linux-kernel - Re: [PATCH v7 06/11] nohz: task_isolation: allow tick to be fully disabled

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Message-ID: <20151001130700.GE3432@lerouge>
Date:	Thu, 1 Oct 2015 15:07:01 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Chris Metcalf <cmetcalf@...hip.com>,
	Gilad Ben Yossef <giladb@...hip.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>, Tejun Heo <tj@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Christoph Lameter <cl@...ux.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v7 06/11] nohz: task_isolation: allow tick to be fully
 disabled

On Mon, Sep 28, 2015 at 04:40:56PM -0400, Andy Lutomirski wrote:
> On Mon, Sep 28, 2015 at 11:17 AM, Chris Metcalf <cmetcalf@...hip.com> wrote:
> > While the current fallback to 1-second tick is still helpful for
> > maintaining completely correct kernel semantics, processes using
> > prctl(PR_SET_TASK_ISOLATION) semantics place a higher priority on
> > running completely tickless, so don't bound the time_delta for such
> > processes.  In addition, due to the way such processes quiesce by
> > waiting for the timer tick to stop prior to returning to userspace,
> > without this commit it won't be possible to use the task_isolation
> > mode at all.
> >
> > Removing the 1-second cap was previously discussed (see link
> > below) and Thomas Gleixner observed that vruntime, load balancing
> > data, load accounting, and other things might be impacted.
> > Frederic Weisbecker similarly observed that allowing the tick to
> > be indefinitely deferred just meant that no one would ever fix the
> > underlying bugs.  However it's at least true that the mode proposed
> > in this patch can only be enabled on a nohz_full core by a process
> > requesting task_isolation mode, which may limit how important it is
> > to maintain scheduler data correctly, for example.
> 
> What goes wrong when a task enables this?  Presumably either tasks
> that enable it experience problems or performance issues or it should
> always be enabled.

We need to make the scheduler resilient to 0Hz tick. Currently it doesn't
even correctly support 1Hz or any dynticks behaviour that isn't idle.

See update_cpu_load_active() for exemple.

> 
> One possible issue: __vdso_clock_gettime with any of the COARSE clocks
> as well as __vdso_time will break if the timekeeping code doesn't run
> somewhere with reasonable frequency on some core.  Hopefully this
> always works.
> 
> --Andy
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