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

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Message-ID: <20180622121722.GW30522@ZenIV.linux.org.uk>
Date:   Fri, 22 Jun 2018 13:17:22 +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 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()
	}