Re: [PATCH] mm: mlock: remove lru_add_drain_all()

[Shakeel Butt <shakeelb@google.com>]

On Thu, Oct 19, 2017 at 5:32 AM, Michal Hocko <mhocko@kernel.org> wrote:
> On Wed 18-10-17 16:17:30, Shakeel Butt wrote:
>> Recently we have observed high latency in mlock() in our generic
>> library and noticed that users have started using tmpfs files even
>> without swap and the latency was due to expensive remote LRU cache
>> draining.
>
> some numbers would be really nice
>

On a production workload, customers complained that single mlock()
call took around 10 seconds on mapped tmpfs files and the perf profile
showed lru_add_drain_all as culprit.

I wasn't able to replicate the workload on my test machine but a
simple workload of calling mlock() many type on a free machine shows
significant difference. Other than workload, the machine size (number
of cores) also matters.

>> Is lru_add_drain_all() required by mlock()? The answer is no and the
>> reason it is still in mlock() is to rapidly move mlocked pages to
>> unevictable LRU.
>
> Is this really true? lru_add_drain_all will flush the previously cached
> LRU pages. We are not flushing after the pages have been faulted in so
> this might not do anything wrt. mlocked pages, right?
>

Sorry for the confusion. I wanted to say that if the pages which are
being mlocked are on caches of remote cpus then lru_add_drain_all will
move them to their corresponding LRUs and then remaining functionality
of mlock will move them again from their evictable LRUs to unevictable
LRU.

>> Without lru_add_drain_all() the mlocked pages which
>> were on pagevec at mlock() time will be moved to evictable LRUs but
>> will eventually be moved back to unevictable LRU by reclaim. So, we
>> can safely remove lru_add_drain_all() from mlock(). Also there is no
>> need for local lru_add_drain() as it will be called deep inside
>> __mm_populate() (in follow_page_pte()).
>
> Anyway, I do agree that lru_add_drain_all here is pointless. Either we
> should drain after the memory has been faulted in and mlocked or not at
> all. So the patch looks good to me I am just not sure about the
> changelog.
>
>> Signed-off-by: Shakeel Butt <shakeelb@google.com>
>> ---
>>  mm/mlock.c | 5 -----
>>  1 file changed, 5 deletions(-)
>>
>> diff --git a/mm/mlock.c b/mm/mlock.c
>> index dfc6f1912176..3ceb2935d1e0 100644
>> --- a/mm/mlock.c
>> +++ b/mm/mlock.c
>> @@ -669,8 +669,6 @@ static __must_check int do_mlock(unsigned long start, size_t len, vm_flags_t fla
>>       if (!can_do_mlock())
>>               return -EPERM;
>>
>> -     lru_add_drain_all();    /* flush pagevec */
>> -
>>       len = PAGE_ALIGN(len + (offset_in_page(start)));
>>       start &= PAGE_MASK;
>>
>> @@ -797,9 +795,6 @@ SYSCALL_DEFINE1(mlockall, int, flags)
>>       if (!can_do_mlock())
>>               return -EPERM;
>>
>> -     if (flags & MCL_CURRENT)
>> -             lru_add_drain_all();    /* flush pagevec */
>> -
>>       lock_limit = rlimit(RLIMIT_MEMLOCK);
>>       lock_limit >>= PAGE_SHIFT;
>>
>> --
>> 2.15.0.rc1.287.g2b38de12cc-goog
>>
>
> --
> Michal Hocko
> SUSE Labs

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