Re: [PATCH] mm: prevent RCU stalls in kswapd by adding cond_resched()

[Hillf Danton <hdanton@sina.com>]

On Sat, 9 Aug 2025 11:59:16 +0000 Subrata Nath (Nokia) wrote:
> The kswapd0 thread can spend extended time in
> page_vma_mapped_walk() -> queued_spin_lock_slowpath() without
> yielding the CPU. Even with CONFIG_PREEMPTION=3Dy, the rcu_preempt
> kthread cannot preempt kswapd0 because preemption and interrupts
> are disabled while holding the spinlock.
> 
> Example stall report:
>   rcu: INFO: rcu_preempt self-detected stall on CPU
>   rcu: rcu_preempt kthread starved for 65939907 jiffies!
>   Call trace:
>     queued_spin_lock_slowpath
>     page_vma_mapped_walk
>     folio_referenced_one
>     kswapd
> 
> Similar stalls occur in shrink_zones(), where long-running loops
> prevent CPUs from reporting a quiescent state during the RCU grace
> period. Without such reports, RCU stall warnings can escalate to
> soft lockups or OOM kills.
> 
> A quiescent state is reported when a CPU exits an RCU read-side
> critical section, enters idle/user mode, performs a context switch,
> or voluntarily reschedules.
> 
> Fix this by adding cond_resched() after all spinlock release points
> in page_vma_mapped_walk() and in the main loop of shrink_zones().

Given spinlock in calltrace, this fixes nothing at best.