Messages in this thread |  | | Date | Thu, 31 Jan 2013 13:14:05 +0530 | Subject | Re: [PATCH] printk: Avoid softlockups in console_unlock() | From | anish singh <> |
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On Fri, Jan 18, 2013 at 3:09 AM, Andrew Morton <akpm@linux-foundation.org> wrote: > On Thu, 17 Jan 2013 22:04:42 +0100 > Jan Kara <jack@suse.cz> wrote: > >> ... >> >> So I played a bit with this. To make things easier for me I added >> artificial mdelay(len*10) (effectively simulating console able to print 100 >> characters per second) just after call_console_drivers() so that I can >> trigger issues even on a machine easily available to me. Booting actually >> doesn't trigger any problems because there aren't enough things happening >> in parallel on common machine during boot but >> echo t >/proc/sysrq-trigger & >> for i in /lib/modules/3.8.0-rc3-0-default/kernel/fs/*/*.ko; do >> name=`basename $i`; name=${name%.ko}; modprobe $name >> done >> easily triggers the problem (as modprobe uses both RCU & IPIs to signal all >> CPUs). >> >> Adding >> touch_nmi_watchdog(); >> touch_all_softlockup_watchdogs(); >> rcu_cpu_stall_reset(); > > I'm not sure that touch_all_softlockup_watchdogs() is needed? > touch_nmi_watchdog() itself calls touch_softlockup_watchdog(). > > If the rcu_cpu_stall_reset() is needed here then presumably it is > needed elsewhere and we should put a call to rcu_cpu_stall_reset() into > (the increasingly misnamed) touch_nmi_watchdog(). > >> into the printk loop did stop all the warnings and the machine eventually >> came alive again after finishing printing sysrq-t output (while printing >> the machine was responding to ping but ssh stopped working after a while - >> not sure what was happening but apparently some IO requests weren't >> completing and maybe networking started dropping packets because it >> accumulated too much RCU work). >> >> So your suggestion seems certainly plausible. I was just wondering >> a) Above three lines can be pretty expensive on big machines as they >> iterate over all CPUs. So we should probably limit it to once per jiffy or >> something like that? > > I guess so - is it hard to test the effects of such a change? Maybe do > a few MB of printks with the output disabled with `dmesg -n' and see > what effect such a patch has? > > If it does need ratelimiting, I'd worry about using jiffies for that. > If the kernel is spending a long time with interrupts disabled, jiffies > might not be incrementing. Using the CPU timestamp would be better > (eg, sched_clock()).
http://lxr.free-electrons.com/source/kernel/sched/clock.c#L75 I am puzzled because of this definition(above link).Sched_clock is dependent on jiffies and jiffies is blocked so how sched_clock would be better(I am 100% missing something very obvious)?
Is it that sched_clock is not dependent on jiffies? > >> b) Above three lines can make softlockup detection pretty useless if >> there's enough printk traffic (it would be enough to printk() something >> every 10s or so which can happen with netfilter logging packets or so). > > Yes, that is a concern. > >> But if we touch the watchdogs only if we spend more than 1 jiffy in the >> console_unlock() we should hopefully touch those watchdogs only in rare >> cases of heavy printk traffic. > > yup. Another option might be to do the touch_nmi_watchdog() only if > there is a "large" amount of data being emitted within > console_unlock(). Like your 1000 character threshold. > >> PS: sysrq-t was ~200 KB on that freshly booted machine so on a busy machine >> that would trigger softlockups with 115200 serial console as well (I >> actually do remember seeing it in some customer's reports). So it's not >> just boot. > > Yes, we have hit this before. Large printks over slow console devices. > See the sad little touch_nmi_watchdog() in lib/show_mem.c, for > example. I actually thought we'd fixed this in printk itself but it > seems not - the two touch_nmi_watchdog()s in there are for special > cases. > > Actually, a bit of grepping for touch_nmi_watchdog() is interesting. A > number of serial drivers are doing it, which makes me wonder why your > customers weren't saved by that - using the wrong driver, perhaps? If > we can get this fixed centrally then a lot of those calls should be > removeable. > > The presence of all those touch_nmi_watchdog() calls around the place > has implications for the testing of your patch, btw ;) > > -- > 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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