linux-kernel - Re: [PATCH v3] sched/debug: Use sched_debug_lock to serialize use of cgroup

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Date:   Sun, 4 Apr 2021 12:02:31 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Bharata B Rao <bharata@...ux.vnet.ibm.com>,
        Phil Auld <pauld@...hat.com>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] sched/debug: Use sched_debug_lock to serialize use
 of cgroup_path[] only

On Fri, 2 Apr 2021 23:09:09 -0400
Waiman Long <longman@...hat.com> wrote:

> The main problem with sched_debug_lock is that under certain 
> circumstances, a lock waiter may wait a long time to acquire the lock 
> (in seconds). We can't insert touch_nmi_watchdog() while the cpu is 
> waiting for the spinlock.

The problem I have with the patch is that it seems to be a hack (as it
doesn't fix the issue in all cases). Since sched_debug_lock is
"special", perhaps we can add wrappers to take it, and instead of doing
the spin_lock_irqsave(), do a trylock loop. Add lockdep annotation to
tell lockdep that this is not a try lock (so that it can still detect
deadlocks).

Then have the strategically placed touch_nmi_watchdog() also increment
a counter. Then in that trylock loop, if it sees the counter get
incremented, it knows that forward progress is being made by the lock
holder, and it too can call touch_nmi_watchdog().

-- Steve