Message-ID: <094fcf02-37bd-c9b9-7156-77b9b34955e0@virtuozzo.com>
Date:   Wed, 11 Mar 2020 14:42:40 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     Shile Zhang <shile.zhang@...ux.alibaba.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Pavel Tatashin <pasha.tatashin@...een.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH v2 1/1] mm: fix interrupt disabled long time inside
 deferred_init_memmap()

On 11.03.2020 04:44, Shile Zhang wrote:
> Hi Kirill,
> 
> Sorry for late to reply!
> I'm not fully understood the whole thing about deferred page init, so I
> just force on the jiffies update issue itself.
> 
> Maybe I'm in wrong path, it seems make no sense that deferred page init in 1 CPU system,
> it cannot be initialize memory parallel.

Yes, it can't be initialized in parallel. But scheduler interprets deferred thread as a separate
task, so CPU time will be shared between that thread and the rest of tasks. This still should
make boot faster.

> It might be better to disable deferred page init in 'deferred_init' in case of 1 CPU
> (or only one memory node).
> 
> In other word, seems the better way to solve this issue is do not bind 'pgdatinit' thread
> on boot CPU.
> 
> I also refactor the patch based on your comment, please help to check, thanks!
> 
> 
> On 2020/3/4 18:47, Kirill Tkhai wrote:
>> On 04.03.2020 05:34, Shile Zhang wrote:
>>> Hi Kirill,
>>>
>>> Thanks for your quickly reply!
>>>
>>> On 2020/3/4 00:52, Kirill Tkhai wrote:
>>>> On 03.03.2020 19:15, Shile Zhang wrote:
>>>>> When 'CONFIG_DEFERRED_STRUCT_PAGE_INIT' is set, 'pgdatinit' kthread will
>>>>> initialise the deferred pages with local interrupts disabled. It is
>>>>> introduced by commit 3a2d7fa8a3d5 ("mm: disable interrupts while
>>>>> initializing deferred pages").
>>>>>
>>>>> The local interrupt will be disabled long time inside
>>>>> deferred_init_memmap(), depends on memory size.
>>>>> On machine with NCPUS <= 2, the 'pgdatinit' kthread could be pined on
>>>>> boot CPU, then the tick timer will stuck long time, which caused the
>>>>> system wall time inaccuracy.
>>>>>
>>>>> For example, the dmesg shown that:
>>>>>
>>>>>     [    0.197975] node 0 initialised, 32170688 pages in 1ms
>>>>>
>>>>> Obviously, 1ms is unreasonable.
>>>>> Now, fix it by restore in the pending interrupts inside the while loop.
>>>>> The reasonable demsg shown likes:
>>>>>
>>>>> [    1.069306] node 0 initialised, 32203456 pages in 894ms
>>>> The way I understand the original problem, that Pavel fixed:
>>>>
>>>> we need disable irqs in deferred_init_memmap() since this function may be called
>>>> in parallel with deferred_grow_zone() called from interrupt handler. So, Pavel
>>>> added lock to fix the race.
>>>>
>>>> In case of we temporary unlock the lock, interrupt still be possible,
>>>> so my previous proposition returns the problem back.
>>>>
>>>> Now thought again, I think we have to just add:
>>>>
>>>>      pgdat_resize_unlock();
>>>>      pgdat_resize_lock();
>>>>
>>>> instead of releasing interrupts, since in case of we just release them with lock held,
>>>> a call of interrupt->deferred_grow_zone() bring us to a deadlock.
>>>>
>>>> So, unlock the lock is must.
>>> Yes, you're right! I missed this point.
>>> Thanks for your comment!
>>>
>>>>> Signed-off-by: Shile Zhang<shile.zhang@...ux.alibaba.com>
>>>>> ---
>>>>>    mm/page_alloc.c | 6 +++++-
>>>>>    1 file changed, 5 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>>>> index 3c4eb750a199..d3f337f2e089 100644
>>>>> --- a/mm/page_alloc.c
>>>>> +++ b/mm/page_alloc.c
>>>>> @@ -1809,8 +1809,12 @@ static int __init deferred_init_memmap(void *data)
>>>>>         * that we can avoid introducing any issues with the buddy
>>>>>         * allocator.
>>>>>         */
>>>>> -    while (spfn < epfn)
>>>>> +    while (spfn < epfn) {
>>>>>            nr_pages += deferred_init_maxorder(&i, zone, &spfn, &epfn);
>>>>> +        /* let in any pending interrupts */
>>>>> +        local_irq_restore(flags);
>>>>> +        local_irq_save(flags);
>>>>> +    }
>>>>>    zone_empty:
>>>>>        pgdat_resize_unlock(pgdat, &flags);
>>>> I think we need here something like below (untested):
>>>>
>>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>>> index 79e950d76ffc..323afa9a4db5 100644
>>>> --- a/mm/page_alloc.c
>>>> +++ b/mm/page_alloc.c
>>>> @@ -1828,7 +1828,7 @@ static int __init deferred_init_memmap(void *data)
>>>>    {
>>>>        pg_data_t *pgdat = data;
>>>>        const struct cpumask *cpumask = cpumask_of_node(pgdat->node_id);
>>>> -    unsigned long spfn = 0, epfn = 0, nr_pages = 0;
>>>> +    unsigned long spfn = 0, epfn = 0, nr_pages = 0, prev_nr_pages = 0;
>>>>        unsigned long first_init_pfn, flags;
>>>>        unsigned long start = jiffies;
>>>>        struct zone *zone;
>>>> @@ -1869,8 +1869,18 @@ static int __init deferred_init_memmap(void *data)
>>>>         * that we can avoid introducing any issues with the buddy
>>>>         * allocator.
>>>>         */
>>>> -    while (spfn < epfn)
>>>> +    while (spfn < epfn) {
>>>>            nr_pages += deferred_init_maxorder(&i, zone, &spfn, &epfn);
>>>> +        /*
>>>> +         * Release interrupts every 1Gb to give a possibility
>>>> +         * a timer to advance jiffies.
>>>> +         */
>>>> +        if (nr_pages - prev_nr_pages > (1UL << (30 - PAGE_SHIFT))) {
>>>> +            prev_nr_pages = nr_pages;
>>>> +            pgdat_resize_unlock(pgdat, &flags);
>>>> +            pgdat_resize_lock(pgdat, &flags);
>>>> +        }
>>>> +    }
>>>>    zone_empty:
>>>>        pgdat_resize_unlock(pgdat, &flags);
>>>>   (I believe the comment may be improved more).
>>> Yeah, your patch is better!
>>> I test your code and it works!
>>> But it seems that 1G is still hold the interrupts too long, about 40ms in my env
>>> with Intel(R) Xeon(R) 2.5GHz). I tried other size, it is OK to use 1024 pages (4MB),
>>> which suggested by Andrew's before.
>>>
>>> Could you please help to review it again?
>>>
>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>> index 3c4eb750a199..5def66d3ffcd 100644
>>> --- a/mm/page_alloc.c
>>> +++ b/mm/page_alloc.c
>>> @@ -1768,7 +1768,7 @@ static int __init deferred_init_memmap(void *data)
>>>   {
>>>          pg_data_t *pgdat = data;
>>>          const struct cpumask *cpumask = cpumask_of_node(pgdat->node_id);
>>> -       unsigned long spfn = 0, epfn = 0, nr_pages = 0;
>>> +       unsigned long spfn = 0, epfn = 0, nr_pages = 0, prev_nr_pages = 0;
>>>          unsigned long first_init_pfn, flags;
>>>          unsigned long start = jiffies;
>>>          struct zone *zone;
>>> @@ -1809,8 +1809,17 @@ static int __init deferred_init_memmap(void *data)
>>>           * that we can avoid introducing any issues with the buddy
>>>           * allocator.
>>>           */
>>> -       while (spfn < epfn)
>>> +       while (spfn < epfn) {
>>>                  nr_pages += deferred_init_maxorder(&i, zone, &spfn, &epfn);
>>> +               /*
>>> +                * Restore pending interrupts every 1024 pages to give
>>> +                * the chance tick timer to advance jiffies.
>>> +                */
>>> +               if (nr_pages - prev_nr_pages > 1024) {
>>> +                       pgdat_resize_unlock(&flags);
>>> +                       pgdat_resize_lock(&flags);
>> Here is problem: prev_nr_pages must be updated.
>>
>> Anyway, releasing every 4M looks wrong for me, since you removes the fix that Pavel introduced.
>> He protected against big allocations made from interrupt content. But in case of we unlock
>> the lock after 4Mb, only 4Mb will be available for allocations from interrupts. pgdat->first_deferred_pfn
>> is updated at the start of function, so interrupt allocations won't be able to initialize
>> mode for themselve.
> 
> Yes, you're right. I missed this point since I'm not fully understood the code before.
> Thanks for your advice!
>> In case of you want unlock interrupts very often, you should make some creativity with first_deferred_pfn.
>> We should update it sequentially. Something like below (untested):
> 
> I got your point now, thanks!
>> ---
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 79e950d76ffc..be09d158baeb 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -1828,7 +1828,7 @@ static int __init deferred_init_memmap(void *data)
>>   {
>>       pg_data_t *pgdat = data;
>>       const struct cpumask *cpumask = cpumask_of_node(pgdat->node_id);
>> -    unsigned long spfn = 0, epfn = 0, nr_pages = 0;
>> +    unsigned long spfn = 0, epfn = 0, nr_pages;
>>       unsigned long first_init_pfn, flags;
>>       unsigned long start = jiffies;
>>       struct zone *zone;
>> @@ -1838,7 +1838,7 @@ static int __init deferred_init_memmap(void *data)
>>       /* Bind memory initialisation thread to a local node if possible */
>>       if (!cpumask_empty(cpumask))
>>           set_cpus_allowed_ptr(current, cpumask);
>> -
>> +again:
>>       pgdat_resize_lock(pgdat, &flags);
>>       first_init_pfn = pgdat->first_deferred_pfn;
>>       if (first_init_pfn == ULONG_MAX) {
>> @@ -1850,7 +1850,6 @@ static int __init deferred_init_memmap(void *data)
>>       /* Sanity check boundaries */
>>       BUG_ON(pgdat->first_deferred_pfn < pgdat->node_start_pfn);
>>       BUG_ON(pgdat->first_deferred_pfn > pgdat_end_pfn(pgdat));
>> -    pgdat->first_deferred_pfn = ULONG_MAX;
>>         /* Only the highest zone is deferred so find it */
>>       for (zid = 0; zid < MAX_NR_ZONES; zid++) {
>> @@ -1864,14 +1863,30 @@ static int __init deferred_init_memmap(void *data)
>>                            first_init_pfn))
>>           goto zone_empty;
>>   +    nr_pages = 0;
> 
> 'nr_pages' used to mark the total init pages before, so it cannot be zerolized each round.
> seems we need one more to count the pages init each round.
> 
>> +
>>       /*
>>        * Initialize and free pages in MAX_ORDER sized increments so
>>        * that we can avoid introducing any issues with the buddy
>>        * allocator.
>> +     * Final iteration marker is: spfn=ULONG_MAX and epfn=0.
>>        */
>> -    while (spfn < epfn)
>> +    while (spfn < epfn) {
>>           nr_pages += deferred_init_maxorder(&i, zone, &spfn, &epfn);
>> +        if (!epfn)
>> +            break;
> 
> Seems 'epfn' never goes to 0 since it is "end page frame number", right?
> So this is needless.
>> +        pgdat->first_deferred_pfn = epfn;
> 
> I think first_deferred_pfn update wrong value here, it seems should be the spfn, the start pfn right?
>> +        /*
>> +         * Restore pending interrupts every 128Mb to give
>> +         * the chance tick timer to advance jiffies.
>> +         */
>> +        if (nr_pages > (1UL << 27 - PAGE_SHIFT)) {
>> +            pgdat_resize_unlock(pgdat, &flags);
>> +            goto again;
>> +        }
>> +    }
>>   zone_empty:
>> +    pgdat->first_deferred_pfn = ULONG_MAX;
>>       pgdat_resize_unlock(pgdat, &flags);
>>         /* Sanity check that the next zone really is unpopulated */
>>
>>
> I update the patch based on your comment, it passed the test.
> Could you please help to review it again? Thanks!

The patch is OK for me. It may be sent into ml with an appropriate description.

Feel free to not forget to add my:

Co-developed-by: Kirill Tkhai <ktkhai@...tuozzo.com>

> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 3c4eb750a199..841c902d4509 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1763,12 +1763,17 @@ deferred_init_maxorder(u64 *i, struct zone *zone, unsigned long *start_pfn,
>         return nr_pages;
>  }
> 
> +/*
> + * Release the tick timer interrupts for every TICK_PAGE_COUNT pages.
> + */
> +#define TICK_PAGE_COUNT        (32 * 1024)
> +
>  /* Initialise remaining memory on a node */
>  static int __init deferred_init_memmap(void *data)
>  {
>         pg_data_t *pgdat = data;
>         const struct cpumask *cpumask = cpumask_of_node(pgdat->node_id);
> -       unsigned long spfn = 0, epfn = 0, nr_pages = 0;
> +       unsigned long spfn = 0, epfn = 0, nr_pages = 0, prev_nr_pages = 0;
>         unsigned long first_init_pfn, flags;
>         unsigned long start = jiffies;
>         struct zone *zone;
> @@ -1779,6 +1784,7 @@ static int __init deferred_init_memmap(void *data)
>         if (!cpumask_empty(cpumask))
>                 set_cpus_allowed_ptr(current, cpumask);
> 
> +again:
>         pgdat_resize_lock(pgdat, &flags);
>         first_init_pfn = pgdat->first_deferred_pfn;
>         if (first_init_pfn == ULONG_MAX) {
> @@ -1790,7 +1796,6 @@ static int __init deferred_init_memmap(void *data)
>         /* Sanity check boundaries */
>         BUG_ON(pgdat->first_deferred_pfn < pgdat->node_start_pfn);
>         BUG_ON(pgdat->first_deferred_pfn > pgdat_end_pfn(pgdat));
> -       pgdat->first_deferred_pfn = ULONG_MAX;
> 
>         /* Only the highest zone is deferred so find it */
>         for (zid = 0; zid < MAX_NR_ZONES; zid++) {
> @@ -1809,9 +1814,23 @@ static int __init deferred_init_memmap(void *data)
>          * that we can avoid introducing any issues with the buddy
>          * allocator.
>          */
> -       while (spfn < epfn)
> +       while (spfn < epfn) {
>                 nr_pages += deferred_init_maxorder(&i, zone, &spfn, &epfn);
> +               /*
> +                * Release the interrupts for every TICK_PAGE_COUNT pages
> +                * (128MB) to give the chance that tick timer to advance
> +                * the jiffies.
> +                */
> +               if ((nr_pages - prev_nr_pages) > TICK_PAGE_COUNT) {
> +                       prev_nr_pages = nr_pages;
> +                       pgdat->first_deferred_pfn = spfn;
> +                       pgdat_resize_unlock(pgdat, &flags);
> +                       goto again;
> +               }
> +       }
> +
>  zone_empty:
> +       pgdat->first_deferred_pfn = ULONG_MAX;
>         pgdat_resize_unlock(pgdat, &flags);
> 
>         /* Sanity check that the next zone really is unpopulated */
> 

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