Re: [PATCH v7 14/36] function_graph: Use a simple LRU for fgraph_array index number

Steven Rostedt
2024-02-14T18:02:40Z

On Wed, 7 Feb 2024 00:10:04 +0900
"Masami Hiramatsu (Google)" <mhiramat@kernel.org> wrote:

> diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
> index ae42de909845..323a74623543 100644
> --- a/kernel/trace/fgraph.c
> +++ b/kernel/trace/fgraph.c
> @@ -99,10 +99,44 @@ enum {
> DEFINE_STATIC_KEY_FALSE(kill_ftrace_graph);
> int ftrace_graph_active;
>
> -static int fgraph_array_cnt;
> -
> static struct fgraph_ops *fgraph_array[FGRAPH_ARRAY_SIZE];
>
> +/* LRU index table for fgraph_array */
> +static int fgraph_lru_table[FGRAPH_ARRAY_SIZE];
> +static int fgraph_lru_next;
> +static int fgraph_lru_last;
> +
> +static void fgraph_lru_init(void)
> +{
> + int i;
> +
> + for (i = 0; i < FGRAPH_ARRAY_SIZE; i++)
> + fgraph_lru_table[i] = i;
> +}
> +
> +static int fgraph_lru_release_index(int idx)
> +{
> + if (idx < 0 || idx >= FGRAPH_ARRAY_SIZE ||
> + fgraph_lru_table[fgraph_lru_last] != -1)

Can fgraph_lru_table[fgraph_lru_last] != -1 ever happen? If not, we should
probably add a:

WARN_ON_ONCE(fgraph_lru_table[fgraph_lru_last] != -1))

As the size of fgraph_lru_table is the same size as the available indexes,
if we hit this I would think we had a fgraph_lru_relaese_index() without a
fgraph_lru_alloc_index() associated with it.

> + return -1;
> +
> + fgraph_lru_table[fgraph_lru_last] = idx;
> + fgraph_lru_last = (fgraph_lru_last + 1) % FGRAPH_ARRAY_SIZE;
> + return 0;
> +}
> +
> +static int fgraph_lru_alloc_index(void)
> +{
> + int idx = fgraph_lru_table[fgraph_lru_next];
> +
> + if (idx == -1)
> + return -1;
> +
> + fgraph_lru_table[fgraph_lru_next] = -1;
> + fgraph_lru_next = (fgraph_lru_next + 1) % FGRAPH_ARRAY_SIZE;
> + return idx;
> +}
> +
> static inline int get_ret_stack_index(struct task_struct *t, int offset)
> {
> return t->ret_stack[offset] & FGRAPH_RET_INDEX_MASK;
> @@ -367,7 +401,7 @@ int function_graph_enter(unsigned long ret, unsigned long func,
> if (index < 0)
> goto out;
>
> - for (i = 0; i < fgraph_array_cnt; i++) {
> + for (i = 0; i < FGRAPH_ARRAY_SIZE; i++) {
> struct fgraph_ops *gops = fgraph_array[i];
>
> if (gops == &fgraph_stub)
> @@ -935,21 +969,17 @@ int register_ftrace_graph(struct fgraph_ops *gops)
> /* The array must always have real data on it */
> for (i = 0; i < FGRAPH_ARRAY_SIZE; i++)
> fgraph_array[i] = &fgraph_stub;
> + fgraph_lru_init();
> }
>
> - /* Look for an available spot */
> - for (i = 0; i < FGRAPH_ARRAY_SIZE; i++) {
> - if (fgraph_array[i] == &fgraph_stub)
> - break;
> - }
> - if (i >= FGRAPH_ARRAY_SIZE) {
> + i = fgraph_lru_alloc_index();
> + if (i < 0 ||
> + WARN_ON_ONCE(fgraph_array[i] != &fgraph_stub)) {

The above can nicely fit on one column. No need to break it up:

if (i < 0 || WARN_ON_ONCE(fgraph_array[i] != &fgraph_stub)) {

> ret = -EBUSY;
> goto out;
> }
>
> fgraph_array[i] = gops;
> - if (i + 1 > fgraph_array_cnt)
> - fgraph_array_cnt = i + 1;
> gops->idx = i;
>
> ftrace_graph_active++;
> @@ -979,25 +1009,22 @@ int register_ftrace_graph(struct fgraph_ops *gops)
> void unregister_ftrace_graph(struct fgraph_ops *gops)
> {
> int command = 0;
> - int i;
>
> mutex_lock(&ftrace_lock);
>
> if (unlikely(!ftrace_graph_active))
> goto out;
>
> - if (unlikely(gops->idx < 0 || gops->idx >= fgraph_array_cnt))
> + if (unlikely(gops->idx < 0 || gops->idx >= FGRAPH_ARRAY_SIZE))
> + goto out;
> +
> + if (WARN_ON_ONCE(fgraph_array[gops->idx] != gops))
> goto out;
>
> - WARN_ON_ONCE(fgraph_array[gops->idx] != gops);
> + if (fgraph_lru_release_index(gops->idx) < 0)
> + goto out;

Removing the above WARN_ON_ONCE() is more reason to add it to the release
function.

-- Steve

>
> fgraph_array[gops->idx] = &fgraph_stub;
> - if (gops->idx + 1 == fgraph_array_cnt) {
> - i = gops->idx;
> - while (i >= 0 && fgraph_array[i] == &fgraph_stub)
> - i--;
> - fgraph_array_cnt = i + 1;
> - }
>
> ftrace_graph_active--;
>