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

[Leon Hwang <leon.hwang@linux.dev>]

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
>