Re: [PATCH] posix-timers: cond_resched() during exit_itimers()

[Thomas Gleixner <tglx@linutronix.de>]

On Fri, Feb 14 2025 at 14:12, Benjamin Segall wrote:
> exit_itimers() loops through every timer in the process to delete it.
> This requires taking the system-wide hash_lock for each of these locks,
> and contends with other processes trying to create or delete timers.
> When a process creates hundreds of thousands of timers, and then exits
> while other processes contend with it, this can trigger softlockups on
> CONFIG_PREEMPT=n.
>
> Ideally this will some day be better solved by eliminating the global
> hashtable, but until that point mitigate the issue by doing
> cond_resched in that loop.

It won't help for a PREEMPT_NONE kernel because the loop will be equally
long as before. Only the hash lock contention will be smaller, but that
does not mean that mopping up 100k timers won't be able to take ages.

We really need to get this PREEMPT_LAZY thing going and kill all of this
cond_resched() nonsense.

Thanks,

        tglx