linux-kernel - Re: [PATCH v6 2/2] Output stall data in debugfs

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Date:	Thu, 11 Aug 2011 21:43:11 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Alex Neronskiy <zakmagnus@...omium.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Don Zickus <dzickus@...hat.com>,
	Mandeep Singh Baines <msb@...omium.org>,
	Alex Neronskiy <zakmagnus@...omium.com>
Subject: Re: [PATCH v6 2/2] Output stall data in debugfs

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.


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