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

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Message-ID: <20180622115300.GA14654@lst.de>
Date:   Fri, 22 Jun 2018 13:53:00 +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 12:01:17PM +0100, Al Viro wrote:
> For fuck sake, if you want vfs_poll() inlined, *make* *it* *inlined*.

That is not going to help with de-virtualizing _qproc, which was
the whole idea of that change.  At least not without a compiler
way smarter than gcc.

But if you want it inline that is fine with me, it just seems little
large for inlining.

None that I plan to actually remove all calls except for poll and select
for vfs_poll in a pending series, at which point it would become static
anyway.

> Said that, you are not attacking the worst part of it - it's a static
> branch, not the considerably more costly indirect ones.  Remember when
> I asked you about the price of those?  Method calls are costly.

And back then it did not show up even in poll heavy workloads.  But
since then something new happened - spectre mitigations, which make
indirect calls exorbitantly more expensive.

> Now, ->sk_wq is modified only in sock_init_data() and sock_graft();
> the latter, IIRC, is ->accept() helper.  Do we ever call either of
> those on a sock of already opened file?  IOW, is there any real
> reason for socket ->get_poll_head() not to be constant, other
> than wanting to keep POLL_BUSY_LOOP handling out of ->poll_mask()?
> I agree that POLL_BUSY_LOOP is ugly as hell, but you *still* have
> sock_poll_mask() not free from it...

I'd have to defer to networking folks if busy looping after pollwait
is what they want, but I suspect the answer is no, by the time
we are already waiting for the queue busy waiting seems pointless.