From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f173.google.com (mail-pf0-f173.google.com [209.85.192.173]) by kanga.kvack.org (Postfix) with ESMTP id 98EE66B0258 for ; Mon, 29 Feb 2016 12:27:47 -0500 (EST) Received: by mail-pf0-f173.google.com with SMTP id 124so34011792pfg.0 for ; Mon, 29 Feb 2016 09:27:47 -0800 (PST) Received: from mail-pf0-x22a.google.com (mail-pf0-x22a.google.com. [2607:f8b0:400e:c00::22a]) by mx.google.com with ESMTPS id ua9si22195412pab.25.2016.02.29.09.27.46 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 29 Feb 2016 09:27:46 -0800 (PST) Received: by mail-pf0-x22a.google.com with SMTP id 4so17387978pfd.1 for ; Mon, 29 Feb 2016 09:27:46 -0800 (PST) Subject: Re: [RFC PATCH] writeback: move list_lock down into the for loop References: <1456505185-21566-1-git-send-email-yang.shi@linaro.org> <20160229150618.GA16939@dhcp22.suse.cz> From: "Shi, Yang" Message-ID: <56D47F90.9050903@linaro.org> Date: Mon, 29 Feb 2016 09:27:44 -0800 MIME-Version: 1.0 In-Reply-To: <20160229150618.GA16939@dhcp22.suse.cz> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: tj@kernel.org, jack@suse.cz, axboe@fb.com, fengguang.wu@intel.com, akpm@linux-foundation.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linaro-kernel@lists.linaro.org On 2/29/2016 7:06 AM, Michal Hocko wrote: > On Fri 26-02-16 08:46:25, Yang Shi wrote: >> The list_lock was moved outside the for loop by commit >> e8dfc30582995ae12454cda517b17d6294175b07 ("writeback: elevate queue_io() >> into wb_writeback())", however, the commit log says "No behavior change", so >> it sounds safe to have the list_lock acquired inside the for loop as it did >> before. >> Leave tracepoints outside the critical area since tracepoints already have >> preempt disabled. > > The patch says what but it completely misses the why part. I'm just wondering the finer grained lock may reach a little better performance, i.e. more likely for preempt, lower latency. Thanks, Yang > >> >> Signed-off-by: Yang Shi >> --- >> Tested with ltp on 8 cores Cortex-A57 machine. >> >> fs/fs-writeback.c | 12 +++++++----- >> 1 file changed, 7 insertions(+), 5 deletions(-) >> >> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c >> index 1f76d89..9b7b5f6 100644 >> --- a/fs/fs-writeback.c >> +++ b/fs/fs-writeback.c >> @@ -1623,7 +1623,6 @@ static long wb_writeback(struct bdi_writeback *wb, >> work->older_than_this = &oldest_jif; >> >> blk_start_plug(&plug); >> - spin_lock(&wb->list_lock); >> for (;;) { >> /* >> * Stop writeback when nr_pages has been consumed >> @@ -1661,15 +1660,19 @@ static long wb_writeback(struct bdi_writeback *wb, >> oldest_jif = jiffies; >> >> trace_writeback_start(wb, work); >> + >> + spin_lock(&wb->list_lock); >> if (list_empty(&wb->b_io)) >> queue_io(wb, work); >> if (work->sb) >> progress = writeback_sb_inodes(work->sb, wb, work); >> else >> progress = __writeback_inodes_wb(wb, work); >> - trace_writeback_written(wb, work); >> >> wb_update_bandwidth(wb, wb_start); >> + spin_unlock(&wb->list_lock); >> + >> + trace_writeback_written(wb, work); >> >> /* >> * Did we write something? Try for more >> @@ -1693,15 +1696,14 @@ static long wb_writeback(struct bdi_writeback *wb, >> */ >> if (!list_empty(&wb->b_more_io)) { >> trace_writeback_wait(wb, work); >> + spin_lock(&wb->list_lock); >> inode = wb_inode(wb->b_more_io.prev); >> - spin_lock(&inode->i_lock); >> spin_unlock(&wb->list_lock); >> + spin_lock(&inode->i_lock); >> /* This function drops i_lock... */ >> inode_sleep_on_writeback(inode); >> - spin_lock(&wb->list_lock); >> } >> } >> - spin_unlock(&wb->list_lock); >> blk_finish_plug(&plug); >> >> return nr_pages - work->nr_pages; >> -- >> 2.0.2 >> >> -- >> 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 > -- 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