Re: [lkp-robot] [fs] 3deb642f0d: will-it-scale.per_process_ops -8.8% regression

[Christoph Hellwig <hch@lst.de>]

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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.