Re: [PATCH v5] sched/debug: Use sched_debug_lock to serialize use of cgroup_path[] only

From: Peter Zijlstra
Date: Fri Apr 16 2021 - 13:30:27 EST

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@xxxxxxxxxxxxx>
> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>