Messages in this thread | | | Date | Thu, 11 Aug 2011 13:10:48 -0700 | Subject | Re: [PATCH v6 2/2] Output stall data in debugfs | From | Alex Neronskiy <> |
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Please don't send email to zakmagnus@chromium.com. That does not exist. The correct address is zakmagnus@chromium.org. I messed up my own email address somewhere somehow.
On Thu, Aug 11, 2011 at 12:35 PM, Peter Zijlstra <peterz@infradead.org> wrote: > 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. One change here is to use raw_spin functions. Okay, sure. Another is to use a mutex instead of a spinlock among the readers. Makes a lot of sense.
Another change is to allow concurrent writers. The readers are serialized but the writers are concurrent; isn't that a strange design? The way the "main" index is changed also looks problematic. A writer will switch the index before anything useful is even known to be in the buffer, and then a reader can go ahead and get that lock and read something potentially very old and misleading. I don't think that's okay. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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