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Subject: Re: [PATCH 1/3] printk: Release console_sem after logbuf_lock
From: Peter Zijlstra <peterz@infradead.org>
To: Ingo Molnar <mingo@elte.hu>
Cc: Hugh Dickins <hughd@google.com>, Andrew Morton <akpm@linux-foundation.org>,
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Date: Fri, 10 Jun 2011 14:42:44 +0200
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On Fri, 2011-06-10 at 14:41 +0200, Peter Zijlstra wrote:
> On Fri, 2011-06-10 at 14:34 +0200, Ingo Molnar wrote:
> > * Peter Zijlstra <peterz@infradead.org> wrote:
> > 
> > > thinking that that would have my printk stmts appear on both the 
> > > fbcon as well as the serial line. But they fail to appear on the 
> > > latency tracer (current max was 165us waking an idle cpu).
> > 
> > Have you removed this bit:
> > 
> >                 spin_unlock(&logbuf_lock);
> >                 stop_critical_timings();        /* don't trace print latency */
> >                 call_console_drivers(_con_start, _log_end);
> >                 start_critical_timings();
> >                 local_irq_restore(flags);
> > 
> > which hides the latencies from the latency tracer?
> 
> it shouldn't those flags come from spin_lock_irqsave(), which already
> has IRQs disabled, so the restore shouldn't re-enable them.
> 
> Hmm,. that might actually already be true for mainline too, yeah, looks
> like we call vprintk()->console_unlock() with IRQs-disabled.
> 
> Hohumm..

Also, I used the preemptirqoff tracer, so even if it did re-enable
interrupts we should still have preemption disabled and still catch the
latency.