linux-kernel - Re: [lkp-robot] [fs] 3deb642f0d: will-it-scale.per_process

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Message-ID: <20180622115613.GV30522@ZenIV.linux.org.uk>
Date:   Fri, 22 Jun 2018 12:56:13 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Christoph Hellwig <hch@....de>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        kernel test robot <xiaolong.ye@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        LKP <lkp@...org>
Subject: Re: [lkp-robot] [fs] 3deb642f0d: will-it-scale.per_process_ops -8.8%
 regression

On Fri, Jun 22, 2018 at 01:53:00PM +0200, Christoph Hellwig wrote:

> > Now, ->sk_wq is modified only in sock_init_data() and sock_graft();
> > the latter, IIRC, is ->accept() helper.  Do we ever call either of
> > those on a sock of already opened file?  IOW, is there any real
> > reason for socket ->get_poll_head() not to be constant, other
> > than wanting to keep POLL_BUSY_LOOP handling out of ->poll_mask()?
> > I agree that POLL_BUSY_LOOP is ugly as hell, but you *still* have
> > sock_poll_mask() not free from it...
> 
> I'd have to defer to networking folks if busy looping after pollwait
> is what they want, but I suspect the answer is no, by the time
> we are already waiting for the queue busy waiting seems pointless.

	So mark that in ->f_mode - I strongly suspect that
sk_can_busy_loop(sock->sk) can't change while an opened file is there.
And lift that (conditional on new FMODE_BUSY_LOOP) into do_poll()
and do_select() - we *already* have bits of pieces of that logics in
there and that way they'd at least be gathered in one place.

	Then replace ->get_poll_head() with file->f_poll_head and
see what it gives.