On Mon, Apr 12, 2021 at 08:54:03PM +0200, Thomas Gleixner wrote: > 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 :)
But it was simple! It just hit all the CPUs. However, you (quite rightly) pointed out that this simple approach had a few shortcomings. ;-) > > 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. Will do! Of course, it turns out that lockdep also doesn't like waited-on smp_call_function_single() invocations from timer handlers, so I am currently looking at other options for dealing with that potential use-after-free. I am starting to like the looks of "only set CLOCK_SOURCE_VERIFY_PERCPU on statically allocated clocksource structures and let KASAN enforce this restriction", but I have not quite given up on making it more general. Thanx, Paul