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

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Message-ID: <20180622123307.GA16699@lst.de>
Date:   Fri, 22 Jun 2018 14:33:07 +0200
From:   Christoph Hellwig <hch@....de>
To:     Al Viro <viro@...IV.linux.org.uk>
Cc:     Christoph Hellwig <hch@....de>,
        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 01:17:22PM +0100, Al Viro wrote:
> > 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()

What I have for now is slightly different:

	if ((events & POLL_BUSY_LOOP) && file->f_op->poll_busy_loop)
		file->f_op->poll_busy_loop(file, events);

	if (file->f_op->poll) {
		return file->f_op->poll(file, pt);
	} else if (file_has_poll_mask(file)) {
		...
	}

returns whatever we want part is something I want to look into
once the basics are done as it probably is non entirely trivial due to
structure of polling in the low-level network protocol.