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Subject: Re: [PATCH v6 2/2] Output stall data in debugfs
From: Peter Zijlstra <peterz@infradead.org>
To: Alex Neronskiy <zakmagnus@chromium.org>
Cc: linux-kernel@vger.kernel.org, Ingo Molnar <mingo@elte.hu>,
        Don Zickus <dzickus@redhat.com>,
        Mandeep Singh Baines <msb@chromium.org>,
        Alex Neronskiy <zakmagnus@chromium.com>
Date: Thu, 11 Aug 2011 21:43:11 +0200
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	 <1312999364-21104-2-git-send-email-zakmagnus@chromium.org>
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zakmagnus seems to bounce, so who am I talking to anyway..

On Thu, 2011-08-11 at 21:35 +0200, Peter Zijlstra wrote:
> 
> 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. 

That also cures another problem you have, a seq_file buffer is only 1
page large, you should be using the seqfile iterator interface and print
one line at a time.. now clearly that won't work with preemption
disabled either.