at the following segment of iomap_finish_ioend(): > > > > > > > > ... > > > > /* walk each page on bio, ending page IO on them */ > > > > bio_for_each_segment_all(bv, bio, iter_all) > > > > iomap_finish_page_writeback(inode, bv->bv_page, error); > > > > > > > > I suppose we could add a cond_resched(), but is that safe directly > > > > inside of a ->bi_end_io() handler? Another option could be to dump large > > > > chains into the completion workqueue, but we may still need to track the > > > > length to do that. Thoughts? > > > > > > We have ioend completion merging that will run the compeltion once > > > for all the pending ioend completions on that inode. IOWs, we do not > > > need to build huge chains at submission time to batch up completions > > > efficiently. However, huge bio chains at submission time do cause > > > issues with writeback fairness, pinning GBs of ram as unreclaimable > > > for seconds because they are queued for completion while we are > > > still submitting the bio chain and submission is being throttled by > > > the block layer writeback throttle, etc. Not to mention the latency > > > of stable pages in a situation like this - a mmap() write fault > > > could stall for many seconds waiting for a huge bio chain to finish > > > submission and run completion processing even when the IO for the > > > given page we faulted on was completed before the page fault > > > occurred... > > > > > > Hence I think we really do need to cap the length of the bio > > > chains here so that we start completing and ending page writeback on > > > large writeback ranges long before the writeback code finishes > > > submitting the range it was asked to write back. > > > > > > > Ming pointed out separately that limiting the bio chain itself might not > > be enough because with multipage bvecs, we can effectively capture the > > same number of pages in much fewer bios. Given that, what do you think > > about something like the patch below to limit ioend size? This > > effectively limits the number of pages per ioend regardless of whether > > in-core state results in a small chain of dense bios or a large chain of > > smaller bios, without requiring any new explicit page count tracking. > > > > Brian > > Dave was asking on IRC if I was going to pull this patch in. I'm unsure > of its status (other than it hasn't been sent as a proper [PATCH]) so I > wonder, is this necessary, and if so, can it be cleaned up and > submitted? Maybe it is lost somewhere, but what is the point of this patch? What does the magic number try to represent?