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Message-ID: <20071026012342.GV19691@waste.org> Date: Thu, 25 Oct 2007 20:23:42 -0500 From: Matt Mackall <mpm@...enic.com> To: Tim Bird <tim.bird@...sony.com> Cc: Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>, linux kernel <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...hat.com>, Jon Smirl <jonsmirl@...oo.com> Subject: Re: IRQ off latency of printk is very high On Thu, Oct 25, 2007 at 04:41:55PM -0700, Tim Bird wrote: > Matt Mackall wrote: > > On Thu, Oct 25, 2007 at 03:52:28PM -0700, Tim Bird wrote: > >> Mathieu Desnoyers wrote: > >>> It might help to read this thread I posted on LKML in January 2006 > >>> explaining the problem, which led to some discussion about the issue. > >>> > >>> http://lkml.org/lkml/2006/6/3/48 > >> This is very helpful. Jon Smirl's answer seems to give the > >> rationale for supporting printk output in interrupt context. > >> I'm not sure, however, if extending the interrupt off period > >> to cover the console output is required. It didn't until > >> Ingo changed it in 2.6.17. > > > > Hmm, I see this at the beginning of the post-BK era (2.6.12-rc2): > > > > spin_lock_irqsave(&logbuf_lock, flags); > > ... > > spin_unlock(&logbuf_lock); > > call_console_drivers(_con_start, _log_end); > > local_irq_restore(flags); > > > > Well, I need to do some more research. This must be in > release_console_sem(). I was looking at vprintk, through > the ages. At 2.6.16, it looked like this: > > spin_lock_irqsave(&logbuf_lock, flags); > ... > spin_unlock_irqrestore(&logbuf_lock, flags); > console_may_schedule = 0; > release_console_sem(); > > but the irq restore has been moving around to different places > in that function over the last few years. I suspect that in the > common case the irqsave in vprintk is the one that disables > ints. > > It appears that formerly interrupts were enabled in vprintk but > re-disabled immediately upon entering release_console_sem(). > As it is now, they're held during formatting, buffering, > and output, which seems excessive. > > It seems draconian to drain the entire buffer with ints disabled. > Is it possible to break this up and send out smaller chunks > at a time? Maybe by putting a chunk loop in release_console_sem()? Well there are things we can do, yes, but I'd be worried that they've give up the deterministic behavior we rely on quite heavily for debugging. If event A happens before event B, we must see the message from A before the one from B, even if B happens in irq context. And if event B is a hard lock up, we'd also like to be sure the message for A actually gets out. If B happens in the interrupt that comes in when we re-enable them, that won't happen. (This is also a problem on unaccelerated video consoles, where scrolling may actually be slower than 115kbps!) -- Mathematics is the supreme nostalgia of our time. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.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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