linux-kernel - Re: [PATCH v5 03/11] tracing/user_events: Use remote writes for event enablement

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Date:   Mon, 5 Dec 2022 16:28:03 -0500
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Beau Belgrave <beaub@...ux.microsoft.com>, rostedt@...dmis.org,
        mhiramat@...nel.org, dcook@...ux.microsoft.com,
        alanau@...ux.microsoft.com, brauner@...nel.org,
        akpm@...ux-foundation.org
Cc:     linux-trace-devel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 03/11] tracing/user_events: Use remote writes for event
 enablement

On 2022-12-05 16:00, Beau Belgrave wrote:
[...]
>   #ifdef CONFIG_USER_EVENTS
>   struct user_event_mm {
> +	struct list_head link;
> +	struct list_head enablers;
> +	struct mm_struct *mm;
> +	struct user_event_mm *next;
> +	refcount_t refcnt;
> +	refcount_t tasks;
>   };
> -#endif
>   
> +extern void user_event_mm_dup(struct task_struct *t,
> +			      struct user_event_mm *old_mm);
> +
> +extern void user_event_mm_remove(struct task_struct *t);
> +
> +static inline void user_events_fork(struct task_struct *t,
> +				    unsigned long clone_flags)
> +{
> +	struct user_event_mm *old_mm;
> +
> +	if (!t || !current->user_event_mm)
> +		return;
> +
> +	old_mm = current->user_event_mm;
> +
> +	if (clone_flags & CLONE_VM) {
> +		t->user_event_mm = old_mm;
> +		refcount_inc(&old_mm->tasks);
> +		return;
> +	}
> +
> +	user_event_mm_dup(t, old_mm);
> +}
> +
> +static inline void user_events_execve(struct task_struct *t)
> +{
> +	if (!t || !t->user_event_mm)
> +		return;
> +
> +	user_event_mm_remove(t);
> +}
> +
> +static inline void user_events_exit(struct task_struct *t)
> +{
> +	if (!t || !t->user_event_mm)
> +		return;
> +
> +	user_event_mm_remove(t);
> +}

So this is adding user_event_mm_remove() calls on each execve and each 
process exit, correct ?

[...]


> +
> +void user_event_mm_remove(struct task_struct *t)
> +{
> +	struct user_event_mm *mm;
> +	unsigned long flags;
> +
> +	might_sleep();
> +
> +	mm = t->user_event_mm;
> +	t->user_event_mm = NULL;
> +
> +	/* Clone will increment the tasks, only remove if last clone */
> +	if (!refcount_dec_and_test(&mm->tasks))
> +		return;
> +
> +	/* Remove the mm from the list, so it can no longer be enabled */
> +	spin_lock_irqsave(&user_event_mms_lock, flags);
> +	list_del_rcu(&mm->link);
> +	spin_unlock_irqrestore(&user_event_mms_lock, flags);
> +
> +	/*
> +	 * Put for mm must be done after RCU sync to handle new refs in
> +	 * between the list_del_rcu() and now. This ensures any get refs
> +	 * during rcu_read_lock() are accounted for during list removal.
> +	 *
> +	 * CPU A			|	CPU B
> +	 * ---------------------------------------------------------------
> +	 * user_event_mm_remove()	|	rcu_read_lock();
> +	 * list_del_rcu()		|	list_for_each_entry_rcu();
> +	 * synchronize_rcu()		|	refcount_inc();
> +	 * .				|	rcu_read_unlock();
> +	 * user_event_mm_put()		|	.
> +	 */
> +	synchronize_rcu();

This means a synchronize_rcu() is added on each execve and each process 
exit ? I am really worried about the performance impact of this big 
hammer synchronization in those key points of process lifetime.

Thanks,

Mathieu

> +
> +	/*
> +	 * We need to wait for currently occurring writes to stop within
> +	 * the mm. This is required since exit_mm() snaps the current rss
> +	 * stats and clears them. On the final mmdrop(), check_mm() will
> +	 * report a bug if these increment.
> +	 *
> +	 * All writes/pins are done under mmap_read lock, take the write
> +	 * lock to ensure in-progress faults have completed. Faults that
> +	 * are pending but yet to run will check the task count and skip
> +	 * the fault since the mm is going away.
> +	 */
> +	mmap_write_lock(mm->mm);
> +	mmap_write_unlock(mm->mm);
> +
> +	/* MM is still alive, but won't be updated anymore */
> +	user_event_mm_put(mm);
> +}
> +
> +void user_event_mm_dup(struct task_struct *t, struct user_event_mm *old_mm)
>   {
> -	int i = user->index;
> +	struct user_event_mm *mm = user_event_mm_create(t);
> +	struct user_event_enabler *enabler;
> +
> +	if (!mm)
> +		return;
> +
> +	rcu_read_lock();
>   
> -	user->group->register_page_data[MAP_STATUS_BYTE(i)] |= MAP_STATUS_MASK(i);
> +	list_for_each_entry_rcu(enabler, &old_mm->enablers, link)
> +		if (!user_event_enabler_dup(enabler, mm))
> +			goto error;
> +
> +	rcu_read_unlock();
> +
> +	return;
> +error:
> +	rcu_read_unlock();
> +	user_event_mm_remove(t);
>   }
>   
-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com