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Message-ID: <20241024210051.2a436bc4@rorschach.local.home>
Date: Thu, 24 Oct 2024 21:00:51 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, Mark
Rutland <mark.rutland@....com>, Mathieu Desnoyers
<mathieu.desnoyers@...icios.com>, Andrew Morton
<akpm@...ux-foundation.org>, Thomas Gleixner <tglx@...utronix.de>, Peter
Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 1/2] fgraph: Free ret_stacks when graph tracing is done
On Fri, 25 Oct 2024 00:00:44 +0900
Masami Hiramatsu (Google) <mhiramat@...nel.org> wrote:
> > ---
> > kernel/trace/fgraph.c | 113 ++++++++++++++++++++++++++++++++++++------
> > 1 file changed, 99 insertions(+), 14 deletions(-)
> >
> > diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
> > index 0b7cf2507569..3c7f115217b4 100644
> > --- a/kernel/trace/fgraph.c
> > +++ b/kernel/trace/fgraph.c
> > @@ -1144,6 +1144,7 @@ void ftrace_graph_init_task(struct task_struct *t)
> > t->curr_ret_stack = 0;
> > t->curr_ret_depth = -1;
> >
> > + mutex_lock(&ftrace_lock);
> > if (ftrace_graph_active) {
> > unsigned long *ret_stack;
> >
> > @@ -1155,6 +1156,7 @@ void ftrace_graph_init_task(struct task_struct *t)
> > return;
>
> The above `return;` shows that you miss unlocking ftrace_lock. B^)
Bah, I added this locking after doing most my tests and then seeing
this needed protection. The return was here before the mutex, but I
missed it when I added the mutexes. I'll switch this with guard.
>
> > graph_init_task(t, ret_stack);
> > }
> > + mutex_unlock(&ftrace_lock);
> > }
> >
> > void ftrace_graph_exit_task(struct task_struct *t)
> > @@ -1292,19 +1294,106 @@ static void ftrace_graph_disable_direct(bool disable_branch)
> > fgraph_direct_gops = &fgraph_stub;
> > }
> >
> > -/* The cpu_boot init_task->ret_stack will never be freed */
> > -static int fgraph_cpu_init(unsigned int cpu)
> > +static void __fgraph_cpu_init(unsigned int cpu)
> > {
> > if (!idle_task(cpu)->ret_stack)
> > ftrace_graph_init_idle_task(idle_task(cpu), cpu);
> > +}
> > +
> > +static int fgraph_cpu_init(unsigned int cpu)
> > +{
> > + if (ftrace_graph_active)
> > + __fgraph_cpu_init(cpu);
> > return 0;
> > }
> >
> > +struct ret_stack_free_data {
> > + struct list_head list;
> > + struct task_struct *task;
> > +};
> > +
> > +static void remove_ret_stack(struct task_struct *t, struct list_head *head, int list_index)
> > +{
> > + struct ret_stack_free_data *free_data;
> > +
> > + /* If the ret_stack is still in use, skip this */
> > + if (t->curr_ret_depth >= 0)
> > + return;
> > +
> > + free_data = (struct ret_stack_free_data*)(t->ret_stack + list_index);
> > + list_add(&free_data->list, head);
> > + free_data->task = t;
> > +}
> > +
> > +static void free_ret_stacks(void)
> > +{
> > + struct ret_stack_free_data *free_data, *n;
> > + struct task_struct *g, *t;
> > + LIST_HEAD(stacks);
> > + int list_index;
> > + int list_sz;
> > + int cpu;
> > +
> > + /* Calculate the size in longs to hold ret_stack_free_data */
> > + list_sz = DIV_ROUND_UP(sizeof(struct ret_stack_free_data), sizeof(long));
> > +
> > + /*
> > + * We do not want to race with __ftrace_return_to_handler() where this
> > + * CPU can see the update to curr_ret_depth going to zero before it
> > + * actually does. As tracing is disabled, the ret_stack is not going
> > + * to be used anymore and there will be no more callbacks. Use
> > + * the top of the stack as the link list pointer to attach this
>
> > + * ret_stack to @head. Then at the end, run an RCU trace synthronization
> > + * which will guarantee that there are no more uses of the ret_stacks
> > + * and they can all be freed.
>
> Just a comment.
> This part can mislead, the ret_stacks here are the ret_stacks which can be
> used by currently running callbacks on other CPUs. Some other ret_stack are
> still used and the owner tasks are in sleep.
OK, I'll update the comment.
>
> > + */
> > + list_index = SHADOW_STACK_MAX_OFFSET - list_sz;
> > +
> > + read_lock(&tasklist_lock);
> > + for_each_process_thread(g, t) {
> > + if (t->ret_stack)
> > + remove_ret_stack(t, &stacks, list_index);
> > + }
> > + read_unlock(&tasklist_lock);
> > +
> > + cpus_read_lock();
> > + for_each_online_cpu(cpu) {
> > + t = idle_task(cpu);
> > + if (t->ret_stack)
> > + remove_ret_stack(t, &stacks, list_index);
> > + }
> > + cpus_read_unlock();
> > +
> > + /* Make sure nothing is using the ret_stacks anymore */
> > + synchronize_rcu_tasks_trace();
> > +
> > + list_for_each_entry_safe(free_data, n, &stacks, list) {
> > + unsigned long *stack = free_data->task->ret_stack;
> > +
> > + free_data->task->ret_stack = NULL;
> > + kmem_cache_free(fgraph_stack_cachep, stack);
> > + }
> > +}
> > +
> > +static __init int fgraph_init(void)
> > +{
> > + int ret;
> > +
> > + ret = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "fgraph_idle_init",
>
> nit: Shouldn't we update the name first?
Heh, I guess ;-)
Thank for the review!
-- Steve
>
>
> Thank you,
>
> > + fgraph_cpu_init, NULL);
> > + if (ret < 0) {
> > + pr_warn("fgraph: Error to init cpu hotplug support\n");
> > + return ret;
> > + }
> > + return 0;
> > +}
> > +core_initcall(fgraph_init)
> > +
> > int register_ftrace_graph(struct fgraph_ops *gops)
> > {
> > - static bool fgraph_initialized;
> > int command = 0;
> > int ret = 0;
> > + int cpu;
> > int i = -1;
> >
> > mutex_lock(&ftrace_lock);
> > @@ -1319,17 +1408,6 @@ int register_ftrace_graph(struct fgraph_ops *gops)
> > }
> > }
> >
> > - if (!fgraph_initialized) {
> > - ret = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "fgraph_idle_init",
> > - fgraph_cpu_init, NULL);
> > - if (ret < 0) {
> > - pr_warn("fgraph: Error to init cpu hotplug support\n");
> > - return ret;
> > - }
> > - fgraph_initialized = true;
> > - ret = 0;
> > - }
> > -
> > if (!fgraph_array[0]) {
> > /* The array must always have real data on it */
> > for (i = 0; i < FGRAPH_ARRAY_SIZE; i++)
> > @@ -1346,6 +1424,12 @@ int register_ftrace_graph(struct fgraph_ops *gops)
> >
> > ftrace_graph_active++;
> >
> > + cpus_read_lock();
> > + for_each_online_cpu(cpu) {
> > + __fgraph_cpu_init(cpu);
> > + }
> > + cpus_read_unlock();
> > +
> > if (ftrace_graph_active == 2)
> > ftrace_graph_disable_direct(true);
> >
> > @@ -1418,6 +1502,7 @@ void unregister_ftrace_graph(struct fgraph_ops *gops)
> > ftrace_graph_entry = ftrace_graph_entry_stub;
> > unregister_pm_notifier(&ftrace_suspend_notifier);
> > unregister_trace_sched_switch(ftrace_graph_probe_sched_switch, NULL);
> > + free_ret_stacks();
> > }
> > out:
> > gops->saved_func = NULL;
> > --
> > 2.45.2
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