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SubjectRe: [PATCH v6 2/2] Output stall data in debugfs
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On Wed, 2011-08-10 at 11:02 -0700, Alex Neronskiy wrote:
> @@ -210,22 +236,27 @@ void touch_softlockup_watchdog_sync(void)
> /* watchdog detector functions */
> static void update_hardstall(unsigned long stall, int this_cpu)
> {
> if (stall > hardstall_thresh && stall > worst_hardstall) {
> unsigned long flags;
> + spin_lock_irqsave(&hardstall_write_lock, flags);
> + if (stall > worst_hardstall) {
> + int write_ind = hard_read_ind;
> + int locked = spin_trylock(&hardstall_locks[write_ind]);
> + /* cannot wait, so if there's contention,
> + * switch buffers */
> + if (!locked)
> + write_ind = !write_ind;
> +
> worst_hardstall = stall;
> + hardstall_traces[write_ind].nr_entries = 0;
> + save_stack_trace(&hardstall_traces[write_ind]);
>
> + /* tell readers to use the new buffer from now on */
> + hard_read_ind = write_ind;
> + if (locked)
> + spin_unlock(&hardstall_locks[write_ind]);
> + }
> + spin_unlock_irqrestore(&hardstall_write_lock, flags);
> }
> }

That must be the most convoluted locking I've seen in a while.. OMG!

What's wrong with something like:

static void update_stall(struct stall *s, unsigned long stall)
{
if (stall <= s->worst)
return;
again:
if (!raw_spin_trylock(&s->lock[s->idx])) {
s->idx ^= 1;
goto again;
}
if (stall <= s->worst)
goto unlock;
s->worst = stall;
s->trace[s->idx].nr_entries = 0;
save_stack_trace(&s->trace[s->idx]);

unlock:
raw_spin_unlock(&s->lock[s->idx]);
}

And have your read side do:


static void show_stall_trace(struct seq_file *f, void *v)
{
struct stall *s = f->private;
int i, idx = ACCESS_ONCE(s->idx);
mutex_lock(&stall_mutex);

raw_spin_lock(&s->lock[idx]);
seq_printf(f, "stall: %d\n", s->worst);
for (i = 0; i < s->trace[idx].nr_entries; i++) {
seq_printf(f, "[<%pK>] %pS\n",
(void *)s->trace->entries[i],
(void *)s->trace->entries[i]);
}
raw_spin_unlock(&s->lock[idx]);
mutex_unlock(&stall_mutex);
}

Yes its racy on s->worst, but who cares (if you do care you can keep a
copy in s->delay[idx] or so). Also, it might be better to not do the
spinlock but simply use an atomic bitop to set an in-use flag, there is
no reason to disable preemption over the seq_printf() loop.




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