linux-kernel - Re: [mm/writeback] 8d92890bd6: will-it-scale.per_process

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Date:   Wed, 14 Oct 2020 12:19:04 +0200
From:   Jan Kara <jack@...e.cz>
To:     kernel test robot <rong.a.chen@...el.com>
Cc:     NeilBrown <neilb@...e.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@....de>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Chuck Lever <chuck.lever@...cle.com>,
        LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org,
        lkp@...el.com, ying.huang@...el.com, feng.tang@...el.com,
        zhengjun.xing@...el.com
Subject: Re: [mm/writeback] 8d92890bd6: will-it-scale.per_process_ops -15.3%
 regression

On Wed 14-10-20 16:47:06, kernel test robot wrote:
> Greeting,
> 
> FYI, we noticed a -15.3% regression of will-it-scale.per_process_ops due
> to commit:
> 
> commit: 8d92890bd6b8502d6aee4b37430ae6444ade7a8c ("mm/writeback: discard
> NR_UNSTABLE_NFS, use NR_WRITEBACK instead")
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git master

Thanks for report but it doesn't quite make sense to me. If we omit
reporting & NFS changes in that commit (which is code not excercised by
this benchmark), what remains are changes like:

        nr_pages += node_page_state(pgdat, NR_FILE_DIRTY);
-       nr_pages += node_page_state(pgdat, NR_UNSTABLE_NFS);
        nr_pages += node_page_state(pgdat, NR_WRITEBACK);
...
-               nr_reclaimable = global_node_page_state(NR_FILE_DIRTY) +
-                                       global_node_page_state(NR_UNSTABLE_NFS);
+               nr_reclaimable = global_node_page_state(NR_FILE_DIRTY);
...
-       gdtc->dirty = global_node_page_state(NR_FILE_DIRTY) +
-                     global_node_page_state(NR_UNSTABLE_NFS);
+       gdtc->dirty = global_node_page_state(NR_FILE_DIRTY);

So if there's any negative performance impact of these changes, they're
likely due to code alignment changes or something like that... So I don't
think there's much to do here since optimal code alignment is highly specific
to a particular CPU etc.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR