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

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

[Leon Hwang]

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.

Reported-by: Lance Yang <lance.yang@linux.dev>
Tested-by: Lance Yang <lance.yang@linux.dev>
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
---
 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

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

[Michal Hocko]

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?

> Reported-by: Lance Yang <lance.yang@linux.dev>
> Tested-by: Lance Yang <lance.yang@linux.dev>
> Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
> ---
>  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

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

[Leon Hwang]

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].

[1]
https://lore.kernel.org/linux-mm/20260818162430.0a51522ac9bd671cce62650f@linux-foundation.org/

Thanks,
Leon

> 
>> Reported-by: Lance Yang <lance.yang@linux.dev>
>> Tested-by: Lance Yang <lance.yang@linux.dev>
>> Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
>> ---
>>  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
> 

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@suse.com>

> 
> [1]
> https://lore.kernel.org/linux-mm/20260818162430.0a51522ac9bd671cce62650f@linux-foundation.org/
> 
> Thanks,
> Leon
> 
> > 
> >> Reported-by: Lance Yang <lance.yang@linux.dev>
> >> Tested-by: Lance Yang <lance.yang@linux.dev>
> >> Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
> >> ---
> >>  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

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

[Mike Rapoport]

On Tue, Aug 18, 2026 at 09:49:34PM +0800, 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.
> 
> Reported-by: Lance Yang <lance.yang@linux.dev>
> Tested-by: Lance Yang <lance.yang@linux.dev>
> Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
> ---
>  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);

This is really obscure, can't it be in the loop body?
>  
>  	start &= P4D_MASK;
>  	if (start < floor)
> -- 
> 2.55.0
> 
> 

-- 
Sincerely yours,
Mike.

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

[Leon Hwang]

On 18/8/26 22:09, Mike Rapoport wrote:
> On Tue, Aug 18, 2026 at 09:49:34PM +0800, 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.
>>
>> Reported-by: Lance Yang <lance.yang@linux.dev>
>> Tested-by: Lance Yang <lance.yang@linux.dev>
>> Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
>> ---
>>  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);
> 
> This is really obscure, can't it be in the loop body?

Agreed. It should be in the loop body, after free_pmd_range().

Thanks,
Leon

>>  
>>  	start &= P4D_MASK;
>>  	if (start < floor)
>> -- 
>> 2.55.0
>>
>>
>