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Date: Wed, 03 Mar 2021 16:09:05 -0600
From: ebiederm@...ssion.com (Eric W. Biederman)
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
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>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
Oleg Nesterov <oleg@...hat.com>,
Matt Fleming <matt@...eblueprint.co.uk>
Subject: Re: [PATCH] signal: Allow RT tasks to cache one sigqueue struct
Sebastian Andrzej Siewior <bigeasy@...utronix.de> writes:
> From: Thomas Gleixner <tglx@...utronix.de>
>
> Allow realtime tasks to cache one sigqueue in task struct. This avoids an
> allocation which can increase the latency or fail.
> Ideally the sigqueue is cached after first successful delivery and will be
> available for next signal delivery. This works under the assumption that the RT
> task has never an unprocessed signal while a one is about to be queued.
>
> The caching is not used for SIGQUEUE_PREALLOC because this kind of sigqueue is
> handled differently (and not used for regular signal delivery).
What part of this is about real time tasks? This allows any task
to cache a sigqueue entry.
Either the patch is buggy or the description is. Overall caching one
sigqueue entry doesn't look insane. But it would help to have a clear
description of what is going on.
Eric
> [bigeasy: With a fix from Matt Fleming <matt@...eblueprint.co.uk>]
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
> include/linux/sched.h | 1 +
> include/linux/signal.h | 1 +
> kernel/exit.c | 2 +-
> kernel/fork.c | 1 +
> kernel/signal.c | 65 +++++++++++++++++++++++++++++++++++++++---
> 5 files changed, 65 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index ef00bb22164cd..7009b25f48160 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -985,6 +985,7 @@ struct task_struct {
> /* Signal handlers: */
> struct signal_struct *signal;
> struct sighand_struct __rcu *sighand;
> + struct sigqueue *sigqueue_cache;
> sigset_t blocked;
> sigset_t real_blocked;
> /* Restored if set_restore_sigmask() was used: */
> diff --git a/include/linux/signal.h b/include/linux/signal.h
> index 205526c4003aa..d47a86790edc8 100644
> --- a/include/linux/signal.h
> +++ b/include/linux/signal.h
> @@ -265,6 +265,7 @@ static inline void init_sigpending(struct sigpending *sig)
> }
>
> extern void flush_sigqueue(struct sigpending *queue);
> +extern void flush_task_sigqueue(struct task_struct *tsk);
>
> /* Test if 'sig' is valid signal. Use this instead of testing _NSIG directly */
> static inline int valid_signal(unsigned long sig)
> diff --git a/kernel/exit.c b/kernel/exit.c
> index 04029e35e69af..346f7b76cecaa 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -152,7 +152,7 @@ static void __exit_signal(struct task_struct *tsk)
> * Do this under ->siglock, we can race with another thread
> * doing sigqueue_free() if we have SIGQUEUE_PREALLOC signals.
> */
> - flush_sigqueue(&tsk->pending);
> + flush_task_sigqueue(tsk);
> tsk->sighand = NULL;
> spin_unlock(&sighand->siglock);
>
> diff --git a/kernel/fork.c b/kernel/fork.c
> index d66cd1014211b..a767e4e49a692 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1993,6 +1993,7 @@ static __latent_entropy struct task_struct *copy_process(
> spin_lock_init(&p->alloc_lock);
>
> init_sigpending(&p->pending);
> + p->sigqueue_cache = NULL;
>
> p->utime = p->stime = p->gtime = 0;
> #ifdef CONFIG_ARCH_HAS_SCALED_CPUTIME
> diff --git a/kernel/signal.c b/kernel/signal.c
> index ba4d1ef39a9ea..d99273b798085 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -20,6 +20,7 @@
> #include <linux/sched/task.h>
> #include <linux/sched/task_stack.h>
> #include <linux/sched/cputime.h>
> +#include <linux/sched/rt.h>
> #include <linux/file.h>
> #include <linux/fs.h>
> #include <linux/proc_fs.h>
> @@ -404,13 +405,30 @@ void task_join_group_stop(struct task_struct *task)
> task_set_jobctl_pending(task, mask | JOBCTL_STOP_PENDING);
> }
>
> +static struct sigqueue *sigqueue_from_cache(struct task_struct *t)
> +{
> + struct sigqueue *q = t->sigqueue_cache;
> +
> + if (q && cmpxchg(&t->sigqueue_cache, q, NULL) == q)
> + return q;
> + return NULL;
> +}
> +
> +static bool sigqueue_add_cache(struct task_struct *t, struct sigqueue *q)
> +{
> + if (!t->sigqueue_cache && cmpxchg(&t->sigqueue_cache, NULL, q) == NULL)
> + return true;
> + return false;
> +}
> +
> /*
> * allocate a new signal queue record
> * - this may be called without locks if and only if t == current, otherwise an
> * appropriate lock must be held to stop the target task from exiting
> */
> static struct sigqueue *
> -__sigqueue_alloc(int sig, struct task_struct *t, gfp_t flags, int override_rlimit)
> +__sigqueue_do_alloc(int sig, struct task_struct *t, gfp_t flags,
> + int override_rlimit, bool fromslab)
> {
> struct sigqueue *q = NULL;
> struct user_struct *user;
> @@ -432,7 +450,10 @@ __sigqueue_alloc(int sig, struct task_struct *t, gfp_t flags, int override_rlimi
> rcu_read_unlock();
>
> if (override_rlimit || likely(sigpending <= task_rlimit(t, RLIMIT_SIGPENDING))) {
> - q = kmem_cache_alloc(sigqueue_cachep, flags);
> + if (!fromslab)
> + q = sigqueue_from_cache(t);
> + if (!q)
> + q = kmem_cache_alloc(sigqueue_cachep, flags);
> } else {
> print_dropped_signal(sig);
> }
> @@ -449,6 +470,13 @@ __sigqueue_alloc(int sig, struct task_struct *t, gfp_t flags, int override_rlimi
> return q;
> }
>
> +static struct sigqueue *
> +__sigqueue_alloc(int sig, struct task_struct *t, gfp_t flags,
> + int override_rlimit)
> +{
> + return __sigqueue_do_alloc(sig, t, flags, override_rlimit, false);
> +}
> +
> static void __sigqueue_free(struct sigqueue *q)
> {
> if (q->flags & SIGQUEUE_PREALLOC)
> @@ -458,6 +486,20 @@ static void __sigqueue_free(struct sigqueue *q)
> kmem_cache_free(sigqueue_cachep, q);
> }
>
> +static void __sigqueue_cache_or_free(struct sigqueue *q)
> +{
> + struct user_struct *up;
> +
> + if (q->flags & SIGQUEUE_PREALLOC)
> + return;
> +
> + up = q->user;
> + if (atomic_dec_and_test(&up->sigpending))
> + free_uid(up);
> + if (!task_is_realtime(current) || !sigqueue_add_cache(current, q))
> + kmem_cache_free(sigqueue_cachep, q);
> +}
> +
> void flush_sigqueue(struct sigpending *queue)
> {
> struct sigqueue *q;
> @@ -470,6 +512,21 @@ void flush_sigqueue(struct sigpending *queue)
> }
> }
>
> +/*
> + * Called from __exit_signal. Flush tsk->pending and
> + * tsk->sigqueue_cache
> + */
> +void flush_task_sigqueue(struct task_struct *tsk)
> +{
> + struct sigqueue *q;
> +
> + flush_sigqueue(&tsk->pending);
> +
> + q = sigqueue_from_cache(tsk);
> + if (q)
> + kmem_cache_free(sigqueue_cachep, q);
> +}
> +
> /*
> * Flush all pending signals for this kthread.
> */
> @@ -594,7 +651,7 @@ static void collect_signal(int sig, struct sigpending *list, kernel_siginfo_t *i
> (info->si_code == SI_TIMER) &&
> (info->si_sys_private);
>
> - __sigqueue_free(first);
> + __sigqueue_cache_or_free(first);
> } else {
> /*
> * Ok, it wasn't in the queue. This must be
> @@ -1807,7 +1864,7 @@ EXPORT_SYMBOL(kill_pid);
> */
> struct sigqueue *sigqueue_alloc(void)
> {
> - struct sigqueue *q = __sigqueue_alloc(-1, current, GFP_KERNEL, 0);
> + struct sigqueue *q = __sigqueue_do_alloc(-1, current, GFP_KERNEL, 0, true);
>
> if (q)
> q->flags |= SIGQUEUE_PREALLOC;
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