linux-kernel - Re: Context switch latency in tickless isolated CPU

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Message-Id: <20160821145302.GP3482@linux.vnet.ibm.com>
Date:   Sun, 21 Aug 2016 07:53:02 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     GeHao Kang <kanghao0928@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Frédéric Weisbecker <fweisbec@...il.com>,
        linux-api@...r.kernel.org, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, mingo@...nel.org
Subject: Re: Context switch latency in tickless isolated CPU

On Sun, Aug 21, 2016 at 07:26:04PM +0800, GeHao Kang wrote:
> On Fri, Aug 19, 2016 at 8:34 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > Why are you wanting to use nohz_full if you do syscalls?
> 
> We hope to reduce the overhead of the tick while the real time
> applications run,
> and these applications might do some syscalls to operate the I/O devices like
> EtherCAT.

If latency is all you care about, one approach is to map the device
registers into userspace and do the I/O without assistance from the
kernel.

Alternatively, use in-memory mailbox/queuing techniques to hand the
I/O off to some other thread.

							Thanx, Paul