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Date:   Fri, 22 Jun 2018 13:17:22 +0100
From:   Al Viro <viro@ZenIV.linux.org.uk>
To:     Christoph Hellwig <hch@lst.de>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        kernel test robot <xiaolong.ye@intel.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        LKP <lkp@01.org>
Subject: Re: [lkp-robot] [fs] 3deb642f0d: will-it-scale.per_process_ops -8.8%
 regression
Message-ID: <20180622121722.GW30522@ZenIV.linux.org.uk>
References: <20180622082752.GX11011@yexl-desktop>
 <CA+55aFw5ghByk_zCN25G6rPPSAQma3Mh0t4s18CtLg=h6U9+Zg@mail.gmail.com>
 <20180622095608.GA12263@lst.de>
 <20180622100014.GA12425@lst.de>
 <20180622110117.GU30522@ZenIV.linux.org.uk>
 <20180622115300.GA14654@lst.de>
 <20180622115613.GV30522@ZenIV.linux.org.uk>
 <20180622120739.GA15351@lst.de>
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On Fri, Jun 22, 2018 at 02:07:39PM +0200, Christoph Hellwig wrote:
> On Fri, Jun 22, 2018 at 12:56:13PM +0100, Al Viro wrote:
> > 	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.
> 
> The problem is that call to sk_busy_loop(), which is going to be indirect
> no matter what.

	if ->f_poll_head is NULL {
		use ->poll
	} else {
		if can ll_poll (checked in ->f_mode)
			call ->ll_poll(), if it returns what we want - we are done
		add to ->f_poll_head
		call ->poll_mask()
	}