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

[Andrew Morton <akpm@linux-foundation.org>]

On Sat, 9 Aug 2025 16:09:12 +0100 Matthew Wilcox <willy@infradead.org> wrote:

> On Sat, Aug 09, 2025 at 11:59:16AM +0000, Subrata Nath (Nokia) wrote:
> > Fix this by adding cond_resched() after all spinlock release points
> > in page_vma_mapped_walk() and in the main loop of shrink_zones().
> > These calls, placed outside spinlock-held sections, allow voluntary
> > scheduling and ensure timely quiescent state reporting, avoiding
> > prolonged RCU stalls.
> 
> No.  We're removing cond_resched().  See
> https://lore.kernel.org/linux-mm/87cyyfxd4k.ffs@tglx/
> and many many other emails over the past few years.

tglx's email was sent two years ago.

Meanwhile we have shipped kernels which are emitting nasty warning
splats (which are indications of possible other misbehavior).  So I
think we should proceed with Subrata's change and give it a cc:stable
also.

We already have 285 cond_resched()s in mm/.  If Thomas's idea ever gets
implemented then six more won't kill us.