netdev - Re: [PATCH v2 0/2] enable hires timer to timeout datagram socket

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Message-ID: <20170920224816.GA73561@amazon.com>
Date:   Wed, 20 Sep 2017 22:48:16 +0000
From:   Vallish Vaidyeshwara <vallish@...zon.com>
To:     Thomas Gleixner <tglx@...utronix.de>
CC:     Eric Dumazet <eric.dumazet@...il.com>,
        Eduardo Valentin <eduval@...zon.com>,
        David Miller <davem@...emloft.net>, <dwmw2@...radead.org>,
        <shuah@...nel.org>, <richardcochran@...il.com>,
        <xiyou.wangcong@...il.com>, <netdev@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <anchalag@...zon.com>,
        <dwmw@...zon.com>
Subject: Re: [PATCH v2 0/2] enable hires timer to timeout datagram socket

On Sat, Sep 16, 2017 at 11:47:56AM +0200, Thomas Gleixner wrote:
> On Fri, 8 Sep 2017, Eric Dumazet wrote:
> > On Fri, 2017-09-08 at 11:55 -0700, Eduardo Valentin wrote:
> > > Hello,
> > > 
> > > On Fri, Sep 08, 2017 at 10:26:45AM -0700, David Miller wrote:
> > > > From: David Woodhouse <dwmw2@...radead.org>
> > > > Date: Fri, 08 Sep 2017 18:23:22 +0100
> > > > 
> > > > > I don't know that anyone's ever tried saying "show me the chapter
> > > and
> > > > > verse of the documentation"
> > > > 
> > > > Do you know why I brought this up?  Because the person I am replying
> > > > to told me that the syscall documentation should have suggested this
> > > > or that.
> > > > 
> > > > That's why.
> > > 
> > > :-) My intention was for sure not to upset anybody.
> > > 
> > > Just to reiterate, the point of patch is simple, there was a change in
> > > behavior in the system call from one kernel version to the other. As I
> > > mentioned, I agree that the userspace could use other means to achieve
> > > the same, but still the system call behavior has changed.
> > > 
> > > > 
> > > > So let's concentrate on the other aspects of my reply, ok?
> > > 
> > > I agree. I would prefer to understand here what is the technical
> > > reason not to accept these patches other than "use other system
> > > calls".
> > 
> > So if we need to replace all 'legacy' timers to high resolution timer,
> > because some application was _relying_ on jiffies being kind of precise,
> > maybe it is better to revert the change done on legacy timers.
> 
> Which would be a major step back in terms of timer performance and system
> disturbance caused by massive recascading operations.
> 
> > Or continue the migration and make them use high res internally.
> > 
> > select() and poll() are the standard way to have precise timeouts,
> > it is silly we have to maintain a timeout handling in the datagram fast
> > path.
> 
> A few years ago we switched select/poll over to use hrtimers because the
> wheel timers were too inaccurate for some operations, so it feels
> consequent to switch the timeout in the datagram rcv path over as well. I
> agree that the whole timeout magic there feels silly, but unfortunately
> it's a documented property of sockets.
> 
> Thanks,
> 
> 	tglx
> 

Hello Thomas,

Thanks for your comments. This patch has been NACK'ed by David Miller. Is
there any other approach to solve this problem with out application code
being recompiled?

Thanks.
-Vallish