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SubjectRe: [PATCH] lockdep: reduce stack_trace usage
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On Fri, 2010-04-23 at 21:40 +0800, Yong Zhang wrote:

> From 6b9d513b7c417c0805ef0acc1cb3227bddba0889 Mon Sep 17 00:00:00 2001
> From: Yong Zhang <yong.zhang0@gmail.com>
> Date: Fri, 23 Apr 2010 21:13:54 +0800
> Subject: [PATCH] lockdep: reduce stack_trace usage
>
> When calling check_prevs_add(), if all validations passed
> add_lock_to_list() will add new lock to dependency tree and
> alloc stack_trace for each list_entry. But at this time,
> we are always on the same stack, so stack_trace for each
> list_entry has the same value. This is redundant and eats up
> lots of memory which could lead to warning on low
> MAX_STACK_TRACE_ENTRIES.
> Using one copy of stack_trace instead.

OK, I like the idea, but I'm a little confused as to why you pull
save_trace() up two functions, from what I can see we can now end up
saving a trace where we previously would not have done one (the whole
recursive lock mess.

So please respin this with save_trace() in check_prev_add() right before
the first add_lock_to_list().

> Signed-off-by: Yong Zhang <yong.zhang0@gmail.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: David S. Miller <davem@davemloft.net>
> ---
> kernel/lockdep.c | 20 ++++++++++++--------
> 1 files changed, 12 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/lockdep.c b/kernel/lockdep.c
> index 2594e1c..097d5fb 100644
> --- a/kernel/lockdep.c
> +++ b/kernel/lockdep.c
> @@ -818,7 +818,8 @@ static struct lock_list *alloc_list_entry(void)
> * Add a new dependency to the head of the list:
> */
> static int add_lock_to_list(struct lock_class *class, struct lock_class *this,
> - struct list_head *head, unsigned long ip, int distance)
> + struct list_head *head, unsigned long ip,
> + int distance, struct stack_trace *trace)
> {
> struct lock_list *entry;
> /*
> @@ -829,11 +830,9 @@ static int add_lock_to_list(struct lock_class *class, struct lock_class *this,
> if (!entry)
> return 0;
>
> - if (!save_trace(&entry->trace))
> - return 0;
> -
> entry->class = this;
> entry->distance = distance;
> + entry->trace = *trace;
> /*
> * Since we never remove from the dependency list, the list can
> * be walked lockless by other CPUs, it's only allocation
> @@ -1635,7 +1634,7 @@ check_deadlock(struct task_struct *curr, struct held_lock *next,
> */
> static int
> check_prev_add(struct task_struct *curr, struct held_lock *prev,
> - struct held_lock *next, int distance)
> + struct held_lock *next, int distance, struct stack_trace *trace)
> {
> struct lock_list *entry;
> int ret;
> @@ -1694,14 +1693,14 @@ check_prev_add(struct task_struct *curr, struct held_lock *prev,
> */
> ret = add_lock_to_list(hlock_class(prev), hlock_class(next),
> &hlock_class(prev)->locks_after,
> - next->acquire_ip, distance);
> + next->acquire_ip, distance, trace);
>
> if (!ret)
> return 0;
>
> ret = add_lock_to_list(hlock_class(next), hlock_class(prev),
> &hlock_class(next)->locks_before,
> - next->acquire_ip, distance);
> + next->acquire_ip, distance, trace);
> if (!ret)
> return 0;
>
> @@ -1732,6 +1731,7 @@ check_prevs_add(struct task_struct *curr, struct held_lock *next)
> {
> int depth = curr->lockdep_depth;
> struct held_lock *hlock;
> + struct stack_trace trace;
>
> /*
> * Debugging checks.
> @@ -1748,6 +1748,9 @@ check_prevs_add(struct task_struct *curr, struct held_lock *next)
> curr->held_locks[depth-1].irq_context)
> goto out_bug;
>
> + if (!save_trace(&trace))
> + return 0;
> +
> for (;;) {
> int distance = curr->lockdep_depth - depth + 1;
> hlock = curr->held_locks + depth-1;
> @@ -1756,7 +1759,8 @@ check_prevs_add(struct task_struct *curr, struct held_lock *next)
> * added:
> */
> if (hlock->read != 2) {
> - if (!check_prev_add(curr, hlock, next, distance))
> + if (!check_prev_add(curr, hlock, next,
> + distance, &trace))
> return 0;
> /*
> * Stop after the first non-trylock entry,



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