lkml.org 
[lkml]   [2021]   [May]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    From
    Subject[PATCH 5.10 345/530] sched/debug: Fix cgroup_path[] serialization
    Date
    From: Waiman Long <longman@redhat.com>

    [ Upstream commit ad789f84c9a145f8a18744c0387cec22ec51651e ]

    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.

    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@infradead.org>
    Signed-off-by: Waiman Long <longman@redhat.com>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Link: https://lkml.kernel.org/r/20210415195426.6677-1-longman@redhat.com
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    kernel/sched/debug.c | 42 +++++++++++++++++++++++++++++-------------
    1 file changed, 29 insertions(+), 13 deletions(-)

    diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
    index 2357921580f9..6264584b51c2 100644
    --- a/kernel/sched/debug.c
    +++ b/kernel/sched/debug.c
    @@ -8,8 +8,6 @@
    */
    #include "sched.h"

    -static DEFINE_SPINLOCK(sched_debug_lock);
    -
    /*
    * This allows printing both to /proc/sched_debug and
    * to the console
    @@ -470,16 +468,37 @@ static void print_cfs_group_stats(struct seq_file *m, int cpu, struct task_group
    #endif

    #ifdef CONFIG_CGROUP_SCHED
    +static DEFINE_SPINLOCK(sched_debug_lock);
    static char group_path[PATH_MAX];

    -static char *task_group_path(struct task_group *tg)
    +static void task_group_path(struct task_group *tg, char *path, int plen)
    {
    - if (autogroup_path(tg, group_path, PATH_MAX))
    - return group_path;
    + if (autogroup_path(tg, path, plen))
    + return;

    - cgroup_path(tg->css.cgroup, group_path, PATH_MAX);
    + cgroup_path(tg->css.cgroup, path, plen);
    +}

    - return group_path;
    +/*
    + * Only 1 SEQ_printf_task_group_path() caller can use the full length
    + * group_path[] for cgroup path. Other simultaneous callers will have
    + * to use a shorter stack buffer. A "..." suffix is appended at the end
    + * of the stack buffer so that it will show up in case the output length
    + * matches the given buffer size to indicate possible path name truncation.
    + */
    +#define SEQ_printf_task_group_path(m, tg, fmt...) \
    +{ \
    + if (spin_trylock(&sched_debug_lock)) { \
    + task_group_path(tg, group_path, sizeof(group_path)); \
    + SEQ_printf(m, fmt, group_path); \
    + spin_unlock(&sched_debug_lock); \
    + } else { \
    + char buf[128]; \
    + char *bufend = buf + sizeof(buf) - 3; \
    + task_group_path(tg, buf, bufend - buf); \
    + strcpy(bufend - 1, "..."); \
    + SEQ_printf(m, fmt, buf); \
    + } \
    }
    #endif

    @@ -506,7 +525,7 @@ print_task(struct seq_file *m, struct rq *rq, struct task_struct *p)
    SEQ_printf(m, " %d %d", task_node(p), task_numa_group_id(p));
    #endif
    #ifdef CONFIG_CGROUP_SCHED
    - SEQ_printf(m, " %s", task_group_path(task_group(p)));
    + SEQ_printf_task_group_path(m, task_group(p), " %s")
    #endif

    SEQ_printf(m, "\n");
    @@ -543,7 +562,7 @@ void print_cfs_rq(struct seq_file *m, int cpu, struct cfs_rq *cfs_rq)

    #ifdef CONFIG_FAIR_GROUP_SCHED
    SEQ_printf(m, "\n");
    - SEQ_printf(m, "cfs_rq[%d]:%s\n", cpu, task_group_path(cfs_rq->tg));
    + SEQ_printf_task_group_path(m, cfs_rq->tg, "cfs_rq[%d]:%s\n", cpu);
    #else
    SEQ_printf(m, "\n");
    SEQ_printf(m, "cfs_rq[%d]:\n", cpu);
    @@ -614,7 +633,7 @@ void print_rt_rq(struct seq_file *m, int cpu, struct rt_rq *rt_rq)
    {
    #ifdef CONFIG_RT_GROUP_SCHED
    SEQ_printf(m, "\n");
    - SEQ_printf(m, "rt_rq[%d]:%s\n", cpu, task_group_path(rt_rq->tg));
    + SEQ_printf_task_group_path(m, rt_rq->tg, "rt_rq[%d]:%s\n", cpu);
    #else
    SEQ_printf(m, "\n");
    SEQ_printf(m, "rt_rq[%d]:\n", cpu);
    @@ -666,7 +685,6 @@ void print_dl_rq(struct seq_file *m, int cpu, struct dl_rq *dl_rq)
    static void print_cpu(struct seq_file *m, int cpu)
    {
    struct rq *rq = cpu_rq(cpu);
    - unsigned long flags;

    #ifdef CONFIG_X86
    {
    @@ -717,13 +735,11 @@ do { \
    }
    #undef P

    - spin_lock_irqsave(&sched_debug_lock, flags);
    print_cfs_stats(m, cpu);
    print_rt_stats(m, cpu);
    print_dl_stats(m, cpu);

    print_rq(m, rq, cpu);
    - spin_unlock_irqrestore(&sched_debug_lock, flags);
    SEQ_printf(m, "\n");
    }

    --
    2.30.2


    \
     
     \ /
      Last update: 2021-05-12 19:01    [W:4.197 / U:0.056 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site