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Subject: Re: [PATCH v2 1/1] mm: fix interrupt disabled long time inside
 deferred_init_memmap()
To:     Shile Zhang <shile.zhang@linux.alibaba.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Pavel Tatashin <pasha.tatashin@soleen.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Vlastimil Babka <vbabka@suse.cz>,
        Michal Hocko <mhocko@suse.com>
References: <20200303161551.132263-1-shile.zhang@linux.alibaba.com>
 <20200303161551.132263-2-shile.zhang@linux.alibaba.com>
 <fc22967d-0803-2e6f-26af-148a24f8f958@virtuozzo.com>
 <386d7d5f-a57d-f5b1-acee-131ce23d35ec@linux.alibaba.com>
 <2d4defb7-8816-3447-3d65-f5d80067a9fd@virtuozzo.com>
 <1856c956-858f-82d4-f3b3-05b2d0e5641c@linux.alibaba.com>
 <094fcf02-37bd-c9b9-7156-77b9b34955e0@virtuozzo.com>
 <59c7f8c7-d2ef-8b59-637e-88a353d4daf0@linux.alibaba.com>
From:   Kirill Tkhai <ktkhai@virtuozzo.com>
Message-ID: <e059d5be-79fe-b2c3-0d77-c33da4e7bce0@virtuozzo.com>
Date:   Wed, 11 Mar 2020 15:04:50 +0300
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On 11.03.2020 15:00, Shile Zhang wrote:
> 
> 
> On 2020/3/11 19:42, Kirill Tkhai wrote:
>> 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.
> 
> Thanks for your quickly reply!
> 
> Sorry, I don't think the CPU time can be shared to the rest of tasks since the CPU be blocked until
> the deferred pages are all initialized, as following code:
> 8<------
> #ifdef CONFIG_DEFERRED_STRUCT_PAGE_INIT
> 
>         /* There will be num_node_state(N_MEMORY) threads */
>         atomic_set(&pgdat_init_n_undone, num_node_state(N_MEMORY));
>         for_each_node_state(nid, N_MEMORY) {
>                 kthread_run(deferred_init_memmap, NODE_DATA(nid), "pgdatinit%d", nid);
>         }
> 
>         /* Block until all are initialised */
>         wait_for_completion(&pgdat_init_all_done_comp);
> 8<-------

Yes, sure. Than, there won't be any profit in runtime.

Anyway, we get maintaince profit in case of all code is written in generic way.
No new corner cases are welcomed.
 
> Maybe I misunderstood this point.
> 
> @Pavel,
> Could you please give any advice on this point? Thanks!
> 
>>> 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@linux.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@virtuozzo.com>
> 
> Yeah, sure!
> Many thanks for your kindly help on this issue!
> I'll send out v3 later for further review.
>>> 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 */
>>>
>