linux-kernel - Re: [PATCH v5] sched/debug: Use sched_debug_lock to serialize use of cgroup

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Message-ID: <YHnJVK5N1QMTlDgm@hirez.programming.kicks-ass.net>
Date:   Fri, 16 Apr 2021 19:28:52 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        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 v5] sched/debug: Use sched_debug_lock to serialize use of
 cgroup_path[] only

On Thu, Apr 15, 2021 at 03:54:26PM -0400, Waiman Long wrote:
> The handling of sysrq key can be activated by echoing the key to
> /proc/sysrq-trigger or via the magic key sequence typed into a terminal
> that is connected to the system in some way (serial, USB or other mean).
> In the former case, the handling is done in a user context. In the
> latter case, it is likely to be in an interrupt context.
> 
> Currently in print_cpu() of kernel/sched/debug.c, sched_debug_lock is
> taken with interrupt disabled for the whole duration of the calls to
> print_*_stats() and print_rq() which could last for the quite some time
> if the information dump happens on the serial console.
> 
> If the system has many cpus and the sched_debug_lock is somehow busy
> (e.g. parallel sysrq-t), the system may hit a hard lockup panic
> depending on the actually serial console implementation of the
> system. For instance,
> 
...
> 
> The purpose of sched_debug_lock is to serialize the use of the global
> cgroup_path[] buffer in print_cpu(). The rests of the printk calls don't
> need serialization from sched_debug_lock.
> 
> Calling printk() with interrupt disabled can still be problematic if
> multiple instances are running. Allocating a stack buffer of PATH_MAX
> bytes is not feasible because of the limited size of the kernel stack.
> 
> The solution implemented in this patch is to allow only one caller at a
> time to use the full size group_path[], while other simultaneous callers
> will have to use shorter stack buffers with the possibility of path
> name truncation. A "..." suffix will be printed if truncation may have
> happened.  The cgroup path name is provided for informational purpose
> only, so occasional path name truncation should not be a big problem.
> 
> Fixes: efe25c2c7b3a ("sched: Reinstate group names in /proc/sched_debug")
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Waiman Long <longman@...hat.com>

Thanks!