Re: [PATCH v2] mm/swap: piggyback lru_add_drain_all() calls

[Konstantin Khlebnikov <khlebnikov@yandex-team.ru>]

On 04/10/2019 16.12, Michal Hocko wrote:
> On Fri 04-10-19 16:09:22, Konstantin Khlebnikov wrote:
>> This is very slow operation. There is no reason to do it again if somebody
>> else already drained all per-cpu vectors while we waited for lock.
>>
>> Piggyback on drain started and finished while we waited for lock:
>> all pages pended at the time of our enter were drained from vectors.
>>
>> Callers like POSIX_FADV_DONTNEED retry their operations once after
>> draining per-cpu vectors when pages have unexpected references.
> 
> This describes why we need to wait for preexisted pages on the pvecs but
> the changelog doesn't say anything about improvements this leads to.
> In other words what kind of workloads benefit from it?

Right now POSIX_FADV_DONTNEED is top user because it have to freeze page
reference when removes it from cache. invalidate_bdev calls it for same reason.
Both are triggered from userspace, so it's easy to generate storm.

mlock/mlockall no longer calls lru_add_drain_all - I've seen here
serious slowdown on older kernel.

There are some less obvious paths in memory migration/CMA/offlining
which shouldn't be called frequently.

> 
>> Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
>> ---
>>   mm/swap.c |   16 +++++++++++++++-
>>   1 file changed, 15 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/swap.c b/mm/swap.c
>> index 38c3fa4308e2..5ba948a9d82a 100644
>> --- a/mm/swap.c
>> +++ b/mm/swap.c
>> @@ -708,9 +708,10 @@ static void lru_add_drain_per_cpu(struct work_struct *dummy)
>>    */
>>   void lru_add_drain_all(void)
>>   {
>> +	static seqcount_t seqcount = SEQCNT_ZERO(seqcount);
>>   	static DEFINE_MUTEX(lock);
>>   	static struct cpumask has_work;
>> -	int cpu;
>> +	int cpu, seq;
>>   
>>   	/*
>>   	 * Make sure nobody triggers this path before mm_percpu_wq is fully
>> @@ -719,7 +720,19 @@ void lru_add_drain_all(void)
>>   	if (WARN_ON(!mm_percpu_wq))
>>   		return;
>>   
>> +	seq = raw_read_seqcount_latch(&seqcount);
>> +
>>   	mutex_lock(&lock);
>> +
>> +	/*
>> +	 * Piggyback on drain started and finished while we waited for lock:
>> +	 * all pages pended at the time of our enter were drained from vectors.
>> +	 */
>> +	if (__read_seqcount_retry(&seqcount, seq))
>> +		goto done;
>> +
>> +	raw_write_seqcount_latch(&seqcount);
>> +
>>   	cpumask_clear(&has_work);
>>   
>>   	for_each_online_cpu(cpu) {
>> @@ -740,6 +753,7 @@ void lru_add_drain_all(void)
>>   	for_each_cpu(cpu, &has_work)
>>   		flush_work(&per_cpu(lru_add_drain_work, cpu));
>>   
>> +done:
>>   	mutex_unlock(&lock);
>>   }
>>   #else
>>
>