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

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Message-ID: <CALCETrXwRYH7x+xUtqyxrqL3Vj6Ez80Ztsc47HLdze5DDAzP8g@mail.gmail.com>
Date:	Mon, 28 Sep 2015 16:40:56 -0400
From:	Andy Lutomirski <luto@...capital.net>
To:	Chris Metcalf <cmetcalf@...hip.com>
Cc:	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>,
	Frederic Weisbecker <fweisbec@...il.com>,
	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 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.

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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