Messages in this thread | | | Subject | Re: [PATCH] watchdog: softdog: fire watchdog even if softirqs do not get to run | From | Guenter Roeck <> | Date | Mon, 20 Feb 2017 07:31:36 -0800 |
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On 02/20/2017 02:03 AM, Niklas Cassel wrote: > On 02/19/2017 05:46 PM, Guenter Roeck wrote: >> Cc: Wolfram for input. >> >> On 02/17/2017 10:25 AM, Niklas Cassel wrote: >>> From: Niklas Cassel <niklas.cassel@axis.com> >>> >>> Checking for timer expiration is done from the softirq TIMER_SOFTIRQ. >>> >>> Since commit 4cd13c21b207 ("softirq: Let ksoftirqd do its job"), >>> pending softirqs are no longer always handled immediately, instead, >>> if there are pending softirqs, and ksoftirqd is in state TASK_RUNNING, >>> the handling of the softirqs are deferred, and are instead supposed >>> to be handled by ksoftirqd, when ksoftirqd gets scheduled. >>> >>> If a user space process with a real-time policy starts to misbehave >>> by never relinquishing the CPU while ksoftirqd is in state TASK_RUNNING, >>> what will happen is that all softirqs will get deferred, while ksoftirqd, >>> which is supposed to handle the deferred softirqs, will never get to run. >>> >>> To make sure that the watchdog is able to fire even when we do not get >>> to run softirqs, replace the timers with hrtimers. >>> >> >> This makes the driver dependent on HIGH_RES_TIMERS, which is not available >> on all architectures. Before adding that restriction, I would like to see >> some discussion if this is the only feasible solution. >> >> Is this driver the only one with this problem, or is anything using >> timers affected ? > > Anything using timers is affected. > The timers will still get incremented, but the code checking for timer > expiration is run from a softirq, which in this case never gets to run, > so the timers will never expire. > > Before 4cd13c21b207 ("softirq: Let ksoftirqd do its job"), softirqs > were never deferred, so they always got to run when exiting an irq. > > So previously with a user space process using all the CPU, like: > chrt -r 99 sh -c "while :; do :; done" > the softdog would still fire. > > If we ask the system to run something all the time, > and the system does that, I don't think we can blame the system. > It is however important that the watchdog can still detect and > fire when this happens. Other drivers, not so much. > > I guess another solution would be to modify the if-statements in > kernel/softirq.c to sometimes do the softirq directly, even if ksoftirqd > is in state TASK_RUNNING, if we also meet some other condition. > However, do we want to add that extra complexity?
No, in that respect using hrtimers is ok. I was more concerned about other users of timers. But, as you say, I guess that is considered ok.
Guenter
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