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

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Message-ID: <20180622200255.GB30522@ZenIV.linux.org.uk>
Date:   Fri, 22 Jun 2018 21:02:55 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Christoph Hellwig <hch@....de>
Cc:     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 06:18:02PM +0200, Christoph Hellwig wrote:
> 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.

It's not just that - there's also an issue of extra indirect call on the
fast path for sockets.  You get this method of yours + ->poll_mask(),
which hits another indirect to per-family ->poll_mask().  It might be
better to have these combined, sparing us an extra indirect call.

Just give it the same calling conventions as ->poll_mask() have...