Paul, On Mon, Apr 12 2021 at 11:20, Paul E. McKenney wrote: > On Mon, Apr 12, 2021 at 03:08:16PM +0200, Thomas Gleixner wrote: >> The reason for irqsave is again historical AFAICT and nobody bothered to >> clean it up. spin_lock_bh() should be sufficient to serialize against >> the watchdog timer, though I haven't looked at all possible scenarios. > > Though if BH is disabled, there is not so much advantage to > invoking it from __clocksource_watchdog_kthread(). Might as > well just invoke it directly from clocksource_watchdog(). > >> > 2. Invoke clocksource_verify_percpu() from its original >> > location in clocksource_watchdog(), just before the call to >> > __clocksource_unstable(). This relies on the fact that >> > clocksource_watchdog() acquires watchdog_lock without >> > disabling interrupts. >> >> That should be fine, but this might cause the softirq to 'run' for a >> very long time which is not pretty either. >> >> Aside of that, do we really need to check _all_ online CPUs? What you >> are trying to figure out is whether the wreckage is CPU local or global, >> right? >> >> Wouldn't a shirt-sleeve approach of just querying _one_ CPU be good >> enough? Either the other CPU has the same wreckage, then it's global or >> it hasn't which points to a per CPU local issue. >> >> Sure it does not catch the case where a subset (>1) of all CPUs is >> affected, but I'm not seing how that really buys us anything. > > Good point! My thought is to randomly pick eight CPUs to keep the > duration reasonable while having a good chance of hitting "interesting" > CPU choices in multicore and multisocket systems. > > However, if a hard-to-reproduce problem occurred, it would be good to take > the hit and scan all the CPUs. Additionally, there are some workloads > for which the switch from TSC to HPET is fatal anyway due to increased > overhead. For these workloads, the full CPU scan is no additional pain. > > So I am thinking in terms of a default that probes eight randomly selected > CPUs without worrying about duplicates (as in there would be some chance > that fewer CPUs would actually be probed), but with a boot-time flag > that does all CPUs. I would add the (default) random selection as a > separate patch.
You can't do without making it complex, right? Keep it simple is not an option for a RCU hacker it seems :) > I will send a new series out later today, Pacific Time. Can you do me a favour and send it standalone and not as yet another reply to this existing thread maze. A trivial lore link to the previous version gives enough context. Thanks, tglx