Messages in this thread | | | Date | Thu, 13 Mar 2014 16:09:15 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH 6/6] kernel: Avoid softlockups in stop_machine() during heavy printing |
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On Thu, 13 Mar 2014 16:58:38 +0100 Jan Kara <jack@suse.cz> wrote:
> When there are lots of messages accumulated in printk buffer, printing > them (especially over serial console) can take a long time (tens of > seconds). stop_machine() will effectively make all cpus spin in > multi_cpu_stop() waiting for the CPU doing printing to print all the > messages which triggers NMI softlockup watchdog and RCU stall detector > which add even more to the messages to print. Since machine doesn't do > anything (except serving interrupts) during this time, also network > connections are dropped and other disturbances may happen. > > Paper over the problem by waiting for printk buffer to be empty before > starting to stop CPUs. In theory a burst of new messages can be appended > to the printk buffer before CPUs enter multi_cpu_stop() so this isn't a 100% > solution but it works OK in practice and I'm not aware of a reasonably > simple better solution. >
Yes it's rather hacky, but it's simple and direct and explicit and obvious. It's the stealth hackiness which causes harm.
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