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=-11.4 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=unavailable 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 A2F60C2B9F7 for ; Mon, 24 May 2021 16:53:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7FDEF6141E for ; Mon, 24 May 2021 16:53:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232107AbhEXQyf (ORCPT ); Mon, 24 May 2021 12:54:35 -0400 Received: from mail.kernel.org ([198.145.29.99]:42366 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233244AbhEXQye (ORCPT ); Mon, 24 May 2021 12:54:34 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 21E9B61405; Mon, 24 May 2021 16:53:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1621875186; bh=eatiY2VwYI6jMYMQPjWcFVSnWA1ru41S8Ad5ObeDT+k=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=hIN8bW+P2zkyyLXx7bxzWELDzPupPeVmdVLq+f3dCwuE8QZQqXtrAvQRg1KUzNXP5 Q1/5uER24+vhIpVNboCToHB0aVHAnUs3n/8/k4GZq/XKrajlFzLhx+IGUcrELQQCLl QTBjJxpZmDKbGMoIZOYgBioj+svfd4EjHRGepxpR74EKoztAYFBbnTH830ngHjqzVv StijQ+QAzyhcv6G1ty69qpf59gVuSOLAOGSU76MzGSdu/6e0tEXVQk3Isc5cYqUtSO lw5ULSnErQf/w7k6Fqj+cPDghppnblPE/z9caO8uIj83/odQ/tCR5vtlGXJHgj/s1a 6PFQVK6+bSNjw== Date: Mon, 24 May 2021 09:53:05 -0700 From: "Darrick J. Wong" To: Brian Foster Cc: Matthew Wilcox , linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org Subject: Re: [PATCH v3 1/3] iomap: resched ioend completion when in non-atomic context Message-ID: <20210524165305.GA202078@locust> References: <20210517171722.1266878-1-bfoster@redhat.com> <20210517171722.1266878-2-bfoster@redhat.com> <20210520215858.GZ9675@magnolia> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Mon, May 24, 2021 at 07:57:31AM -0400, Brian Foster wrote: > On Thu, May 20, 2021 at 02:58:58PM -0700, Darrick J. Wong wrote: > > On Tue, May 18, 2021 at 07:38:01AM -0400, Brian Foster wrote: > > > On Mon, May 17, 2021 at 06:54:34PM +0100, Matthew Wilcox wrote: > > > > On Mon, May 17, 2021 at 01:17:20PM -0400, Brian Foster wrote: > > > > > @@ -1084,9 +1084,12 @@ iomap_finish_ioend(struct iomap_ioend *ioend, int error) > > > > > next = bio->bi_private; > > > > > > > > > > /* walk each page on bio, ending page IO on them */ > > > > > - bio_for_each_segment_all(bv, bio, iter_all) > > > > > + bio_for_each_segment_all(bv, bio, iter_all) { > > > > > iomap_finish_page_writeback(inode, bv->bv_page, error, > > > > > bv->bv_len); > > > > > + if (!atomic) > > > > > + cond_resched(); > > > > > + } > > > > > > > > I don't know that it makes sense to check after _every_ page. I might > > > > go for every segment. Some users check after every thousand pages. > > > > > > > > > > The handful of examples I come across on a brief scan (including the > > > other iomap usage) have a similar pattern as used here. I don't doubt > > > there are others, but I think I'd prefer to have more reasoning behind > > > adding more code than might be necessary (i.e. do we expect additional > > > overhead to be measurable here?). As it is, the intent isn't so much to > > > check on every page as much as this just happens to be the common point > > > of the function to cover both long bio chains and single vector bios > > > with large numbers of pages. > > > > It's been a while since I waded through the macro hell to find out what > > cond_resched actually does, but iirc it can do some fairly heavyweight > > things (disable preemption, call the scheduler, rcu stuff) which is why > > we're supposed to be a little judicious about amortizing each call over > > a few thousand pages. > > > > It looks to me it just checks some state bit and only does any work if > actually necessary. I suppose not doing that less often is cheaper than > doing it more, but it's not clear to me it's enough that it really > matters and/or warrants more code to filter out calls.. > > What exactly did you have in mind for logic? I suppose we could always > stuff a 'if (!(count++ % 1024)) cond_resched();' or some such in the > inner loop, but that might have less of an effect on larger chains > constructed of bios with fewer pages (depending on whether that might > still be possible). I /was/ thinking about a function level page counter until I noticed that iomap_{write,unshare}_actor call cond_resched for every page it touches. I withdraw the comment. :) --D > > Brian > > > --D > > > > > Brian > > > > > >