Re: [PATCH] mm: add cond_resched() to free_pud_range()

[Michal Hocko]

On Wed 19-08-26 09:51:00, Leon Hwang wrote:
> On 18/8/26 22:08, Michal Hocko wrote:
> > On Tue 18-08-26 21:49:34, Leon Hwang wrote:
> >> Packet receive timeouts were seen in production. Tracing showed that an
> >> exiting process with a sparse 2.5 TiB mapping could remain in kernel
> >> context for over 20 ms without reaching a scheduling point while
> >> freeing PTE page-table pages. Hard IRQs could still be handled, but the
> >> per-CPU ksoftirqd thread and other runnable tasks could not run during
> >> that interval, delaying NET_RX softirq work queued to ksoftirqd.
> >>
> >> Like zap_pud_range(), add cond_resched() to free_pud_range() so
> >> ksoftirqd and other runnable tasks can run between PUD entries. Testing
> >> with PREEMPT_NONE showed that the maximum interval between scheduling
> >> points fell from over 20 ms to below 2 ms.
> > 
> > PREEMPT_NONE is effectivelly dead and most cond_resched will/should be
> > removed. Is there any reason why you are not using full preemption when
> > requiring low latencies?
> 
> Seems that we have reached a consensus about adding cond_resched() [1].

I disagree. It makes zero sense to add more cond_resched calls unless
they are essentially necessary. In this case you are talking about
latency requirements while using non-preemptible kernel which is in
stark contradiction. So either you have other very good reasons or just
base on the current changelog
Nacked-by: Michal Hocko <mhocko@xxxxxxxx>

> 
> [1]
> https://lore.kernel.org/linux-mm/20260818162430.0a51522ac9bd671cce62650f@xxxxxxxxxxxxxxxxxxxx/
> 
> Thanks,
> Leon
> 
> > 
> >> Reported-by: Lance Yang <lance.yang@xxxxxxxxx>
> >> Tested-by: Lance Yang <lance.yang@xxxxxxxxx>
> >> Signed-off-by: Leon Hwang <leon.hwang@xxxxxxxxx>
> >> ---
> >>  mm/memory.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/mm/memory.c b/mm/memory.c
> >> index 4134ac607ee0..68c15449de07 100644
> >> --- a/mm/memory.c
> >> +++ b/mm/memory.c
> >> @@ -231,7 +231,7 @@ static inline void free_pud_range(struct mmu_gather *tlb, p4d_t *p4d,
> >>  		if (pud_none_or_clear_bad(pud))
> >>  			continue;
> >>  		free_pmd_range(tlb, pud, addr, next, floor, ceiling);
> >> -	} while (pud++, addr = next, addr != end);
> >> +	} while (pud++, cond_resched(), addr = next, addr != end);
> >>  
> >>  	start &= P4D_MASK;
> >>  	if (start < floor)
> >> -- 
> >> 2.55.0
> > 

-- 
Michal Hocko
SUSE Labs