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

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Message-ID: <20180622161802.GA18957@lst.de>
Date:   Fri, 22 Jun 2018 18:18:02 +0200
From:   Christoph Hellwig <hch@....de>
To:     Al Viro <viro@...IV.linux.org.uk>
Cc:     Christoph Hellwig <hch@....de>,
        kernel test robot <xiaolong.ye@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>, lkp@...org
Subject: Re: [lkp-robot] [fs]  3deb642f0d:  will-it-scale.per_process_ops
        -8.8% regression

On Fri, Jun 22, 2018 at 05:28:50PM +0200, Christoph Hellwig wrote:
> On Fri, Jun 22, 2018 at 04:14:09PM +0100, Al Viro wrote:
> > >     http://git.infradead.org/users/hch/vfs.git/shortlog/refs/heads/remove-get-poll-head
> > 
> > See objections upthread re "fs,net: move poll busy loop handling into a
> > separate method"; as for the next one... I'd like an ACK from networking
> > folks.  The rest of queue makes sense.
> 
> I want to see basic results first before micro-optimizing.  After that
> I'll send it out to the net folks for feedback.

I looked into this a bit, in the end sk_can_busy_loop does this:

	return sk->sk_ll_usec && !signal_pending(current);

where sk_ll_usec defaults based on a sysctl that needs to be
turned on, but can be overriden per socket.

While at the same time corect poll code already checks net_busy_loop_on
to set POLL_BUSY_LOOP.  So except for sockets where people set the
timeout to 0 the code already does the right thing as-is.  IMHO not
really worth wasting a FMODE_* flag for it, but if you insist I'll add
it.