From: Waiman Long <longman@redhat.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Ingo Molnar <mingo@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
Juri Lelli <juri.lelli@redhat.com>,
Vincent Guittot <vincent.guittot@linaro.org>,
Dietmar Eggemann <dietmar.eggemann@arm.com>,
Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
Daniel Bristot de Oliveira <bristot@redhat.com>,
Bharata B Rao <bharata@linux.vnet.ibm.com>,
Phil Auld <pauld@redhat.com>,
Daniel Thompson <daniel.thompson@linaro.org>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3] sched/debug: Use sched_debug_lock to serialize use of cgroup_path[] only
Date: Sun, 4 Apr 2021 21:29:43 -0400 [thread overview]
Message-ID: <03f8cfd4-bb73-a7e5-83f8-7b0731071ae8@redhat.com> (raw)
In-Reply-To: <4014fe97-5875-f64a-7b68-854a2b08394e@redhat.com>
On 4/4/21 9:27 PM, Waiman Long wrote:
> On 4/4/21 12:02 PM, Steven Rostedt wrote:
>> On Fri, 2 Apr 2021 23:09:09 -0400
>> Waiman Long <longman@redhat.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().
>
> Thanks for the suggestion, but it also sound complicated.
>
> I think we can fix this lockup problem if we are willing to lose some
> information in case of contention. As you have suggested, a trylock
> will be used to acquire sched_debug_lock. If succeeded, all is good.
> Otherwise, a shorter stack buffer will be used for cgroup path. The
> path may be truncated in this case. If we detect that the full length
> of the buffer is used, we assume truncation and add, e.g. "...", to
> indicate there is more to the actual path.
>
> Do you think this is an acceptable comprise?
Actually, I don't really need to disable interrupt under this situation
as deadlock can't happen.
Cheers,
Longman
next prev parent reply other threads:[~2021-04-05 1:30 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-01 18:10 [PATCH v3] sched/debug: Use sched_debug_lock to serialize use of cgroup_path[] only Waiman Long
2021-04-02 20:40 ` Steven Rostedt
2021-04-03 3:09 ` Waiman Long
2021-04-04 16:02 ` Steven Rostedt
2021-04-05 1:27 ` Waiman Long
2021-04-05 1:29 ` Waiman Long [this message]
2021-04-05 15:59 ` Steven Rostedt
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