linux-kernel - Re: [PATCH] Fix /proc/stat freezes (was [PATCH v15] "task

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Date:   Fri, 19 Aug 2016 21:42:19 -0400
From:   Chris Metcalf <cmetcalf@...lanox.com>
To:     Christoph Lameter <cl@...ux.com>,
        Frederic Weisbecker <fweisbec@...il.com>
CC:     Gilad Ben Yossef <giladb@...lanox.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>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Andy Lutomirski <luto@...capital.net>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        "Francis Giraldeau" <francis.giraldeau@...il.com>,
        <linux-doc@...r.kernel.org>, <linux-api@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Fix /proc/stat freezes (was [PATCH v15] "task_isolation"
 mode)

On 8/17/2016 3:37 PM, Christoph Lameter wrote:
> On Tue, 16 Aug 2016, Chris Metcalf wrote:
>
>> - Dropped Christoph Lameter's patch to avoid scheduling the
>>    clocksource watchdog on nohz cores; the recommendation is to just
>>    boot with tsc=reliable for NOHZ in any case, if necessary.
> We also said that there should be a WARN_ON if tsc=reliable is not
> specified and processors are put into NOHZ mode. This is something not
> obvious causing scheduling events on NOHZ processors.

Yes, I agree.  Frederic said he would queue a patch to do that, so I
didn't want to propose another patch that would conflict.

>> Frederic, do you have a sense of what is left to be done there?
>> I can certainly try to contribute to that effort as well.
> Here is a potential fix to the problem that /proc/stat values freeze when
> processors go into NOHZ busy mode. I'd like to hear what people think
> about the approach here. In particular one issue may be that I am
> accessing remote tick-sched structures without serialization. But for
> top/ps this may be ok. I noticed that other values shown by top/os also
> sometime are a bit fuzzy.

This seems pretty plausible to me, but I'm not an expert on what kind
of locking might be required for these data structures.

-- 
Chris Metcalf, Mellanox Technologies
http://www.mellanox.com