linux-kernel - Re: [PATCH] timers/nohz: Update nohz load even if tick already stopped

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Message-ID: <b56f988176fca4f13c310b9dc866baf5408eeadd.camel@redhat.com>
Date:   Fri, 08 Nov 2019 02:13:44 -0600
From:   Scott Wood <swood@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Frederic Weisbecker <frederic@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] timers/nohz: Update nohz load even if tick already
 stopped

On Tue, 2019-11-05 at 13:43 +0100, Peter Zijlstra wrote:
> On Tue, Nov 05, 2019 at 01:30:58AM -0600, Scott Wood wrote:
> > As for the warning in sched_tick_remote(), it seems like a test for time
> > since the last tick on this cpu (remote or otherwise) would be better
> > than
> > relying on curr->se.exec_start, in order to detect things like this.
> 
> I don't think we have a timestamp that is shared between the remote and
> local tick. 

Why wouldn't rq_clock_task() work on the local tick?  It's what
->task_tick() itself uses.

> Also, there is a reason this warning uses the task time
> accounting, there used to be (as in, I can't find it in a hurry) code
> that could not deal with >u32 (~4s) clock updates.

Detecting a 3 second interval between ticks for a given cpu should assert in
a superset of the situations the current check asserts in -- it just avoids
the false negative of exec_runtime getting updated by something other than
the tick.

-Scott